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SemTest Pt.2: By JIM ROWE Check all those semiconductors in your collection with this easy-to-build test set! This month we present the full circuit of this versatile unit which can test all those semiconductors in your collection. It employs a PIC16F877A microcontroller to run all the different tests and displays the results on the 2-line LCD panel. T HERE’S QUITE A LOT of circuitry in the new SemTest, despite the fact that most of its operation is managed by a microcontroller. For that reason, the circuitry is accommodated on two PCBs which are stacked inside the case. To begin the circuit description we will start with the lower or main PCB which carries the micro, the power supplies and metering. This section of the circuit is shown in Fig.5. Microcontroller IC4 forms the brain of the SemTest. We have used a PIC70  Silicon Chip 16F877A because it has five I/O ports, including three that are eight bits wide. It also includes a 10-bit A-D (analogto-digital) converter with a choice of eight input channels. All eight bits of both ports B and D are used to control the various relays which provide the test configurations. The two ports drive IC5 and IC6, which are ULN2803A octal Darlington arrays which in turn drive a total of 16 relays. Most of these are on the upper PCB but four relays are on the main board: • Relay 1, which is used to switch the device test voltage on and off; • Relay 2, which switches the test voltage between its “BV” or “OPV” modes; • Relay 7, which controls the value of the current shunt resistor used to measure device current (and hence switch current ranges); and • Relay 8, which controls the voltage divider ratio for device voltage measurement (ie, to switch voltage measurement ranges). The connections to the relays on the upper PCB are made via CON3 and CON4. siliconchip.com.au Bit lines RC0-RC3 of port C, together with RE0 and RE1 of port E, are used to control the LCD module, which is again on the upper PCB. These connections are made via CON2, which links to CON5 on the upper board via a 10-way IDC ribbon cable. The remaining bit lines RC4-RC7 of port C plus bit line RE2 of port E are used to monitor activity on the five pushbutton switches S3-S7. These mount on the instrument’s front panel and are connected using a 16-way IDC ribbon cable from CON7 on the upper PCB to CON4 on the main board. The same connectors and cable are used to make the connections for relays 3-6, 9, 15 & 16, plus the connection for LED1 (the “Test Volts Present” indicator). High-voltage supply The test voltage power supply circuitry at upper left on Fig.5 has been adapted from the high-voltage power supply in the Electrolytic Capacitor Tester/Reformer described in AugustSeptember 2010. As before, the supply is a stepup DC-DC converter using IC1 (an MC34063) as the controller, with transistors Q1 & Q2 used to drive Mosfet Q3 and transformer T1. The “flyback” voltage pulses developed by T1 are rectified by fast diode D2 and fed to the output filtering and current limiting circuitry. The MC34063 maintains the output voltage at the selected level by means of negative feedback from the four 75kΩ resistors in series with trimpot VR1 in the top leg, plus the 100kΩ resistor connected from pin 5 of IC1 to ground as the “default” bottom leg. This basic divider determines the converter’s nominal “10V” output level. The three other operating test voltages (25V, 50V & 100V) are achieved using switch S2a to bring other resistances in parallel with the 100kΩ lower resistor, while relay 2(a) is used to achieve the converter’s much higher (about 600V) “BV” output voltage by switching in a 680Ω resistor across the 100kΩ resistor instead. Trimpot VR1 is used to set the converter’s OPV output voltages precisely. Bit lines RA4 and RA5 of the micro’s port A are used to sense the setting of switch S2, ie, via poles S2b and S2c. While we’re looking at the DC-DC converter circuitry, note that the second pole of relay 2 (2b) is used siliconchip.com.au Features & Specifications Main Features A compact yet flexible test set for most common discrete semiconductor devices, including diodes (junction and Schottky), LEDs, zeners, diacs, bipolar junction transistors (BJTs), Mosfets, SCRs and thyristors (including Triacs). Based on a PIC16F877A microcontroller, with device and test selection, plus the test results, displayed via a 16x2 alphanumeric LCD readout. Devices to be tested are connected to the test set via an 18-way ZIF socket. Five test voltages are available: a 600V source for avalanche breakdown (BV) testing plus a choice of either 10V, 25V, 50V or 100V for operating voltage (OPV) tests. All test voltages are applied to the DUT via current limiting resistors – 100kΩ in the case of BV tests or 2kΩ in the case of OPV tests. Maximum avalanche current which can flow during BV tests is 6mA (short circuit current). Maximum device/leakage current which can flow with OPV = 100V is 30mA; with OPV = 50V is 25mA; with OPV = 25V is 12.5mA; and with OPV = 10V is 6mA. Minimum leakage current which can be measured = 1μA. Diode tests (1) Reverse avalanche current IR (BV) (2) Reverse leakage current IR (OPV) (3) Forward voltage drop VF (OPV) (4) Zener/avalanche voltage VR (BV) LED tests (1) Reverse leakage current IR (OPV = 10V) (2) Forward voltage drop VF (OPV) BJT tests (1) Breakdown voltage C-B with emitter o/c V(BR)CBO (2) Breakdown voltage C-E with base o/c V(BR)CEO (3) Leakage current C-B with emitter o/c ICBO (OPV) (4) Leakage current C-E with base o/c ICEO (OPV) (5) Forward current gain hFE with a choice of three base current levels: 20μA, 100μA or 500μA Maximum hFE which can be measured with IB = 20μA is 1500 (OPV = 100V) Maximum hFE which can be measured with IB = 100μA is 300 (OPV = 100V) Maximum hFE which can be measured with IB = 500μA is 60 (OPV = 100V) Mosfet tests (1) Breakdown voltage D-S with G-S shorted V(BR)DSS (2) Leakage current D-S with G-S shorted IDSS (OPV) (3) D-S current IDS versus G-S bias voltage VGS (ie, gm) SCR, PUT & Triac tests (1) Breakdown voltage with G-K (SCR) or G-A (PUT) shorted V(BR)AKS (2) Leakage current with G-K (SCR) or G-A (PUT) shorted IAKS (OPV) (3) Current IAKS with gate current applied (20μA, 100μA or 500μA) and OPV applied (4) Voltage drop A-K when conducting VAK (OPV) Note: the test set operates from an external power source of 12V DC. Current drain varies from around 65mA when a test is being set up, to a maximum of approximately 900mA during testing. It can therefore be powered from either a 12V SLA battery or a 12V/1A mains power supply or regulated DC plugpack. March 2012  71 12V DC INPUT POWER D1 1N4004 REG1 7805 +11.4V K A + – S1 IN RELAY1 1000 F 25V CON1 1000 F 25V +5V OUT GND 100nF 68 IC5 PIN18 +11.4V D2 UF4007 A 5W 80T 6 7 8 Vcc Ips DrC 10T SwC IC1 MC34063 Ct TP4 SwE Cin5 GND 4 1nF K +OPV/+BV T1 0.27  3 33k 1W 1.5k 5W 1 C B Q1 BC337 E 2 E 2.2k B C 470nF 630V 390k 75k 1% 100k 390k 75k 1% 100k 100 G 470nF 630V S Q2 BC327 390k 75k 1% 390k SET TEST VOLTS VR1 50k (25T) +1.25V 100k RELAY 2b TPG ZD1 4.7V TPG 47 F 450V 100k K 100k 1% 680  1% 12k 1% 5.1k 1% 1.0k 1W 75k 1% D Q3 IRF540N 1.0k 47 F 1W 450V TPVdev +Vdevice 30  1% 3.9k 1% A 25V +11.4V RELAY 2a 50V 10V 100V SET OP TEST VOLTS S2a 680  1% S2b S2c 600V CON3 +11.4V 7 +Vdevice 6 WARNING! HIGH VOLTAGES (UP TO 600V DC) CAN BE PRESENT AT THE OUTPUT OF THE DC -DC CONVERTER WHEN THIS CIRCUIT IS OPERATING AND FOR SOME TIME ACROSS THE 47 µF 450V CAPACITORS AFTER SWITCH-OFF SC 2012 SEMTEST DISCRETE SEMICONDUCTOR TEST SET CONNECTS TO CON6 ON UPPER BOARD 1 Vgs 2 3 Idevice 11 13 14 12 10 4,5,8,9 15 16 +11.4V MAIN CIRCUIT (LOWER BOARD) Fig.5: the main part of the SemTest circuit is built on the lower PCB and includes microcontroller IC4, the power supplies and metering. IC4 controls the relays via IC5 & IC6, performs A-D conversion of the measurements applied to its inputs and drives the LCD on the upper board via CON2. The test voltages (up to 600V) are generated by a DC-DC converter circuit based on IC1, transformer T1 and Mosfet Q3 at upper left. 72  Silicon Chip siliconchip.com.au +5V 1,14 300k 1% 33k 1W 1.6k 1% 160k 1% Vdd 1 2.4k 1% 6 300k 1% 11 RELAY 8 2 7,8 2 ADJ 1 4 RE1 AN2/RA2 RE0 RC0 RC1 RC2 3.0k RC3 7 +5V K D9 1k Idevice RS 1 8 EN 3 15 D7 2 16 D6 4 17 D5 6 18 D4 39 RB5 4 3 RB4 RB3 AN1/RA1 RB2 560 RB0 RELAY 7 1,14 6 RB1 +11.4V 2 7,8 1 1B 1C 18 39 2 2B 2C 17 38 3 3B 3C 16 RLY5 COIL 6 37 4 4B 4C 15 RLY3 COIL 4 36 5 5B 5C 14 RLY4 COIL 2 35 6 6B 6C 13 RLY16 COIL 1 33 7 7B 7C 12 RLY15 COIL 3 34 8 8B 9 18 1C 1B 1 22 17 2C 2B 2 21 RELAY 12 COIL 16 3C 3B 3 27 RELAY 14 COIL 15 4C 4B 4 28 RELAY 13 COIL 14 5C 5B 5 29 13 6C 6B 6 30 12 7C 7B 7 19 11 8C 8B 8 20 RELAY 8 COIL 10 COM E 9 RD3 RC4 RD2 RC5 RD4 RC6 RD5 RC7 RD6 RE2 RD7 OSC2 RD0 OSC1 RD1 Vss 12 A A siliconchip.com.au E 12 S3 14 S5 16 25 S6 15 26 S4 13 S7 10 10 14 13 27pF B K – + ADJ E 9 7805 D GND IN G C 8 LED1 27pF BC327, BC337 1N4004, UF4007 RELAY 6 COIL X1 8.0MHz IRF540N A 11 23 31 LM336Z–2.5 K K 5 24 Vss D3-D4, D9: 1N4148 ZD1, ZD2 7 8C 11 RLY9 COIL COM 10 5x 10k RELAY 10 COIL RELAY 2 COIL CON4 +5V IC6 ULN2803A RELAY 11 COIL +11.4V A = 1.205 3.0k IC5 PIN17 10 40 56 10k 8 9 IC5 ULN2803A RB6 7 IC3b 7 9 RA4 RA5 RB7 6 CON2 5 +5V RELAY 7 COIL 5 10nF VR2 10k (10T) RELAY 1 COIL IC3: LM358 A A SET 2.49V REFERENCE D4 IC4 PIC16F877A Vgs K – TPG A 6 2.7M D3 + IC7 LM336Z –2.5 560 A = 1.205 470k A AN0/RA0 56 2x 10k +2.49V 5 K IC3a 2 10nF 10k Vref+ MCLR 8 3 20k K ZD2 6.2V 1W 100 F 100nF TP1 IC6 PIN11 +5V 32 Vdd 100nF 22  1% +11.4V 2.4k 100nF 2.2k CONNECTS TO CON5 ON UPPER BOARD 300k 1% 33k 1W 47 F CONNECTS TO CON7 ON UPPER BOARD 240k 1% D S GND OUT March 2012  73 +11.4V +5.0V RELAY 9 220 F CON5 RELAY 15 5 7 1 4 2 15 Vdd B-L A RS 16 x 2 LCD MODULE 3 CONNECTS TO CON2 ON MAIN BOARD RELAY 16 22 6 CONTRAST 3 VR11 10k LCD CONTRAST CON7 3 EN D7 D6 D5 D4 D3 D2 D1 D0 GND 1 14 13 12 11 10 9 8 7 2 R/W 5 1 B-L K 16 7 4 6 14 8 13 9 16 15 10 10 S3 S5 TEST 100nF D5 12 ADJ – VR3 10k 4.7k 4.7k 1% LED1 68k 1% 9 A  K SET +500 A 1k A 56k 1% 2 K +8.75V (TP+ –2.49V) 3 +2.49V IC2a 1 E 1k B A 6 IC2b 7 1k B – VR4 10k A K 120k 1% 4.7k 1% RELAY 5 RELAY3b 6 2 14 –Ibias 1 7 2 6 RELAY 3a SET –100 A 68k 1% 4.7k 8 4 RELAY6 VR7 5k 6 7,8 +Ibias Q5 BC549 56k 1% D8 TPG C E 4 K ADJ Q4 BC559 100 C VR6 5k SET +100 A 100 5 D7 + 8 IC2: LM358 6.8k TP3 RELAY 4b 120k 1% 5 TEST VOLTS ON VR5 5k D6 TP2 620 K + 11 S7 S6 +11.4V A IC9 LM336Z 2.5 S4 ENTER +11.4V TP+ IC8 LM336Z 2.5 DOWN UP CONNECTS TO CON4 ON MAIN BOARD MENU 2 +/–Ibias SET –500 A VR8 5k RELAY 4a 620 SC 2012 SEMTEST DISCRETE SEMICONDUCTOR TEST SET UPPER BOARD CIRCUIT Fig.6: the upper board circuit carries the LCD module, an 18-pin ZIF (zero insertion force) socket to connect the device under test (DUT), the control pushbuttons and various relays to switch the device connections to CON6. to adjust the value of the current limiting resistance in series with the converter’s output, to suit the various output voltage levels. So for the four output voltage settings selected by S2a, the total current limiting resistance is (1.5kΩ + (500Ω//99kΩ)), or just under 2kΩ. This limits the device current to 74  Silicon Chip about 50mA on the 100V range. On the “BV” setting (relay 2 off), the current limiting resistance jumps up to 100.5kΩ – limiting the maximum current to less than 6mA even if the device under test is shorted. Crystal X1 and its two associated 27pF capacitors are used to run the micro at 8MHz, which gives an instruction cycle time of 500ns. Analog-to-digital conversion Now let’s turn to the micro’s ADC module and how it’s used to perform the SemTest’s various metering functions. Starting with the ADC’s input siliconchip.com.au +11.4V RELAY 9 RELAY 16 RELAY 15 RELAY 14 10k 1W K ZD3 12V 1W 10k 1W K VR10b 10k VR10a 10k A A ZD4 12V 1W 68 10k 1W +11.4V 10k 1W +Vdevice RELAY 14 Vgs 22 G 2 17 K D 3 16 A G 4 15 G S 5 14 K 6 13 C K 7 12 8 11 A 9 10 K E RELAY 15 +/– Ibias S E Vgs 18 G B RELAY 10 18-PIN ZIF SOCKET 1 Idevice +Vdevice RELAY 12 Idevice RELAY 16 SCRs & PUTs MOSFETS 1M +Vdevice RELAY 13 A BIPOLAR JUNCTION TRANSISTORS DIODES & LEDS RELAY 9 RELAY 11 RELAY 14 +/– Ibias +Vdevice 68 RELAY 13 RELAY 10 +11.4V RELAY 12 RELAY 11 12 10 14 13 11 Idevice 16 15 1 2 3 4 5 6 7 8 9 CON6 CONNECTS TO CON3 ON MAIN BOARD LED D5–D8: 1N4148 A K ZD3, ZD4 A K K A BC549, BC559 LM336Z–2.5 B – + ADJ E C 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 (tens of milliamps). 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 circuit (lower board). 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. Provided the unit is built and fully-enclosed in a case as described, it’s safe to operate. Exercise extreme caution if testing the unit with the lid opened and always allow time for the 47μF capacitors to discharge before working on the circuit. siliconchip.com.au March 2012  75 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 three IDC header cables. channel AN0 (IC4 pin 2), this is used to measure the voltage VDEV being applied to the device under test. Relay 8 is used to switch the upper leg of the voltage divider feeding AN0 to give the VDEV voltmeter two ranges: 0-1028V in the case of the higher “breakdown voltage” or BV range; and 0-102.8V for the lower “OPV” voltage range. The second ADC input channel AN1 (IC4 pin 3) is used to measure the current I DEV passing through the device under test (DUT). It does this by measuring the voltage drop across a shunt resistance connected between the negative end of the DUT and ground. Here, relay 7 is used to switch the value of the shunt resistor, 76  Silicon Chip to provide two current ranges. When relay 7 is activated, it shorts the bottom end of the 39Ω “upper” shunt resistor to ground, giving an effective shunt resistance of 39Ω; this provides a 0-50mA current range. However, if the micro turns off relay 7, this removes the short across the par­alleled 2.7MΩ and 10kΩ resistors, bumping up the effective shunt resistance to almost exactly 10kΩ and providing a 0-200µA current range. On both ranges, the voltage drop developed across the shunt resistance is fed to the micro’s AN1 input via buffer amplifier IC3b, which provides a gain of 1.205. This is used for scaling. The third ADC input channel, AN2 (IC4 pin 4), is used to measure VGS, the gate-source voltage for Mosfets. It does this by using another input voltage divider, with the top leg formed by the series 470kΩ and 20kΩ resistors, and the bottom leg by the 10kΩ resistor from pin 3 of IC3a to ground. This gives a 50:1 input division, which together with the gain of buffer amplifier IC3a (again 1.205) gives a voltage range of 0-103.3V. This may seem too high for measuring voltages lower than 20V but it was only possible to give the AN2 voltmeter a single range and this needs to measure voltages up to around 100V to cope with VGS measurements on P-channel devices (where the effective VGS must siliconchip.com.au Parts List 1 ABS enclosure, 222 x 146 x 55mm (Jaycar HB-6130 or similar) 2 control knobs, 19mm diameter 5 SPST pushbutton switches, panel-mount (Altronics S1084, Jaycar SP-0700) 1 18-pin ZIF socket (Altronics P0590, Jaycar PI-6480) 1 18-pin machined IC socket 1 18-pin IC socket, wire-wrap type 4 M3 x 15mm tapped metal spacers 2 M3 x 6mm M3 tapped Nylon spacers 4 M3 x 25mm machine screws 2 M3 x 15mm machine screws 10 M3 x 6mm machine screws 10 M3 hex nuts 2 M3 flat washers, Nylon 4 16-way (8x2) IDC header sockets 2 10-way (5x2) IDC header sockets 4 16-way (8x2) pin headers, vertical PCB-mount 2 10-way (5x2) pin headers, vertical PCB-mount 1 300mm length of 16-way IDC ribbon cable 1 180mm length of 10-way IDC ribbon cable 1 16-way length of SIL pin header strip Main board 1 PCB, code 04103121, 210 x 134mm 2 19mm square heatsinks (Altronics H 0630, Jaycar HH-8502) 1 6V SPDT mini relay (Jaycar SY-4058) 1 12V DPDT mini relay (Altronics S4150) 2 SPST mini DIL relay (Altronics S4101A) 1 Ferrite pot core, 25mm dia x 16mm high (Altronics L5300 or similar) 1 moulded bobbin to suit (L 5305) 1 M3 x 25mm Nylon machine screw plus nut and washer be found by subtracting the actual VGS from the device voltage VDEV). ADC reference voltage The ADC reference voltage for all three of these measuring ranges is siliconchip.com.au 1 3-pole 4-position rotary switch 1 SPDT sub mini toggle switch, PCB-mount (Altronics S 1320) 1 8.0MHz crystal, HC-49S (X1) 2 8-pin DIL IC sockets, PCB-mount 1 40-pin DIL IC socket, PCB-mount 1 2.1mm concentric DC connector, PCB-mount (Altronics P 0620) 2 Nylon cable ties 4 1mm PCB terminal pins 1 50kΩ multi-turn vertical trimpot (VR1) 1 10kΩ multi-turn horizontal trimpot (VR2) 1 1m length of 0.8mm-diameter enamelled copper wire 1 10m length of 0.25mm-diameter enamelled copper wire Semiconductors 1 MC34063 switchmode controller (IC1) 1 LM358 dual op amp (IC3) 1 PIC16F877A microcontroller (IC4) 2 ULN2803A octal driver (IC5,IC6) 1 LM336Z-2.5 voltage reference (IC7) 1 7805 5V regulator (REG1) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 1 IRF540N N-channel Mosfet (Q3) 1 4.7V 1W zener diode (ZD1) 1 6.2V 1W zener diode (ZD2) 1 1N4004 1A diode (D1) 1 UF4007 fast 1A diode (D2) 3 1N4148 100mA diode (D3,D4,D9) Capacitors 2 1000µF 25V RB electrolytic 1 100µF 10V RB electrolytic 2 47µF 450V RB electrolytic 1 47µF 16V RB electrolytic 2 470nF 630V MKT capacitor 4 100nF MKT capacitor 2 10nF MKT capacitor 1 1nF MKT or polyester capacitor 2 27pF NP0 ceramic Resistors (0.25W, 1%) 1 2.7MΩ 2 2.4kΩ 1 470kΩ 1 2.2kΩ 4 390kΩ 1 1.6kΩ provided at pin 5 of IC4, by the voltage reference circuit based on IC7, trimpot VR2 and diodes D3 & D4, together with a 2.4kΩ load resistor. During set-up, VR2 is adjusted to bring the reference voltage across IC7 as close as possible 3 300kΩ 1 240kΩ 1 160kΩ 5 100kΩ 4 75kΩ 3 33kΩ 1W 1 20kΩ 1 12kΩ 9 10kΩ 1 5.1kΩ 1 3.9kΩ 2 3.0kΩ 1 1.5kΩ 5W 1 1kΩ 2 1kΩ 1W 2 680Ω 2 560Ω 1 100Ω 1 68Ω 2 56Ω 1 39Ω 1 30Ω 1 22Ω 1 0.27Ω 5W Upper (Display) board 1 PCB, code 04103122, 200 x 124mm 1 16x2 LCD module (Altronics Z 7013; Jaycar QP-5512) 2 6V SPDT mini relays (Jaycar SY-4058) 8 12V DPDT mini relays (Altronics S4150) 2 SPST mini DIL relay (Altronics S4101A) 1 8-pin DIL IC socket 4 1mm PCB terminal pins 1 10kΩ mini horizontal trimpot 2 10kΩ multi-turn horizontal trimpot 1 10kΩ linear 16mm dual-gang pot 4 5kΩ multi-turn horizontal trimpot Semiconductors 1 LM358 dual op amp (IC2) 2 LM336Z-2.5 (IC8,IC9) 1 BC559 PNP transistor (Q4) 1 BC549 NPN transistor (Q5) 2 12V 1W zener diodes (ZD3,ZD4) 1 5mm red LED (LED1) 4 1N4148 100mA diode (D5-D8) Capacitors 1 220µF 10V RB electrolytic 1 100nF MKT capacitor Resistors (0.25W, 1%) 1 1MΩ 2 2.2kΩ 2 120kΩ 3 1kΩ 2 68kΩ 2 620Ω 2 56kΩ 2 100Ω 4 10kΩ 1W 2 68Ω 1 6.8kΩ 1 22Ω 4 4.7kΩ to 2.490V, where it has a temperature coefficient that’s very close to zero. All three ADC input circuits have been designed to give the most accurate readings with this reference voltage, so this one adjustment perMarch 2012  77 Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $A10.00 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ 78  Silicon Chip forms the instrument’s basic metering calibration. That’s just about it for the main PCB circuitry. However, before moving on, we should point out that the complete instrument runs from an external 12V DC supply which comes in via CON1, polarity protection diode D1 and power switch S1. The resulting +11.4V rail then feeds regulator REG1 (a 7805), which provides +5V to power IC3, IC4, the LCD module and their associated circuitry. The +11.4V rail itself is also used to supply the various relays and the DC-DC converter based on IC1 – when the micro turns on relay 1 to begin a test. It’s also used to power the IBIAS circuitry on the upper PCB, as we shall see in a moment. Incidentally, the overall current drain of the SemTest from the external 12V supply ranges from around 65mA when a test is being set up to between 150mA and 900mA during testing (depending on the test concerned). A regulated 12V/1A plugpack supply or a 12V SLA battery would be suitable. The upper (display) PCB Now we can turn our attention to the circuitry on the upper display PCB – see Fig.6. This has the LCD module and control pushbutton switch area at upper left. Trimpot VR11 is used to set the LCD’s contrast for maximum readability and the four data lines (D4-D7) and two control lines (EN and RS) are fed from various pins on CON5 and linked back to CON2 on the main board. Similarly, the five pushbutton switch­es S3-S7 are simply wired to CON7, which is linked to CON4 on the main board and then to pins 23-26 & 10 of microcontroller IC4, so the micro can monitor them. At lower left in Fig.6 are two programmable current sources, used to provide the base current IBIAS for testing BJTs as well as the gate current for testing SCRs and PUTs. IC8, a 2.490V reference, together with op amp IC2a and PNP transistor Q4, is the positive IBIAS source. Similarly, voltage reference IC9, IC2b and NPN transistor Q5 is the negative IBIAS source (or “sink”, if you prefer). The 2.490V voltage references (IC8 or IC9) are connected to the non-inverting input of their respective op amps, ie, IC2a or IC2b. The output of each op amp drives the base of the current pass transistor (Q4 or Q5), while feedback to the inverting input of each op amp is taken from the emitter of its pass transistor. Then the emitter of each transistor is taken either to the +11.4V rail (in the case of Q4) or to ground (in the case of Q5), via a series resistance whose value is carefully chosen to have a voltage drop of 2.490V when the transistor’s emitter current is at the desired level. For example, when relays 3 and 4 are both off, the emitter resistances for Q4 and Q5 are both equal at 124.7kΩ (120kΩ + 4.7kΩ). As a result, the current passed by either transistor will tend to stabilise at 20µA, ie, the level which results in a voltage drop of 2.490V across its emitter resistance. Relays 3 & 4 are used to switch in different values of emitter resistance for transistors Q4 and Q5, to change the operating currents. For example when relay 3 is energised by the micro, the 68kΩ and 56kΩ resistors plus trimpot VR6 (or VR7) are switched in parallel with the fixed emitter resistors, changing the current level of each source to 100µA. Similarly, when relay 4 is energised, the combinations of 4.7kΩ and 620Ω resistors plus trimpot VR5 (or VR8) are switched in parallel with the fixed emitter resistors, changing the current level of each source to 500μA. So that’s how we program the IBIAS current sources for currents of either 20µA, 100µA or 500µA. Relays 5 & 6 are used to switch the output of either the upper +IBIAS source or the lower -IBIAS source, to the device test circuitry. By the way, when either relay 5 or relay 6 (or both) are off, the current sources obviously can’t provide any of the three preset current levels. The op amp comparators simply bias their pass transistors “hard on”, ready to pass the appropriate current when current is able to flow. That covers pretty well all of the circuitry on the lefthand side of Fig.6, apart from LED1, the “Test Volts On” indicator. This is connected between pin 9 of CON7 and ground, via a series 1kΩ resistor. If you refer back to Fig.5, you’ll see that the LED is connected to the +11.4V rail whenever relay 1 switches on the DC-DC converter circuitry, to perform a test. ZIF socket In the centre of the righthand side of Fig.6 you’ll see the 18-pin ZIF socket siliconchip.com.au that’s used to connect the various types of discrete semiconductor device to the SemTest. The socket’s pin clips are divided into four groups: four for BJTs at lower left, five for Mosfets at upper left, five for SCRs and PUTs at upper right and the remaining four for diodes and LEDs at lower right. You’ll also note that within each device group there are some clips connected together; this has been done to provide for as many pin-out configurations as possible, for each type of device. Upper board relays Shown around the ZIF socket are the various relays used to set up the connections for each device type: relay 9 for diodes and LEDs, relays 10 & 11 for BJTs, relays 12, 13 & 14 for Mosfets and relays 15 & 16 for SCRs and PUTs. If you want to trace out the four separate relay circuits you will find this easier by referring back to the simplified circuits given in Figs.1-4 in the first article. The only other part of the circuitry on the righthand side of Fig.6 is that at top centre, associated with zener diodes ZD3 and ZD4 and pots VR10a and VR10b. These are used to adjust the gate bias voltage, VGS, for Mosfets, which was also shown in Fig.3 of the first article. VR10a is used to adjust the positive VGS for N-channel Mosfets, while VR10b is used to adjust the “negative” VGS for P-channel Mosfets. In operation, the microcontroller Another view inside the prototype SemTest. The full assembly details will be described in Pt.3 next month. works out the effective VGS for the latter devices by subtracting the actual voltage at VR10b’s wiper from the device voltage VDEV (which in this case corresponds to the source voltage). That completes the circuit description. Next month, we will present the SC construction details. Australia’s Lowest Priced DSOs Shop On-Line at emona.com.au Now you’ve got no excuse ... update your old analogue scopes! Whether you’re a hobbyist, TAFE/University, workshop or service technician, the Rigol DS-1000E guarantee Australia’s best price. RIGOL DS-1052E 50MHz RIGOL DS-1102E 100MHz 50MHz Bandwidth, 2 Ch 1GS/s Real Time Sampling 512k Memory Per Channel USB Device & Host Support 100MHz Bandwidth, 2 Ch 1GS/s Real Time Sampling 512k Memory Per Channel USB Device & Host Support ONLY $ Sydney Melbourne Tel 02 9519 3933 Tel 03 9889 0427 Fax 02 9550 1378 Fax 03 9889 0715 email testinst<at>emona.com.au siliconchip.com.au Brisbane Tel 07 3275 2183 Fax 07 3275 2196 362 Adelaide Tel 08 8363 5733 Fax 08 8363 5799 inc GST Perth ONLY $ Tel 08 9361 4200 Fax 08 9361 4300 web www.emona.com.au 439 inc GST EMONA March 2012  79