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Building our new SemTest Pt.3: By JIM ROWE Now that we have looked at the full circuit of our new Discrete Semiconductor Test Set, it’s time to describe its construction and the setting-up procedure. We also describe how to fit a crowbar circuit to quickly discharge the HT after making high-voltage measurements. A S SHOWN IN the photos, the SemTest is built in an ABS enclosure measuring 222 x 146 x 55mm. Apart from VR10 (the Mosfet VGS pot) and the five pushbutton switches (which mount directly on the front panel), all the components are mounted on one of two PCBs. Both boards are double-sided, so there is no need to fit any wire links. Incidentally, our prototype photos in the February & March 2012 issues showed numerous link positions on both PCBs. These have now been 84  Silicon Chip incorporated into the copper patterns on the top layers of both boards so that is one less tedious task needing to be done. The main board (coded 04103121) mounts in the bottom of the enclosure, while the display board (coded 04103122) sits behind the front panel and is spaced 18mm from it. The two boards are linked via three flat ribbon cables fitted with IDC connectors. Rotary switch S2 is mounted on the lower PCB. Its control shaft is 42mm long, so that when the case is assembled, it passes through clearance holes in both the display PCB and the front panel. Power switch S1 and 12V input connector CON1 are both located on the righthand end of the main board, towards the rear, and pass through holes in the righthand end of the enclosure. A small hole nearer the front of the enclosure provides access to trimpot VR2, which is used to set the micro’s 2.490V reference voltage. Six similar holes along the top edge of the righthand end of the enclosure siliconchip.com.au COMMON DISCRETE SEMICONDUCTOR DEVICE CONNECTIONS DIODES B A A E A K A K A A A C C K A C103B, BT149 G D S G A 2N7000, VK10KN K BT169D, 2N5060 C106D (TO-225) (DO-247) S (TO-225) E D S (TO-264) B C A C E B LEDS C (TO-218) E A B CATHODE BAND A E K A (TO-220) G C106D1, C122E (TO-263A D-PAK) PUTS 2N6027 G S K (SOT-93/ TO-264) G S A TRIACS D IGBTS A1 A2 C FGA25N120ANTD G BT137F, SC141D, SC151D, TAG225 A2 C E B (TO-5) B K A K (TO-262) D (TOP-3) K A D BD135-6-7-8, BD139-140, BD681-2, BF469-470, MJE340-350 G D G D (TO-220) C G BS170, BS250 G C D S (TO-92/72) E PN100, PN200, C8050 ETC A K S(1) C K B A K S G A K A G D(2) B (TO-220) K G D G1(4) E B BC639, BC640, 2SC3242 K (TO-220) K G2(3) (TO-92/14) K SCRS 2N7002, DMP2215L BF998 (TO-92/17) BC327-8, BC337-8, BC546-7-8-9, BC550, BC556-7-8-9, BC560, 2N2222A, 2N3638 MBR735 K MOSFETS BJTS K K C E G C E (TO-3PN) A1 C (TO-3) A2 (TOP-3) G ABOVE: this handy table shows the pin connections for many discrete semiconductor devices. The ZIF socket on the front of the SemTest makes it easy to connect devices for testing. are used to access various trimpots mounted on the righthand end of the display board. Main PCB assembly Use the layout diagram of Fig.10 as a guide to assembling this board. Begin by fitting all the smaller resistors, which should be of 1% tolerance. Note that one of these resistors (which mounts about 20mm above and to the right of IC3) is marked “0Ω/68Ω”, because its value depends on the type of relay you use for Relay 1. If you use a relay with a 12V coil, this resistor can be replaced with a wire link (or zero ohm resistor). With a 6V relay (eg Jaycar SY-4058), the resistor should be 68Ω. The 1W and 5W resistors are next. Mount the 5W resistors about 1.5mm above the surface of the board, to allow some ventilation if they become hot in operation. siliconchip.com.au Follow with trimpots VR1 and VR2. VR1 (50kΩ) mounts near IC1 while VR2 is a horizontal multi-turn 10kΩ unit which mounts at lower right. Once these are in, fit the capacitors. The two 47µF 450V electrolytics need to be laid on their sides and secured with small cable ties. Now fit the DC input connector CON1, followed by power switch S2, DIL pin headers CON2, CON3 and CON4, the 40-pin DIL socket for IC4 and the 8-pin DIL sockets for IC1 and IC3. Relay drivers IC5 and IC6 do not need sockets and are soldered direct to the PCB later during the assembly. The six 1mm PCB terminal pins, used for the various test points can now go in. The four relays can then be installed. Note that RLY7 and RLY8 are mini-DIL reed relays, which should be mounted with the orientation shown in Fig.10. The next step is to wind T1, the step-up transformer for the SemTest’s DC-DC converter. The winding and assembly details are shown in Fig.11; follow this exactly (or else!). Wind each layer as closely and evenly as possible; wind them all in the same direction and cover each layer with a layer of insulating tape (to both hold that layer in place and provide insulation between it and the layer above it). Before T1 is assembled don’t forget the “gap” washer, cut from a small piece of 0.06mm thick plastic sheet. T1 can now be mounted on the main PCB. It is held in place (as well as being held together) by an M3 x 25mm long Nylon screw and nut. Note that the primary start (S), tap (T) and secondary finish (F) wires all connect to the PCB, just to the right of the transformer itself. Semiconductors Now for the semiconductors, startMay 2012  85 HT crowbar – a safety refinement +HV 1 2 3 A 10nF K A Vin 4 5 10k D1 1N4004 V+ LK1 AG 100 F 16V PUT1 2N6027 A SCR1 TYN816 KG K K 100k A A AG K HV DC CROWBAR TYN816 2N6027 1N4004 SC 100  1W GND CON1 2012 330  1W K A K A KG CENTRE LEAD CUT SHORT IN THIS PROJECT Fig.7: the circuit monitors the converter’s power supply rail in the SemTest & when Vin drops below 6V, PUT1 & SCR1 turn on to discharge the 47μF capacitors across the high-voltage output. S INCE PRODUCING our proto­ type SemTest presented in the February & March issues, we have developed a further refinement – an add-on crowbar module which in- Parts List 1 PCB, code 04105121, 56 x 40.5mm (available from SILICON CHIP) 1 M3 x 6mm machine screw & nut 2 M3 shakeproof washers 1 100mm length red heavy duty mains-rated hook-up wire 1 200mm length black heavy duty mains-rated hook-up wire 1 200mm length yellow hook-up wire 1 70mm length 30mm diameter heatshrink tubing (Jaycar WH5658, Altronics W0919A) Semiconductors 1 TYN816 SCR (SCR1; Altronics Z1778) 1 2N6027 PUT (PUT1; Jaycar ZT2397, Altronics Z1410) 1 1N4004 1A diode (D1) Capacitors 1 100µF 16V electrolytic 1 10nF monolithic multi-layer ceramic Resistors (0.25W, 1%) 1 100kΩ 1 330Ω 1W (5%) 1 10kΩ 1 100Ω 1W (5%) 86  Silicon Chip stantly kills the high voltage applied to the ZIF socket at the conclusion of any breakdown voltage test. As a further safety measure, it also kills the high voltage in the event that the SemTest is inadvertently turned off before a test has properly concluded. This minimises the chance of the user getting a shock from the test terminals when removing the DUT or a possible breakdown of the DUT itself when the power is inadvertently removed. The crowbar module is wired to three points on the main (lower) SemTest PCB. On our prototype, these wires have been soldered to specific component leads but the final SemTest PCB has pads for these wires. The crowbar board senses the 11.4V supply rail to the MC34063 DC/DC converter IC1. This drops very quickly to around 6V when a test finishes or more slowly if the unit is switched off during a test. Either way, this is the trigger for the crowbar to discharge the capacitor bank from 600V to a few volts in around 20ms. Circuit description Fig.7 shows the full crowbar circuit. It could potentially be used in other devices but for use with the SemTest, link LK1 is installed, to short Vin (the sense input) and V+ (its power supply) together. The +HV and GND terminals at CON1 are connected across the SemTest’s high voltage capacitor bank. Fig.8 shows a fragment of the SemTest circuit and demonstrates how the crowbar module is connected. The V+ terminal goes to pin 6 of IC1, which is at around +11.4V when the DC/DC converter is running and drops to 0V when it is switched off. While the DC/DC converter is running, current flows from this rail, through diode D1, charging the 100µF capacitor. As this capacitor charges, the gate (AG) of programmable unijunction transistor PUT1 is pulled up too, via the 10kΩ and 100kΩ resistors. At the same time, the anode (A) is pulled up via a 330Ω resistor. The 10nF capacitor between PUT1’s anode and gate is initially discharged and this helps to keep the gate at anode potential, preventing false triggering if there are any initial glitches in IC1’s power supply (eg, due to relay contact bounce). A PUT is essentially a small anodegate SCR. While a conventional SCR is turned on when its gate is pulled above its cathode, a PUT turns on when its gate is pulled below its anode, sinking current from the gate. Both SCRs and PUTs remain on once triggered until their anode-cathode current flow drops below the “holding” current, in this case much less than a milliamp. As long as V+/Vin are held at around 11.4V, the crowbar circuit remains deactivated. But once Vin drops precipitously, the 10nF capacitor begins to charge while the 100µF capacitor retains its charge, by virtue of diode D1. Once Vin drops below the ~6V threshold, sufficient current flows from PUT1’s gate to trigger it on. It then dumps the charge in the 100µF capacitor into SCR1’s gate (KG), via the 330Ω current-limiting resistor. This happens in less than 100μs if Vin drops fast, as when a test ends normally. The 330Ω resistor limits the current into SCR1’s gate to around 25siliconchip.com.au +11.4V RELAY1 CROWBAR 3 +HV V+ 68 IC5 PIN18 1 GND 5 D2 UF4007 A 5W 80T 6 7 8 Vcc Ips DrC 10T SwC Ct IC1 MC34063 GND 4 1nF 33k 1W 1.0k 1W 33k 1W +OPV/+BV T1 0.27  3 1.5k 5W K 33k 1W SwE Cin5 TP4 1 B C E 2 E 2.2k B C Q1 BC337 470nF 630V 390k 75k 1% 100k 390k 75k 1% 100k Q3 IRF540N G Q2 BC327 390k 470nF 630V S +Vdevice 75k 1% D 100 1.0k 47 F 1W 450V 75k 1% 390k SET TEST VOLTS VR1 50k (25T) +1.25V 100k 100k 47 F 450V RELAY 2b TO S2a Fig.8: this diagram shows how the HV Crowbar module is connected to the SemTest circuit. Only three connections are required, as indicated by the lines highlighted in red. © 2012 CON1 +HV 04105121 100 1W 330 1W +V Vin GND D1 100F 4004 LK1 100k 10k SCR1 TYN816 10nF 12150140 30mA, enough to trigger it reliably. SCR1 then rapidly discharges the high voltage capacitor bank through the 100Ω resistor. The peak discharge current is 600V ÷ 100Ω = 6A. PUT1 switches off as soon as it has finished dumping the charge of the 100µF cap into SCR1’s gate. But SCR1 stays on until the current through it drops below 40mA (its holding current) so the capacitor bank discharges to around 4V. The specified TYN816 SCR is rated for 800V & 16A. Do not use an SCR with lower ratings. PUT1 2N6027 HIGH VOLTAGE RABW CROWBAR ORC EGATLOV HGIH Fig.9: follow this layout diagram and photo to build the HV Crowbar. For the SemTest, leave out the screw terminal block and install a wire link for LK1. Refer to the overlay diagram, Fig.9. Fit the two small resistors first, followed by diode D1, with its cathode stripe towards the right side of the board. Use a lead off-cut for LK1 and solder it in place. Then install the two 1W resistors. Wiggle the middle lead of SCR1 back and forth until it snaps off. If there is any lead remaining, remove it with side-cutters. Bend the remaining two leads down and insert them through the holes on the PCB, then use the machine screw to attach the tab with a shakeproof washer both under the screw head and under the nut. Do it up tightly since the screw conducts the current when the crowbar activates. Then solder the two pins. Fit the 10nF capacitor and then PUT1, bending its leads out with pliers to suit the pad spacing. Push it down as far as it will go before soldering and trimming the leads. Then mount the 100µF capacitor, with its longer (positive) lead towards the left side of the PCB. Bend its leads so that it lays down flat on the board before soldering them. Don’t fit a terminal block for CON1 since we have limited clearance to fit the unit into the SemTest. Instead, solder a red wire to HV, a yellow wire to V+ and a black wire to 0V. Make sure there are no stray copper strands. Wire the unit up to the SemTest as shown in the main overlay diagram (Fig.10). Trim each lead so that you don’t have a lot of extra length. The photos show the best place to fit it. Once it’s wired up, slip the crowbar module into the heatshrink tubing and apply gentle heat. Make sure there is no exposed metal when you are finished. Some silicone sealant can then be used to hold the unit in place, so it doesn’t rattle around inside the case. Once the SemTest unit itself is complete, the HV crowbar must now be tested for correct operation, as described in the main article. ing with the diodes and zener diodes. Make sure that these are all installed the correct way around. The same goes for transistors Q1 & Q2. Make sure Q1 is a BC337 and Q2 is a BC327. Note that IC7, the metering voltage reference IC, is in the same TO-92 package as Q1 and Q2 – be careful not to install it in the wrong position. Two devices come in TO-220 packages – REG1, the 7805 5V regulator and Q3, the IRF540N switching Mosfet. Construction and testing siliconchip.com.au May 2012  87 300k V4.11+ 240k 160k 4D 15 16 +Vdev C ON3 8.0MHz X1 ULN2803A 27pF 27pF 7D 1 2 IC 6 C ON2 V5+ S T F 75k + – 2 1 IC 4 Q3 IRF540N 47 F Q1 BC 337 Q2 BC 327 390k sgV 10k vedI + 100nF V5+ 1nF 1k 39 470k VR1 50k 15 16 + – 1DEL D9 4148 2 1 IC 5 V01,R 680 12k 5.1k 30 RLY1 10k 100k PUT1 2N6027 10nF 100 F 220 F REG1 7805 1000 F 1000 F FER+ ZD2 V52 6V2 RLY7 10k 2.7M 2.4k S2 V001 SET TEST VOLTS V05 DNG 2102 © ULN2803A 100 F 0.27  5W C ON4 7S 3.0k 10k IC 3 LM358 3.9k LK1 4004 D1 330  1W SC R1 TYN816 © 2012 4004 D1 S1 C ON1 NOTE: W IRE LINK FITTED FOR LK1 C ROW BAR MODULE (IN HEATSHRINK SLEEVING) D3 TP1 4148 VR2 TPG D4 10k SET 2.49V REF 4148 IC 7 LM336Z-2.5 100nF LOW ER BOARD R AB W OR C EBAR GATL OV H GI H HIGH VOLTAGE C ROW DRA OB R E W OL GND GND Vin V+ 100  1W 04105121 22 +HV C ON1 ET ER C SI D R OT CU D N O CI M E S T E S T S ET 47 F 450V TP4 V+ C ROW BAR 20k TPG 470nF 630V 390k GND C ROW BAR IC 1 34063 390k PIC 16F877A 12130140 100nF 470nF 630V 390k 47 F 450V Vgs Idev POW ER Fig.10: follow this parts layout diagram to build the main (lower) PCB assembly. Use a socket for IC4 and take care to ensure that all semiconductors and electrolytic capacitors are correctly orientated. Take care also when installing the three IDC headers – they must go in with their key-way slots positioned as shown. The two switches are mounted directly on the PCB but be sure to use the specified switch for rotary switch S2 to ensure that its control shaft is long enough (see text and panel). 300k RLY8 TPVdev 9 10 33k 1W 33k 1W TPG RLY2 33k 1W T1 75k D2 UF4007 75k !VH+ 75k 1.5k 5W 1.6k HV+ 100 10k 100k C ROW BAR 680 ZD1 4V7 100k 2.2k 10k 12V IN WARNING! HIGH VOLTAGES (UP TO 600V DC) CAN BE PRESENT WHEN THE CIRCUIT IS OPERATING. CHECK TO ENSURE THAT THE 47F 450V CAPACITORS HAVE FULLY DISCHARGED BEFORE WORKING ON THE CIRCUIT. 2.2k 100k 100k 56 1.0k 1W 300k 0 /68  1.0k 1W C OM 10k C OIL 10k 10k NO 10k NC 10nF 3.0k 100k 560 NC 560 56 C OIL 10nF NO 2.4k C OMMON + 88  Silicon Chip + 100nF 04105121 siliconchip.com.au The view shows the completed main board assembly before the HV crowbar module is added. It carries the PIC microcontroller (IC4), the power supply components and the test voltage selector switch (S2). Both are mounted with their leads bent down by 90° at a distance of 6mm from their bodies, so they pass down through the corresponding holes in the board to be soldered. Both devices are mounted on standard 19mm-square U-shaped heatsinks and secured using M3 x 10mm machine screws and nuts. Having installed the semiconductors, install crystal X1. It’s mounted just to the left of IC4’s socket. That done, install the 3-pole 4-position rotary switch. This switch must have a 42mm long shaft and the one to use is a metric switch made by Lorlin (CK1051). We sourced ours from Element14 (Cat. 112-3697). IC5 & IC6 can then be soldered in place and IC1, IC3 & IC4 plugged into their respective sockets. The main PCB assembly can then be completed by wiring the HV crowbar PCB to it, as shown in Fig.10 Display PCB assembly Fig.12 is the component overlay for this PCB. Begin by fitting the resistors. As before, two of these are shown with a value of 0Ω/68Ω, to suit 6V or 12V mini SPDT relays: with a 6V relay, use a 68Ω resistor; for a 12V relay, use a wire link. The seven trimpots can now go in. VR11 is a 10kΩ mini horizontal type near relay RLY3. The remaining six multi-turn trimpots have values of 5kΩ and 10kΩ; don’t mix them up. VR10, the 10kΩ dual-gang pot, is wired with short flying leads and will be bolted to the front panel later. Note that it should have its shaft cut to 15mm long, to suit the knob. Follow with the two capacitors and the relays. Make sure the two mini-DIL reed relays are correctly orientated, as you would for DIL ICs. Now fit the semiconductors. There are four TO-92 devices: transistors Q4 & Q5 and voltage references IC8 & IC9; don’t siliconchip.com.au UPPER SECTION OF FERRITE POT CORE BOBBIN WITH WINDING (10T OF 0.8mm DIAMETER ENAMELLED COPPER WIRE WITH END BROUGHT OUT. THEN START OF 0.25mm DIA ECW TWISTED TO IT, BEFORE WINDING 4 x 20T LAYERS OF SECONDARY. NOTE THAT ALL FIVE LAYERS SHOULD BE COVERED WITH INSULATING TAPE) FINISH (OF SECONDARY) TAP (END OF PRIMARY, START OF SECONDARY) START (OF PRIMARY) 'GAP' WASHER OF 0.06mm PLASTIC FILM LOWER SECTION OF FERRITE POT CORE (ASSEMBLY HELD TOGETHER & SECURED TO PCB USING 25mm x M3 NYLON SCREW & NUT) Fig.11: here are the winding details for the step-up transformer (T1) on the main PCB. Note the “gap” washer which is cut from 0.06mm plastic sheet. May 2012  89 This view shows the assembled display PCB with the ZIF socket and potentiometer VR10 removed for clarity. Note that this is a prototype board and there are some differences between this and the final version depicted in Fig.12. mix them up. Don’t fit LED1 at this stage; do it just before the display PCB is attached to the front panel. The three DIL pin headers CON5, CON6 and CON7 are next, followed by the 8-pin DIL socket for IC2. Then fit the four PCB terminal pins near IC2. Next comes the ZIF socket. It’s not mounted directly on the board but needs to be “jacked up” so that it will protrude through the matching hole in the front panel. The ZIF socket also needs to clear the front panel by almost 8mm, to allow its actuator lever to swing down into the horizontal position. Fig.13 shows how two 18-pin DIL sockets, piggy-backed together, are used to mount the ZIF socket. Most of the “jacking up” is done by an 18-pin DIL IC socket with long wire-wrap tails. However, because the machined clips of this type of socket are not able to accept the rectangular pins of the ZIF socket, we have to use a “production” type 18-pin DIL socket (having bent sheet metal clips) between the two, as an adaptor. The ZIF socket is plugged into this 90  Silicon Chip intermediate socket first and the two are then plugged into the machinedclip socket. After this the 3-socket assembly is held together using fillets of epoxy adhesive – see Fig.13. When the epoxy cement has cured you can fit the whole ZIF socket assembly to the display PCB. Note that the assembly should be installed with the actuator lever towards the LCD module position on the PCB. Make sure also that the bottom of the ZIF socket itself is exactly 18mm (or 19.5mm if you are using a PCB front panel) above the top surface of the PCB before you solder the 18 wire-wrap pins of the bottom socket to the pads on the PCB. You can ensure this by using an 18mm-wide strip of stout cardboard underneath the assembly as a temporary spacer. It’s best to initially tacksolder one pin at either end, then do a final check of the spacing and vertical positioning. This will allow you to make any last-minute adjustments that may be necessary before soldering the remaining 16 pins. The next step is to mount the LCD module – see Fig.14. The connections between this module and the PCB are made via a 16-way section of SIL pin header strip, which should be fitted to the PCB (long pin sides uppermost) before the module is attached. Don’t solder its pins at this stage, though. The module itself is mounted on the PCB on two M3 x 6mm tapped Nylon spacers. These are secured using M3 x 15mm machine screws which pass up from under the board, with a flat Nylon flat washer under each screw head. The LCD module is then carefully slipped down over the screws, with the SIL strip pins passing up through the matching holes at bottom left. M3 nuts are then fitted to the top ends of the screws to fasten the module in position, after which the bottom ends of the SIL strip pins are soldered to the display PCB pads underneath. Finally, their top ends are soldered to the pads on the top of the LCD module. Use a fine-tipped iron for this job and solder as quickly as possible to prevent heat damage. Once the LCD module is in position, fit LED1 to the display board. It’s siliconchip.com.au IC9 S7 TEST ON/OFF TPG LM336Z-2.5 10k VR4 TP3 e4Q RLY6 1k 100 Q5 BC549 BC559 + RLY6 RLY5 Q4 4148 D8 IC8 4148 IC2 LM358 D7 D6 TP2 1k e5Q TP+ RLY4 RLY3 SET 2.49V LM336Z-2.5 10k 4148 6.8k VR3 SET 8.75V (TP+ –2.49V) D5 4148 VR5 4.7k 620 G LCD CONT 5k SET +500 A 5k VR8 4.7k 620 SET -500 A 5k VR7 SET -100 A 68k 5k VR6 68k SET +100 A 56k VR11 10k UPPER BOARD 220 F COIL COIL 4YLR RLY5 15 16 S6 ENTER RLY9 RLY15 S5 S4 UP E RLY11 S3 MENU E B C G S D G S sgV DOWN G K A G K K A K A SOCKET) 2 2 1 3 0 1 4 0(DUT 2 1 0 2 © R OT CUD N O CI MES ETER CSID DRA O B RE P P U T E S T S E T ZIF1 vedV+ NO NC 2 16 CON6 0 /68  1 sgV 11YLR RLY10 +11.4V RLY12 (VR10a CONNECTIONS) COIL RLY14 RLY12 COIL NC COM NO RLY14 COIL COM 15 COM NC vedI NO vedV+ RLY10 2 COIL NC vedI NO COM 1 RLY13 COMMON 14 13 12 11 10 9 8 7 6 5 4 3 2 1 16 15 COMMON 1M 22 16X2 LCD MODULE NC NO NC ALTRONICS CON7 RLY9 COM NO COIL 14 13 2 1 +11.4V COIL saibI-/+ 0 /68  Z-7013 (B/L) RLY15 NO NC RLY3 RLY16 +Vdev NC COIL (JAYCAR QP-5515 LCD MODULE) RLY16 COM NO WARNING! HIGH VOLTAGES (UP TO 600V DC) CAN BE PRESENT ON THIS PCB WHEN THE CIRCUIT IS OPERATING. +11.4V COIL 22 LED1 10k 1W 12V ZD3 10k 1W (VR10b CONNECTIONS) VR10a/b (2x10k) 12V ZD4 CON5 10k 1W 9 10 10k 1W 1 2 RLY13 1k siliconchip.com.au NC COM 4.7k If you are working from a kit, the lid NC COM 120k Front panel NO 100 If you are building the SemTest from a kit, the case will probably be already laser-cut and screen printed. If you are working from scratch, you will need to download the drilling/ cutting diagrams from the April 2012 downloads section of the SILICON CHIP website and print these out to use as drilling templates. Take care when you are cutting the rectangular holes in the lid of the case for the ZIF socket and the LCD window because any curved or out-of-square edges will be painfully obvious when your SemTest is finished. The best approach is to first drill a series of 2.5mm holes around the inside perimeter of each rectangle and then use small jeweller’s files to complete the job. The easiest way to prepare the six notch holes along the upper edge of the righthand side of the case (and the matching edge of that end of the lid) is to first temporarily fit the lid to the case. You can then drill the holes in both at the same time, using a 2mm drill to first make pilot holes and then enlarging these holes with a 4mm drill. 4.7k Preparing the case NO 120k The details of these are shown in Fig.15. The two 16-way cables are cut from 120mm lengths of ribbon, with 15mm at each end to loop through the top of the IDC connector, leaving approximately 90mm of ribbon between the connectors. The 10-way cable is made from a 190mm length of ribbon, with 15mm again used at each end for the connector loops. This leaves approximately 160mm of cable between the connectors. When you’re fitting the IDC connectors to each end of the cables, make sure you fit them with the orientation as shown in the circled details in Fig.15. 56k Making the ribbon cables 100nF mounted at lower left, with its cathode flat side to the left. At this stage just tack-solder its leads temporarily to the board pads, with the lower surface of the LED body about 16mm above the board. This will enable you to adjust its final height above the board after it’s attached to the front panel. Now plug IC2 into its socket at lower right. That completes the assembly of this board. Fig.12: the display (top) PCB assembly. This PCB carries the ZIF socket, the LCD and most of the relays and is connected to the main board via IDC cables. is likely to be already screen-printed with the label. If not, you can purchase a PCB dress panel from SILICON CHIP. It is secured to the front panel with the same screws which mount the display PCB. Cut a 70 x 25mm rectangle of clear plastic sheet and fasten this to the lid, behind the 51 x 16mm rectangular cut-out for the LCD viewing window. This will protect the LCD from dust and moisture. The plastic sheet can be fastened to the underside of the lid using cellulose tape around its edges. Now mount pushbutton switches S3-S7 on the front panel. That done, fit the four M3 x 25mm machine screws which ultimately attach the front PCB to the rear of the front panel. As shown in Fig.14, each screw May 2012  91 18-pin ZIF SOCKET S4 CON4 on the main board. The unit is now ready for testing. BOX LID (FRONT PANEL) Setting up 18-pin CLIP-TYPE DIL IC SOCKET 18-pin MACHINED-CLIP DIL IC SOCKET WITH WIRE-WRAP TAILS EPOXY CEMENT UPPER (DISPLAY) PCB Fig.13: the ZIF socket is mounted via two 18-pin IC sockets, with the parts piggy-backed together and secured using epoxy cement before the assembly is installed (see text). Note that the bottom of the ZIF socket should be 18mm above the display PCB (or 19.5mm if you are using a PCB front panel). is fitted with an M3 x 15mm tapped spacer. The screws and spacers should be tightened as securely as you can, without causing the screw head to distort the dress front panel. An M3 nut is then added to each screw at the end of each spacer to bring the effective spacer length close to 18mm. Next, solder “extension wires” to the connection lugs on pushbutton switches S3-S7. The extension wires for these switches should all be made from 0.5mm diameter tinned copper wire, with their lengths staggered between about 40mm and 60mm as this will make it easier to later pass them through their matching holes in the upper PCB. You now need to solder some short flying leads (about 50mm long) to the terminals of dual-gang potentiometer VR10. The other ends of these leads can then be soldered to the PCB as shown in Fig.12. That done, temporarily stick the back of the pot to the display board with its shaft sticking up, ready to pass through the front panel. You can use some BluTac or double-sided tape for this job. The three IDC cables should now be plugged into CON5, CON6 & CON7. The next operation is a bit tricky, because you have to dress each of the extension wires from switches S3-S7 so they all go through their respective holes in the display PCB as it is moved up towards the rear of the front panel. You also have to pass the body of the ZIF socket (with its actuator lever vertical) up through its cut-out in the panel, and make sure that LED1 and VR10 are lined up to pass through their clearance holes in the front panel. When you have managed to mate the two together, with the PCB fitted onto the ends of the four mounting screws, you can add a further nut to each screw to hold it all together. Tighten each nut to complete the job. Once it’s in position, solder all of extension wires from switches S3-S7 to their pads on the underside of the PCB. Be sure to trim the excess leads after the wires are soldered. The main board can now be mounted in the case but don’t fit the lid/ upper board assembly to the case just yet. It can be stood up near-vertically just in front of the case, with the front panel buttons and LCD display quite accessible. Now plug the free ends of the three ribbon cables into CON2, CON3 and M3 x 25mm MACHINE SCREWS M3 x 15mm TAPPED SPACERS M3 NUTS M3 NUTS Be careful when testing this device, as high voltages (up to 600V DC) can be present on both PCBs (see panel). Start by setting the voltage selector switch S2 to its 50V position, then connect the SemTest to a 12V DC plugpack rated at 900mA or more and turn on power switch S1. There should be no test devices plugged into the ZIF socket as yet. You should see this initial greeting message in the LCD window: SC Discrete Semi conductor Tester which should be replaced after a couple of seconds with this message: Press Menu Select button to begin: If you only see a clear window or two lines of 16 black rectangles, it probably means that the contrast trimpot VR11 needs adjustment. Adjust VR11 in one direction or the other until you see the messages displayed clearly and with good contrast. Once this has been done, you can use your DMM to check the voltages at the input and output pins of REG1 (at upper right on the main board, just to the left of CON1). With the DMM’s negative lead connected to the TPG pin just below D4 on the same PCB, you should get a reading of about 11.4V on REG1’s upper input pin and a reading very close to 5.00V at its lower output pin. Finishing the set-up Your SemTest is now ready for the final setting-up adjustments. Do the adjustments in this order: • Adjust trimpot VR2, at lower right on the main board, to set the PIC miBOX LID (FRONT PANEL) 16x2 LCD MODULE M3 x 6mm TAPPED NYLON SPACER UPPER (DISPLAY) PCB NYLON FLAT WASHERS M3 x 15mm MACHINE SCREWS 16-WAY SECTION OF SIL PIN HEADER STRIP USED TO MAKE INTER-BOARD CONNECTIONS Fig.14: this diagram shows the mounting arrangement for the LCD module. It’s mounted on four M3 x 6mm tapped Nylon spacers, with the holes along one edge mating with the pins of a 16-pin SIL header strip that’s soldered to the display PCB. Secure the LCD module in place before soldering it to the header pins along the top. The PCB itself is mounted on the box lid using M3 x 15mm spacers, with M3 nuts used to provide additional spacing. 92  Silicon Chip siliconchip.com.au 90mm TWO CABLES REQUIRED cro’s ADC reference voltage to 2.490V. It’s adjusted while monitoring the reference voltage with your DMM, across terminal pins TP1 and TPG, just below D4. This calibrates the SemTest ADC module’s voltage and current measurement ranges. • Adjust trimpots VR3 and VR4, at lower right on the display board. VR3 sets the voltage drop across IC8 to 2.490V, while VR4 is used to set the drop across IC9 to the same figure. IC8 is the voltage reference for the +IBIAS current source, while IC9 does the same job for the -IBIAS current source. To do this, connect the DMM leads between TP+ (+) and TP2 (-) and adjust VR3 to get a reading of 2.490V. VR4 is adjusted while monitoring the voltage between test point pins TP3 (+) and TPG (-) with your DMM, again to get a reading of 2.490V. These adjustments effectively set the lowest current level (20µA) for +IBIAS and -IBIAS. The next four set-up adjustments set the higher current settings for +IBIAS and -IBIAS, using VR5, VR6, VR7 & VR8. To do these adjustments, you need to fit two short lengths of hookup wire into two of the device lead positions on the ZIF socket, and then set up the SemTest for four different device tests. Here’s the procedure: • Take two short lengths of insulated wire with about 15mm of insulation at each end stripped off. Then with the ZIF socket’s actuator lever upright, introduce one end of each wire into the socket’s “B” and “E” lead holes for a BJT. (It doesn’t matter which of the two “E” holes you use). • Push the socket’s actuator lever down into the horizontal position, to lock these temporary base and emitter leads in place. • Switch your DMM to read low DC current levels (say 200µA to begin) and connect its test leads to the two wire leads: the “+” lead to the base wire and 120mm LENGTH OF 16-WAY IDC RIBBON CABLE (15mm LOOP IN CONNECTOR AT EACH END) 190mm LENGTH OF 10-WAY IDC RIBBON CABLE (15mm LOOP IN CONNECTOR AT EACH END) ONE CABLE REQUIRED 160mm Fig.15: here’s how to make up the IDC cables. Be sure to orientate the headers with the locating spigots facing exactly as shown – they face outwards on the 90mm cables and inwards on the 190mm cables. the “-” lead to the emitter wire. Now we need to negotiate SemTest’s menu system to reach a device test setup which will allow us to measure the various IBIAS levels using the DMM. Apply power and press the MENU SELECT button for half a second or so. You should then see the opening device selection display: Device to Test: ˄ 1:Diode/Zener ˅ In case you’re wondering, those “^” and “v” symbols at the right-hand ends of the lines are meant to remind you that you can scroll up or down through a sequence of menu choices, using the UP or DOWN buttons. For the first of these IBIAS adjustments, we actually want to select some BJT (NPN) tests, so press either of these buttons briefly a number of times, until you see this display: Device to Test: ˄ 3:NPN bipolar ˅ Since that’s the type of device we want to set up for (even though there is no actual device plugged into the ZIF socket), confirm this by pressing the ENTER button. This will cause the display to change into: Test parameter:˄ BVcbo (e o/c) ˅ As before, note the symbols at far right on the display, indicating as before that other tests can be selected using the UP and DOWN buttons. So press either of these buttons briefly a few times until you see this display: Test parameter:˄ hFE (Ib=20μA) ˅ This is the first test we want to set up for in order to make these set-up adjustments, so press the ENTER button to confirm it. The display will then become: NPN bipolar: hFE(Ib20μA)=0000 Now, after checking that you have set voltage selector switch S2 to its 50V position, press the TEST ON/OFF button to turn on the DC-DC converter and take a measurement. LED1 should be on, to indicate that the DC-DC converter is operating and providing test WARNING: SHOCK HAZARD! The DC-DC step-up converter used in this project can generate high voltages (up to 600V DC) and can also supply significant current. As a result, it’s capable of delivering a nasty electric shock and there are some situations where such a shock could be potentially lethal. For this reason, DO NOT touch any part of the circuit while it is operating, particularly around transformer T1, diode D1 and the two 47μF 450V electrolytic capacitors on the main (lower) PCB. Note, however, that high voltages can also be applied to the display board (via CON6) during operation, so it’s not safe to touch certain parts on this board either. Exercise caution if testing the unit with the lid opened and always allow time for the 47μF capacitors to discharge before working on the circuit. Note also that high voltages (up to 600V DC) can be present on the component leads when testing for high-voltage breakdown. DO NOT touch any leads while testing is in progress. siliconchip.com.au May 2012  93 Sourcing The Rotary Switch As mentioned in the article, the 3-pole 4-position rotary switch (S2) must have a 42mm-long control shaft, so that when the case is assembled, it passes through the clearance holes in the front panel with enough length left over to attach the control knob. A Lorlin CK1051 switch is suitable and this can be sourced from Element14 (Cat. 112-3697). Note that the shafts on the switches usually available from the kit suppliers will be too short for this project. voltage. The LCD display will also change, but don’t take much notice of the hFE reading because there is no transistor connected at present (it will probably show an hFE reading of either “00” or “01”). Your DMM should now show a figure very close to 20.0µA (the default/ lowest IBIAS level). Now press the TEST ON/OFF button again, and hold it down for a second or so until LED1 goes out, indicating that the DC-DC converter has been turned off. The LCD display will also return to its “Press MenuSelect” message, ready for another test. And when you press the MENU SELECT button, you’ll find that the SemTest has “remembered” that you were testing an NPN bipolar device and will offer the same device test again: Device to Test:˄ 3:NPN bipolar ˅ Confirm this by pressing the ENTER button. Then use either the UP or DOWN buttons until you get this display: Test parameter:˄ hFE (Ib=100μA) ˅ Press the ENTER button to confirm and finally press the TEST ON/OFF button again to turn on the DC-DC converter and take a measurement. As before though, don’t worry about the hFE measurement on the LCD display – pay attention to what the DMM is showing, because this will be reading the actual bias current. This should be close to 100.0µA. Now adjust VR6 with a small screwdriver until it reads 100.0µA. Once that’s done, press and hold down the TEST ON/OFF button until LED1 goes off. Then press the MENU SELECT and ENTER buttons and then UP or DOWN to get: Test parameter:˄ hFE (Ib=500μA) ˅ 94  Silicon Chip Press ENTER to confirm, set your DMM is set to read to over 500µA, then press the TEST ON/OFF button. Your DMM should now read close to 500µA. Adjust VR5 to get that exact figure. Press the TEST ON/OFF button once again until LED1 goes off. That completes the two adjustments for the +IBIAS current levels. Those for the -IBIAS levels are next on the list. This time we use the tests for an PNP bipolar instead of an NPN and we need to reverse the connections to the DMM test leads. Press MENU SELECT again and then press the UP button once, to get: Device to Test:˄ 4:PNP bipolar ˅ Press ENTER to confirm and press either UP or DOWN to select the “hFE (IB=20µA)” test. Press ENTER to confirm and then press TEST ON/ OFF. Your DMM should show close to 20.0µA, confirming the default/lowest -IBIAS level. Now press and hold down TEST ON/OFF to stop this test. Now press MENU SELECT again and you’ll find that the PNP bipolar tests are still being offered. Press ENTER to confirm and then the UP or DOWN buttons until you get: Test parameter:˄ hFE (Ib=100μA) ˅ Confirm this by pressing ENTER and follow by pressing TEST ON/OFF to start the test. Your DMM should now be reading close to 100.0µA. Adjust trimpot VR7 to bring the reading as close as possible to that figure, then press TEST ON/OFF to stop the test. Set the DMM to read more than 500µA and then press MENU SELECT, ENTER and the UP or DOWN buttons until you have selected: Test parameter:˄ hFE (Ib=500μA) ˅ Press ENTER and TEST ON/OFF again and confirm that the DMM reads close to 500µA. Adjust VR8 to obtain that exact figure, then press TEST ON/ OFF again and you have completed all the setting-up adjustments for the SemTest’s IBIAS current levels. One more adjustment remains: using trimpot VR1 to set the DC-DC converter output voltage levels. To do this, check that S2 is set to 50V. Then press MENU SELECT and UP or DOWN until you get: Device to Test:˄ 7:SCR ˅ Press ENTER to confirm and either UP or DOWN until you get: Test parameter:˄ Vak on (OPV) ˅ Now press ENTER and TEST ON/ OFF. The second line of the LCD should now read something like this: Vak(OPV) = 49.6V Adjust VR1 (just above the centre of the main board) until the LCD reading changes to: Vak(OPV) = 50.0V Finally, press the TEST ON/OFF button once. This completes all the set-up adjustments. Final assembly The front panel assembly can now be lowered down onto the case. Make sure that the three ribbon cables are folded neatly into the space above the lower PCB and not caught between the edges of the case or lid. Fasten the case together with four M4 screws into the corner holes, then fit the knobs to the rotary switch and the pot and the assembly is complete. Testing the HV crowbar It’s now necessary to check that the HV crowbar circuit is working correctly. To do this, power up the unit, wait a few seconds and then press the Menu Select button. You will get a display like this: Device to Test: ˄ 1:Diode/Zener ˅ Press Enter and then the Up button. The display will then show: Test parameter:^ Irev(OPV) ˅ Press Enter again. Set the Device Operating Voltage to 25V, using the right-hand knob. Then press the Test On/Off button to start the test. Now carefully measure the voltage across the top and bottom A & K terminals in the Diodes & LEDs section of the test socket. You should get a reading close to 25V. If it’s much lower (say, 12V) then either the crowbar circuit has triggered prematurely or there is a fault in the DC/DC converter circuit. You will need to switch off, open up the unit and check the crowbar and converter circuits for faults such as incorrectly orientated components. If you get a much higher reading than 25V, there is a problem with the DC/DC converter section. Switch off and measure the voltage across the A & K terminals until it drops to a safe level. Then open the unit up and look for the source of the problem. Assuming all is well, press the Test siliconchip.com.au On/Off button to terminate the test. You can now do a high-voltage test. The procedure is similar to before except you want to do an IREV(BV) test. So when you get to this stage: Device to Test: ˄ 1:Diode/Zener ˅ press enter twice and start the test. Carefully measure the voltage across the A & K terminals again. It should be several hundred volts and it will rise to close to 600V after a number of seconds. Now press the Test On/Off button again to terminate the test while monitoring the voltage between the A & K terminals. It should immediately fall to just a few volts when the test is terminated. If it remains high and only decreases slowly, the crowbar has failed to operate and you will need to wait for the capacitors to discharge before opening the unit up and checking for faults. If the crowbar is not working (eg, if it fails), a warning will be displayed on the LCD immediately after performing a high-voltage test. This indicates that there is still a high voltage present at the test socket. If you get this warning then you should open the unit up and repair the crowbar circuit. Using the SemTest The SemTest is used as follows: STEP 1: place DUT in ZIF socket and switch on. STEP 2: Press Menu Select. STEP 3: Use Up/Down buttons to select device type and press Enter. STEP 4: Use Up/Down buttons to select test and press Enter. STEP 5: For OPV tests, use righthand knob to select test voltage. STEP 6: Press Test On/Off to start test (red LED on) and read off result. STEP 7: Press Test On/Off again to finish test (red LED out). STEP 8: check that the red LED is out and that there is no high voltage warning on the LCD before removing DUT. Exercise caution when testing components for high-voltage breakdown. Up to 600V DC is present on the device leads during such tests, so be careful not to touch them! The biggest problem in using the SemTest is knowing the various lead configurations of the devices it can test. To that end, we have prepared a connections chart showing commonly used diodes, LEDs, BJTs, Mosfets, SCRs and PUTs. It can be stuck on a wall or to the underside of the SemTest siliconchip.com.au This view inside the completed prototype shows how it all goes together. The two PCB assemblies are mounted in their respective case halves on spacers and joined together via the three IDC header cables. case for easy reference. For less common devices, you’ll need to look up the connections in a data book or by downloading a data sheet from the manufacturer’s website. Finally, here are a few tips to guide you when you’re doing some of the more specific tests: • When reading the forward voltage drop VF of a diode or LED or the voltage drop VAK of an SCR when it’s conducting, be aware that the accuracy of this measurement is not very high due to measuring circuit limitations. So if you need to make really accurate measurements of VF or VAK, you’ll need to use an external DMM with its leads connected across the device’s “A” and “K” leads. Remember that during the same tests, it’s OK to increase the device operating voltage to a higher setting in order to see the voltage drop at higher current levels. • When you want to measure the hFE of a BJT, start on the setting with the lowest IBIAS level (ie, 20µA), because this is the setting with the highest hFE range. Only swing down to one of the higher IBIAS settings if the hFE reading you get is very low (ie, below 300). This should only be necessary with medium-to-higher power devices, which often have their “peak” hFE at higher currents. • When you want to measure the IDS vs VGS characteristic of a Mosfet to get an idea of its transconductance or “gm”, start by selecting the highest device operating voltage which will not exceed the device’s VDS ratings. That’s because the VGS bias voltage (adjusted via VR10) is derived from the actual device operating voltage, which inevitably tends to drop once the device begins to draw drain-source current (due to voltage drop in the current limiting resistors). If you don’t set the switch for a reasonably high voltage to start with, you’ll find that it won’t be possible to provide much VGS once the device starts to conduct. Actually, although you need to set the operating voltage within the device ratings when you start this test, it’s OK to increase the setting to 100V during the test itself, if you need to do so in order to achieve a higher VGS. This won’t cause any problems if you only increase the voltage setting SC once the device is conducting. May 2012  95