Fullscreen HTML5Med Res (??MB)HTML4

Appliance Insulation Tester By JOHN CLARKE Do you think all your 230VAC-powered tools and appliances are safe because they are double-insulated? If so, you could be in for a rude shock – literally! Or do you think you are safe because your home (or workplace) is fitted with RCDs (Residual Current Devices)? Again, you could still be at risk of a severe electric shock. The only way to be reasonably sure about appliance and power tool safety is to test them regularly. That is where our Appliance Insulation Tester is a crucial tool. L ET’S BE BLUNT: an RCD will not save you from electric shock if you use a faulty power tool or appliance. Nor will it necessarily save you from death. Have we got your full attention now? An RCD (commonly called a safety switch) will switch off the 230VAC power if it detects an imbalance between the Active and Neutral currents in the appliance circuit. That imbalance could mean that current is flowing through your body rather than the mains wiring. At least 30mA of current needs to flow through your 30  Silicon Chip body for a typical RCD to switch off the power – but depending on the fault, the current could be a lot more than 30mA and the time before it is switched off could be up to 150 milliseconds. That’s long enough to experience a very nasty electric shock and one which could possibly kill you! Well hopefully, it would not kill you but you could still be seriously injured. Say you get the shock while using the faulty tool and standing on an aluminium ladder. The shock could throw you off the ladder and you could be seriously injured or killed (again!). And anyway, how you do know the RCDs in your home are working properly? Have they been tested recently? You can now see that appliances and power tools should be tested regularly. So we have produced our Appliance Insulation Tester which checks whether the insulation resistance is adequate to protect you from serious shock on double-insulated or earthed appliances and power tools. It does this by applying 250V or 500V DC between mains Active and Neutral to the Earth on the 3-pin plug of earthed appliances (Class 1 appliance) or to siliconchip.com.au 10-LED BARGRAPH HIGH VOLTAGE GENERATOR (IC1, IC2, Q2, T1, D1–D4, VR1 ENA λ OUT 3.9M FB VOLTAGE FEEDBACK 100n + – OUTPUT ADJUST VR1 λ λ λ λ λ λ λ IC3c BARGRAPH DRIVER (IC5) CALIBRATE VR2 100k 250V λ OVER LED4 22k 500V λ TEST TERMINALS 3.0k 22k λ 100k S2 D6 K 200k FEEDBACK MONITOR (IC4,LED1) GENERATOR DISABLE (Q4, LED2) SAFETY CIRCUIT A POWER OFF DISCHARGE (Q5, S1) DISCHARGE (Q3) K D8 TRIP COMPARATOR A IC3a, D5 REFERENCE (REF1, IC3b) Fig.1: block diagram of the Appliance Insulation Tester. It uses a high-voltage generator (top, left) to produce either 250V or 500V DC which is applied to the test terminals. The resulting leakage current through the appliance under test and the associated 3kΩ resistor is monitored by op amp IC3c which then drives a 10-LED bargraph via IC5. IC3a monitors IC3c’s output and shuts down the high-voltage generator via Mosfet Q4 if the voltage across the 3kΩ resistor exceeds 3V. exposed metal on double insulated appliances (Class 2) and then the insulation resistance is measured. In general, an insulation resistance (IR) below 1MΩ is deemed unsafe. There are a couple of appliances where this 1MΩ value does not apply. The first is with a portable RCD that has a functional earth (ie, requires an earth for correct operation) and the second is for appliances which have mineral insulated metal sheath heating elements. Check with the AS/ NZS3760 standard for more information. Our Appliance Insulation Tester is not suitable for these devices. Another instrument required While our Appliance Insulation Tester will check most appliances, it does not apply 230VAC mains voltage and therefore cannot conduct an IR test on appliances that have a “soft” or a non-mechanical power switch such as in most appliances with remote controls (eg, DVD players and TV sets). These appliances can only be tested with an instrument that permits energising with the normal 230V supsiliconchip.com.au ply voltage to measure the actual earth leakage current. We plan to feature an Appliance Earth Leakage Tester next month as a companion instrument. Testing safely We mentioned that the testing voltage used is 500V or 250V DC. 500V DC is the usual test voltage while 250V DC is used where an appliance has overvoltage protection. These voltages are high enough to give you a nasty shock if you come into contact with both the test probes, so we have incorporated three safety features. The first is the use of shrouded banana sockets for the high voltage output terminals. Secondly, there is a 1mA (or 500µA depending on output voltage) trip current detector that shuts off the high voltage if this current is exceeded. So if you do make contact with both the test probes you will get an unpleasant “tickle” instead of possibly a more severe electric shock. As well, the Tester has a Trip Test pushbutton which verifies that the unit will shut down if you make contact with the probes. It also lights a LED to indicate that it has been tripped. To restore operation, the unit has to be switched off and then on again. Finally, a check LED is included to indicate if the high-voltage generator is not working correctly. Simplified circuit Fig.1 shows the simplified circuit arrangement of the Appliance Insulation Tester. It comprises a high-voltage generator that can be set to produce either 250V or 500V DC, with voltage feedback to maintain the required voltage with varying load. IC4 includes two comparators which detect if there is a fault in the high-voltage output. A high or low voltage is indicated with LED1 (HV Error). The positive high voltage becomes the “+” test output while the negative (-) test output is connected to the supply ground via a 3kΩ resistance. When the test terminals are connected to an appliance to test for insulation resistance, any leakage current will flow through this 3kΩ resistance and so develop a voltage. This voltage is monitored by IC3c, a high input imApril 2015  31 POWER S1 OFF A +9V ON 10 µF 16V 9V BATTERY LOW ESR 1k 6 Q1 IRF540 D 4x 1Ω 470 µF 16V 100nF IC1 MC34063 REVERSE POLARITY PROTECT Ct 3 GND 4 λ LED1 IC4b 5 IC4a A 100nF 630V S VR1 1M (VR25/VR37) 100k (VR25/VR37) 100k 250V 100k (VR25/VR37) +9V 20k Q3 TK7A60W IC3: LMC6484 10k 13 100Ω 12 1.2V IC3d 14 10k 820Ω 100k C 2.2k 1M 10k 4 FEEDBACK MONITOR K S2 6 2 K +500V/ +250V OUTPUT ADJUST 1.3V 3 A 22k 500V IC4: LM393 SC 22k 820Ω 8 K 3.9M +2.5V K 20 1 5 2 A A Q2 IRF540 G 5 VOLTAGE FEEDBACK HV ERROR 1 10Ω 3 IC2 7555 6 1 1nF 7 5 K D 4 10 µF 2.2k A 8 7 1nF +9V T1 2.2k 100nF 1 SwC FB SwE 2 D1–D4 4x UF4007 TRIPPED λ LED2 10k K 7 Ips 8 DrC Vcc G S 2.2k A λ LED3 +2.5V BATTERY VOLTAGE MONITOR LOW BATTERY D Q4 2N7000 S B E Q5 BC337 NC NO D S S3 TRIP TEST G 10Ω D8 1N4148 A K 10Ω G 10k DISCHARGE MOSFET K APPLIANCE INSULATION TESTER Fig.2: the circuit of the Appliance Insulation Tester. The high-voltage generator consists of an MC34063 DC-DC converter (IC1), a 7555 CMOS timer (IC2), Mosfet Q2, step-up transformer T1 and bridge rectifier D1-D4. IC3c monitors the leakage current through the two series 1.5kΩ resistors and drives IC5, while IC3a is the trip comparator for the safety circuit. pedance, low input current op amp. IC3c operates as a unity gain buffer for the 500V setting or with a gain of two when 250V is selected. So for example, a 1MΩ leakage resistance between the test terminals with a 500V DC test voltage would produce a current of 500µA. This gives 1.5V across the 3kΩ resistance and thus 1.5V at IC3c’s output. For 250V DC, the current with the same 1MΩ leakage resistance would give 250µA and there would be 750mV across the 3kΩ resistance. However, we still get 1.5V at IC3c’s output because it now operates with a gain of two. The pin 8 output of IC3c is attenuated and fed to IC5, an LM3915 dot/ bar display driver (used in dot mode) and 10-LED bargraph display. The display shows resistance in 10 3dB steps: 32  Silicon Chip <707kΩ, 1MΩ, 1.4MΩ, 2MΩ, 2.8MΩ, 4MΩ, 5.6MΩ, 8MΩ, 11MΩ and 16MΩ. A separate LED lights for resistance values of more than 16MΩ. Op amp IC3a compares the output of IC3c with a 2.5V reference voltage set by IC3b. If the voltage across the 3kΩ resistance reaches 3V, IC3a’s output goes high to turn on Mosfet Q4 and disable the high-voltage generator. At the same time, Mosfet Q3 discharges the high-voltage generator’s 100nF output capacitor via a 200kΩ resistance and the display will show a low ohm (<707kΩ) reading. In addition, LED2 indicates that the high-voltage generator has been disabled. Finally, since the high-voltage output will be zero, the feedback monitor will turn on the high-voltage error indicator, LED1. As already noted, to restore operation, the unit has to be switched off and then on again. Note that if the unit is switched off, Mosfet Q3 discharges the high-voltage capacitor, under the control of transistor Q5, which monitors the on/off switch. Full circuit The full circuit is shown in Fig.2. The high-voltage generator comprises an MC34063 DC-DC converter (IC1), a 7555 CMOS timer (IC2), Mosfet Q2 and transformer T1. If this circuit did not have the trip current protection feature, IC1 & IC2 could have been used in a slightly simpler configuration, with the 7555 used as a rail-to-rail Mosfet gate driver and with no gating function via Q4. IC1c’s oscillator runs at a nominal siliconchip.com.au + TEST TERMINALS – A λ 16MΩ A λ 8MΩ A λ 11MΩ 4MΩ A λ 5.6MΩ A λ A A A λ λ λ 150Ω A E 10k B K C λ 10-LED BARGRAPH 10k >16MΩ λ LED4 Q6 BC557 A 2.2k 16 15 13 14 10 11 12 17 18 O2 O3 O8 O7 O6 O10 O9 O4 O5 100k 4 10 (VR25/VR37) 9 10k A λ D7 1N4148 2.0MΩ A CLAMP 2.8MΩ K 1MΩ 10 µF 1.4MΩ +9V <707kΩ +9V IC3c 10nF 6.8k 8 5 IN +9V DOT/ 9 BAR 10 µF IC5 LM3915 VREF 7 CALIBRATE DISPLAY VR2 10k 1nF 3 V+ 11 BUFFER/AMPLIFIER 1 O1 RHI 6 RLO 4 REF ADJ 8 V– 2 1.5k 3.3k 100k CURRENT MONITOR RESISTANCE K 1.5k 1W A 1.5k FORCE DISPLAY LOW D6 1N4148 D5 1N4148 IC3: LMC6484 1W K A 20k 5 LATCH +2.5V REF 3 1 IC3a 7 K LEDS A K A K A K REF1 LM285Z-2.5 REFERENCE VOLTAGE BUFFER A 100nF 100k 1N4148 6 2 TRIP COMPARATOR UF4004 IC3b LM 285 Z-2.5 BC 33 7 , BC557 2N7000 B A K NC D G S E IRF540, TK7A60W G C D D S Main Features 30kHz, as set by the 1nF capacitor at pin 3. IC1’s output pins (1&8) are opencollector transistors that are pulled up to the 9V supply by a 2.2kΩ resistor. The 30kHz output signal is coupled to IC2, a 7555 which is mainly used as an inverting buffer/gate which drives Mosfet Q2. When Q2 is switched on, current flows through the primary winding of transformer T1 until it peaks at about 1.2A. This current flows through the four paralleled 1Ω resistors between pins 6 & 7 of IC1 and when it reaches 1.2A, IC1 stops its oscillator and Mosfet Q2 is switched off. Thus, the magnetic field in the transformer core collapses, producing high voltage in the primary winding. The secondary winding steps up the voltage and feeds a bridge rectifier comprising diodes siliconchip.com.au • Displays insulation resistance in 10 steps from 707kΩ to 16MΩ with acceptable resistance in green and unacceptable resistance in orange and red • • • • • • • 500V DC and 250V DC test voltages • Not suitable for mineral insulated metal-sheathed heating elements 1mA/500µA over-current trip for safety Over-current trip test and trip indicator Low battery indicator High voltage fault indicator High voltage discharges to safe levels at power off and over-current trip out Not suitable for portable residual current devices that incorporate a functional earth D1-D4 to produce a 500V (or 250V) DC supply and this is filtered with a 100nF 630V DC capacitor. Note that a single diode could have been used instead of the bridge rectifier. However, a single diode rectifier would require the transformer windings to be correctly phased and this can be problem for constructors winding their own transformers. Using April 2015  33 Appliance Insulation Tester: Parts List 1 double-sided PCB, code 04103151, 86 x 133mm 1 front panel PCB, code 04103152, 90 x 151mm 1 UB1 plastic utility box 158 x 95 x 53mm 1 ferrite pot core and bobbin set (Jaycar LF-1060 & LF-1062, Altronics L 5300 & L 5305) (T1) 1 pot core spacer eg 0.25mm cardboard 11mm OD or similar (see text) 1 9V battery clip lead 1 9V battery 1 9V battery U-shaped holder 1 20-pin wire wrap SIL socket strip for LED bargraph 2 SPDT toggle switches, PCBmount (S1,S2) (Altronics S 1315) 1 SPDT pushbutton PCB-mount switch (S3) (Altronics S 1393) 1 red safety banana socket (Jaycar PS-0420) 1 black safety banana socket (Jaycar PS-0421) 1 shrouded safety multimeter test lead set (Altronics P 0404A, Jaycar WT-5325) 3 M3 tapped 9mm Nylon spacers 3 M3 tapped 6mm Nylon spacers 3 M3 x 12mm screws 3 M3 x 5mm screws 1 M3 x 10mm countersink screw 1 M3 x 25mm Nylon screw 8 M3 Nylon washers 2 M3 nuts 1 6.5m length of 0.25mm-dia. enamelled copper wire 1 700mm length of 0.5mm-dia. enamelled copper wire 1 40mm length of 230VAC rated red wire 1 40mm length of 230VAC rated black wire 3 PC stakes 1 1MΩ multiturn trimpot (VR1) 1 10kΩ multiturn trimpot (VR2) a bridge rectifier makes transformer winding and termination easier. Since the load on the high-voltage supply can vary, we have voltage feedback to pin 5 of IC1 via a 3.9MΩ resistor and 100kΩ trimpot VR1, together with a 22kΩ resistor at pin 5 of IC1 to ground. VR1 is adjusted to provide an output of 250V DC. An extra 22kΩ resistor is switched via S2 to provide the 500V setting. Either way, the feedback divider reduces the high voltage to a nominal 1.25V at pin 5 and this is compared against an internal 1.25V reference in IC1. If the output voltage drops, the duty cycle of the output waveform from pins 1 & 8 is increased to compensate (and vice versa). Note that the 3.9MΩ feedback resistor is a VR37 or VR25 type, rated at 3500V DC or 1600V DC, respectively. Note also that IC2 has its pin 4 reset pin connected to other parts of the circuitry. This is used to shut down the high voltage generation when required. Under normal operation, pin 4 is pulled high via a 10kΩ resistor to allow IC2 to operate. 34  Silicon Chip Semiconductors 1 MC34063AP1 DC-DC converter (IC1) 1 7555 CMOS timer (IC2) 1 LMC6484AIN quad CMOS op amp (IC3) 1 LM393N dual comparator (IC4) 1 LM3915N dot/bar display driver (IC5) 1 10-LED green/yellow/red LED bar (Altronics Z 0964) 2 IRF540 100V 33A N Channel Mosfets (Q1,Q2) 1 600V low gate threshold Nchannel Mosfet (Q3) (Toshiba TK7A60W or equivalent) (RS Components Cat. 799-5201) 1 2N7000 Mosfet (Q4) Voltage fault indication As mentioned above, the circuit has voltage fault indication and this comprises IC4, an LM393 configured as a “window” comparator. It drives LED1 when the voltage feedback signal fed to pin 5 of IC1 is outside the limits set at its pins 5 & 2 (of IC4). Normally, with feedback voltage in the range of 1.2-1.3V, the paralleled open-collector outputs of IC4 at pins 1 & 7 will remain high and LED1 will be unlit. If the feedback voltage drops below 1.2V, pin 1 of IC4a will go low to light LED1. Similarly, if the feedback voltage goes above 1.3V, pin 7 of IC4b will go low instead to again turn on LED1. So if LED1 lights, it indicates that the 1 BC337 NPN transistor (Q5) 1 BC557 PNP transistor (Q6) 1 LM285Z 2.5V reference (REF1) 4 UF4007 1000V 1A fast diodes (D1-D4) 4 1N4148 diodes (D5-D8) 3 3mm red high brightness LEDs (LED1-LED3) 1 3mm green high brightness LED (LED4) Capacitors 1 470µF 16V low ESR electrolytic 4 10µF 16V PC electrolytic 3 100nF MKT polyester 1 100nF 630V metallised polyester 1 10nF MKT polyester 3 1nF MKT polyester Resistors (0.25W, 1%) 1 3.9MΩ VR37/VR25 (3500V DC or 1600V DC) 1 1MΩ VR37/VR25 (for calibration) (3500V DC or 1600V DC) 3 100kΩ VR37/VR25 (3500V DC or 1600V DC) 1 1MΩ 1 1.5kΩ 4 100kΩ 2 1.5kΩ 1W 2 22kΩ 1 1kΩ 2 20kΩ 2 820Ω 8 10kΩ 1 150Ω 1 6.8kΩ 1 100Ω 1 3.3kΩ 3 10Ω 5 2.2kΩ 4 1Ω 5% DC-DC converter is not producing the correct high voltage. It is not a completely foolproof check of output voltage because if one of the feedback resistors should fail or change its value, the feedback voltage could be correct but the output voltage will not. However, it is still a useful indicator as it will light up when the DC-DC converter is shut down or if the output cannot provide sufficient voltage under load. Of course, test voltages can be periodically checked with a multimeter. Output terminals As can be seen on the circuit, the positive output of the high voltage generator connects directly to the positive (red) test terminal while the negative test terminal is connected to circuit ground via two 1.5kΩ 1W resistors connected in series. These provide a means of monitoring the load current siliconchip.com.au for the Insulation Tester. IC3c monitors the voltage across the resulting 3kΩ resistance via a 100kΩ resistor. This 100kΩ resistor protects the op amp’s input should one of the 1.5kΩ resistors go open circuit and allow the full 250V or 500V to be applied. Diode D7 clamps the input to just over the +9V supply. IC3c amplifies the voltage across the 3kΩ resistance by a factor of two when switch S2 is in the 250V position. In the 500V setting, S2 disconnects the associated 100kΩ resistor from ground and connects another 22kΩ resistor between pin 5 of IC1 and ground. As well as doubling the output from the high-voltage generator, it converts IC3c to a unity gain voltage follower. So either way, the following LED display circuitry involving IC5 gets the correct signal range which is fed via a 6.8kΩ resistor and 10kΩ trimpot VR2. IC5 is an LM3915 logarithmic dot/ bar driver and this drives the 10-LED display. An internal 1.25V reference at pin 7 sets the full-scale input voltage. IC5 is set in dot mode, meaning that only one LED in the 10-LED bargraph is driven at any one time. For our circuit, full scale is when the LED at pin 10 is lit and this is labelled 707kΩ. Other LEDs show 1MΩ, 1.4MΩ etc, as mentioned above. If any of the 10 LEDs in the bargraph is lit, the resulting LED current through the 150Ω resistor from the +9V rail will produce a voltage to switch on transistor Q6 and it shunts LED4 so it cannot light. If all the bargraph LEDs are off, Q6 will be off and LED4 will light, indicating that the load across the tester’s terminals is more than 16MΩ. So in practice, with nothing across the test terminals, LED4 will be lit. Over-current trip As noted above, the trip circuit shuts down the high voltage if the leakage current exceeds 1mA in the 500V setting and 500µA for the 250V setting. The over-current detection circuitry comprises op amps IC3a, IC3b, Q3 & Q4 and REF1. REF1 is an LM285 2.5V reference and IC3b buffers it and feeds the inverting input of IC3a at pin 2. IC3a’s non-inverting input at pin 3 monitors the output of IC3c via a voltage divider comprising a 20kΩ and 100kΩ resistor. IC3a is connected as a comparator. If IC3c’s output goes above 3V, the load current through the two 1.5kΩ current siliconchip.com.au Fig.3: this scope grab shows the action of the trip circuit when the load current exceeds 1mA (for the 500V setting). The green trace shows the voltage across the 3kΩ monitor resistance and the orange trace shows the resulting exponential drop in the high voltage in less than 40ms. monitoring resistors will evidently be above 1mA (for the 500V DC setting). So with a voltage just above 3V from IC3c, IC3a’s output goes high and D5 pulls pin 3 up even further to ensure IC3a then stays latched. IC3a’s high output then switches on Mosfet Q4 and it pulls down pin 4 (the reset input) of IC2. IC2 now acts as a gate and shuts off the drive to Mosfet Q2 to kill the output of the high voltage generator. At the same time, LED2 lights to indicate that the high voltage is off and the overcurrent circuit has tripped. In addition, diode D8 drives the gate of Mosfet Q3 to discharge the 100nF high-voltage supply capacitor via two 100kΩ resistors. And finally, diode D6 drives the input of IC3c to well over 3V so that the LED display will show a low reading, ie, “<700kΩ”. As mentioned previously, the highvoltage error LED (LED1) will also light to indicate that the high voltage has shut down. To return to normal operation, the Insulation Tester is simply switched off and on again. Pushbutton switch S3 connects the two 100kΩ VR25/VR37 resistors on the positive high-voltage supply to the negative test terminal. The resulting current through the 3kΩ monitor resistance causes the circuit to trip out as described earlier. The trip test current is 2.5mA at 500V and 1.25mA at 250V. These test currents are more than twice the rated trip current but will at least verify that the trip current circuit will work. If you are unfortunate enough to get a shock from this Appliance Insulation Tester, it shuts down the high voltage to safe levels within 40ms (much faster than any RCD is supposed to disconnect the 230VAC mains supply in the event of a fault or shock). 9V battery A 9V battery powers the circuit and it is connected via switch S1 in the positive lead. Mosfet Q1 is connected in the negative lead to the battery and provides protection against reversed polarity (ie, when you connect the battery the wrong way around). If the polarity is correct, the internal diode in Q1 will conduct and Q1’s gate will be driven to 9V via a 1kΩ resistor to switch it on. If the 9V is reversed, Q1’s internal diode will be reverse biased and the Mosfet will remain off. A low battery indicator is provided by op amp IC3d, connected as a comparator. It compares the 2.5V from Specifications Power: 9V at 25mA for a 500V output, 18mA for 250V (with >16MΩ leakage resistance), 110mA at 600kΩ leakage and 500V test voltage Low voltage indication: 7.5V. Circuit operates down to 5V Output voltage: 500V and 250V with <1% variation from no load to 1mA trip point Leakage trip current: 1mA at 500V, 500µA at 250V Trip test current: 2.5mA at 500V, 1.25mA at 250V High voltage discharge rate: the 500V output drops below 50V in 40ms April 2015  35 WIRE STRESS RELIEF LOOP TO BATTERY 10-LED BARGRAPH 1k 470 µF 2N7000 100nF 10k 4148 20k 2.2k 10Ω 2.2k D1-D4 (600V) 10 µF TP GND Q4 SOCKETS PANEL) 10nF 1M 10k 100k 100k 100k Q3 1N4148 1nF 20k – Q5 BC337 1.5k 1W REF1 500V 100k* 10k IC3 LMC6484 S2 D5 10k 1 LM285 100k 22k 1.5k 10k 250V + 100k* 10Ω HV ERR. 100nF 630V 100k* VR1 1M 820Ω * VR25 or VR37 3.9M* 4148 A HV ADJ. 4148 LED1 1 4148 k IC4 LM393 10 µF 22k 2.2k 1nF C UF4007 10Ω Q2 IRF540 820Ω 10k 100nF TWO MORE 1 Ω UNDER NC NO 1 IC2 7555 100Ω 1Ω 1Ω 10k 2.2k 1nF MC34063 Q6 A OVER RANGE TRIP TEST 15130140 PRIMARY--T1--SECONDARY 100nF + Low ESR 1 S3 A 10 µF IC1 LED4 k 6.8k S1 POWER LM3915 TEST LEAD (ON FRONT IC5 1.5k 1W LOW BATT. TRIPPED LED2 k 10k IRF540 LED3 04103151 C 2015 1 BC557 Q1 k A APPLIANCE INSULATION TESTER 3.3k 10k 2.2k – 150Ω VR2 10 µF 9V + DISPLAY CAL. D6 D7 D8 Fig.4: follow this parts layout diagram and the photo at right to build the PCB but don’t solder the LEDs or connect the insulated banana sockets until after the front panel PCB is attached (see text). Figs.5&6 on the following pages show the transformer winding details. REF1 with a sample of the battery voltage fed via the 20kΩ and 10kΩ resistors on pin 13. This will cause the comparator to switch its output to turn on LED3 for battery voltages of less than 7.5V. The 1MΩ resistor between pins 14 & 13 provides 135mV of hysteresis to stop any flickering of the LED. Note that the circuit will continue to operate down to about 5V. Note also that we have not provided a separate power indicator LED since either the LED bargraph or LED4 will light whenever power is on. Assembly The assembly is straightforward, with all parts installed on a doublesided PCB coded 04103151 and meas­ uring 86 x 133mm. A second PCB (coded 04103152, 90 x 151mm) is used 36  Silicon Chip as the front panel and this replaces the lid of the UB1 plastic utility box that’s used to house the unit. The two PCBs can be obtained either as part of a complete kit (ie, from parts retailers) or can be purchased as separate parts from the SILICON CHIP Online Shop (www.siliconchip.com.au). Fig.4 shows the parts layout on the main PCB. Begin by installing the resistors and diodes, taking care to ensure that the latter are correctly orientated. Table 1 overleaf shows the resistor colour codes but you should also check each one with a multimeter before fitting it to the PCB. Two 1Ω resistors must be installed on the underside of the PCB. These are mounted directly under the two 1Ω resistors (located adjacent to IC1) and are installed in parallel with these two resistors. It’s just a matter of soldering their leads directly to the pigtails of the top resistors. Note that VR25 or VR37 resistors must be used in the positions marked with an asterisk (*). In addition, two different diode types are used – 1N4148 and UF4007. Be sure to install the UF4007 diodes adjacent to T1. Once the resistors are in, install the three PC stakes. These are used for the positive and negative terminals adjacent to the test lead sockets and for the TP GND terminal (near Q4 at bottom left). The ICs can now be installed. Make sure that the correct IC goes in each position and that it is orientated as shown on Fig.4. IC1, IC2 & IC4 are all 8-pin devices, so be careful not to get them mixed up. siliconchip.com.au foul the front panel PCB). Q1 and Q2 are both IRF540 types, while Q3 is a TK7A60W type (or equivalent). Begin by soldering these Mosfets in at full lead length, taking care to ensure that each is orientated correctly (Q1 & Q2 face in opposite directions). That done, grip the leads of each device in turn using needle-nose pliers and bend it over so that its body sits horizontally above the PCB. Note that if Q3 has a metal tab, it should be covered with heatshrink tubing as the tab will have a high voltage on it. The specified TK7A60W has a plastic insulated tab and so does not require heatshrink insulation. Installing the LED bargraph The primary & secondary leads that emerge from the bottom of the transformer are soldered directly to their respective pads on the underside of the PCB. Note that two 1Ω resistors (circled) are also soldered to the underside of the PCB. These go directly under the two 1Ω resistors on the top of the PCB and are soldered directly to their solder pads, so that all four resistors are in parallel. Follow with the capacitors, taking care to install the electrolytics with the correct polarity. Note that the 470µF capacitor (near power switch S1) must be a low-ESR type. Note also that the top of each electrolytic capacitor must be no more than 15mm above the PCB, to allow clearance for the front panel PCB. It may be necessary to mount the 470µF low-ESR capacitor on its side to meet this requirement, as shown in the photos. Transistors Q5 & Q6 and Mosfet Q4 can now go in. Q5 is a BC337 NPN type while Q6 is a BC557 PNP type, so don’t get them mixed up. The LM285Z (REF1) be also now be installed – it goes in to the left of IC3. Multi-turn trimpots VR1 & VR2 are next on the list. VR1 (1MΩ) goes in with its adjustment screw towards the siliconchip.com.au top edge of the PCB, while VR2’s adjustment screw goes to the right. VR1 (1MΩ) could be marked as 105, while VR2 (10kΩ) may be marked as 103. Now for the three switches. S1 & S2 can be installed either way around – just push them all the way down onto the PCB and make sure they are seated correctly before soldering their terminals. By contrast, S3 (at top right) must be orientated with its common pin towards the lower edge of the PCB. This pin is marked with a “C” on the switch side. Power Mosfets As shown in the photos, Mosfets Q1-Q3 must be mounted horizontally, with their leads bent down through 90° to go into their respective PCB holes (this is necessary so that they don’t The LED bargraph is mounted using a 20-way wire-wrap socket strip. First, break the socket strip into two 10-way strips and plug these into the bargraph pins. That done, insert the socket strips into the holes on the PCB with the bargraph’s anode at top right, as indicated by the chamfer on one edge (see Fig.4). Finally, solder the pins so that the top of the display is 18mm above the PCB. It’s best to solder one end pin first, the adjust the display as necessary before soldering the diagonally opposite end pin. The remaining pins can then be soldered once everything is correct. Winding the transformer Fig.5 shows the transformer winding details. It’s wound on a plastic bobbin which is then fitted into a pot core assembly. The primary goes on the plastic bobbin first and is wound using 10 turns of 0.5mm enamelled copper wire (ECW). These turns are wound on side-by-side (ie, close-wound), with the wire ends brought out through the notched exit April 2015  37 BOBBIN SECONDARY WINDING 3 LAYERS OF 40 TURNS EACH; 0.25mm ENAMELLED COPPER WIRE (120T TOTAL) PRIMARY WINDING 10 TURNS OF 0.5mm ENAMELLED COPPER WIRE Fig.5: the transformer primary consists of 10 turns of 0.5mm-diameter enamelled copper wire (ECW), while the secondary is wound using 120 turns of 0.25mm-diameter ECW in three 40-turn layers. Note that one lead of each winding is brought out at the top of the bobbin, while the other is brought out at the bottom (see text & photos). M3 x 25mm NYLON SCREW M3 x 5mm SCREWS NYLON WASHERS NYLON WASHERS M3 x 9mm TAPPED NYLON SPACER T1 HALF CORE M3 x 6mm NYLON SPACER T1 HALF CORE M3 x 9mm TAPPED NYLON SPACER PCB M3 x 6mm NYLON SPACER NYLON WASHERS NYLON WASHERS M3 NYLON NUT M3 x 12mm SCREWS Fig.6: this diagram and the two photos at right show the mounting details for the transformer. It’s secured in place using three sets of Nylon spacers, Nylon washers and Nylon screws. points on the top and bottom of the bobbin. Once it’s on, cover the winding with a layer of 10mm-wide insulating tape to hold it in place. By contrast, the secondary consists of 120 turns of 0.25mm ECW and is wound using three 40-turn layers, each separated by a layer of insulation tape. As before, the start and finish windings exit from two notched exit points on the top and bottom on the bobbin. The secondary is also close-wound but note that 40 turns will not fit sideby-side across the bobbin. This means that some of the turns in each layer will have to go directly over the top of the others. Ideally, each layer should start on one side of the bobbin and be wound progressively toward the opposite side of the bobbin. Make sure that all three layers are wound in the same direction. Secure the top secondary winding layer with another layer of insulation tape to hold it in place. The next step is to cut an 11mm OD spacer from 0.25mm-thick cardboard. This spacer is used to separate the two Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o No.   1   2   7   2   2   8   1   1   5   1   1   2   1   1   3   4 38  Silicon Chip Value 3.9MΩ 1MΩ 100kΩ 22kΩ 20kΩ 10kΩ 6.8kΩ 3.3kΩ 2.2kΩ 1.5kΩ 1kΩ 820Ω 150Ω 100Ω 10Ω 1Ω 4-Band Code (1%) orange white green brown brown black green brown brown black yellow brown red red orange brown red black orange brown brown black orange brown blue grey red brown orange orange red brown red red red brown brown green red brown brown black red brown grey red brown brown brown green brown brown brown black brown brown brown black black brown brown black gold brown pot core halves (it produces an air gap which prevents saturation in the ferrite cores). Once you’ve cut the spacer to size, cut a neat 3mm hole in its centre. The pot core halves can now be placed over the bobbin with the spacer between them (ie, the spacer fits inside the bobbin). Check that the four leads Table 2: Capacitor Codes Value 100nF 10nF 1nF µF Value IEC Code EIA Code 0.1µF 100n 104 0.01µF   10n 103 0.001µF    1n 102 5-Band Code (1%) orange white black yellow brown brown black black yellow brown brown black black orange brown red red black red brown red black black red brown brown black black red brown blue grey black brown brown orange orange black brown brown red red black brown brown brown green black brown brown brown black black brown brown grey red black black brown brown green black black brown brown black black black brown brown black black gold brown brown black black silver brown siliconchip.com.au The front panel PCB (with the insulated banana sockets fitted) is secured to the main PCB assembly by fitting it over the three switch shafts and doing up nuts on either side. Once it’s in place, the LEDs are pushed through the front panel and soldered and the banana sockets connected to their respective test terminal pads. from the bobbin exit through the core notches, then secure the core assembly using an M3 x 25mm screw, two Nylon washers (one at the top and one at the bottom) and an M3 nut (see Fig.5). Cut off any excess screw length using side cutters. The transformer is now fitted into its hole in the PCB with its 0.5mm primary leads to the left (ie, near Q2) and its 0.25mm secondary leads to the right (near D1-D4). One lead on each side will exit on the top of the PCB, while the other two leads exit the transformer on the underside of the PCB. Once it’s in position, secure the transformer in place using three M3 x 9mm tapped Nylon spacers, three M3 x 6mm Nylon spacers and M3 screws and washers – see Fig.6. The ends of the windings can then be trimmed, striped of insulation using a sharp knife and soldered to their respective pads on the PCB. All that remains before the calibration procedure is to install the battery snap connector. Loop its leads through the two strain relief holes as shown on Fig.4 before soldering them to their pads. Note that LEDs1-4 and the two banana socket terminals are not installed at this stage. Test & calibration Before going further, note that the This adaptor cable makes it easy to connect one of the Insulation Tester’s probes to both the Active & Neutral leads of the appliance being tested at the same time. It’s made by cutting the socket and about 150mm of lead from the end of an extension cord, then connecting the socket’s Active and Neutral wires together and terminating them in a solder eyelet. The Earth wire is cut back out of the way and the cable sleeved in heatshrink and marked. The appliance to be tested is plugged into this socket and one of the Insulation Tester’s probes connected to the solder lug while the other probe goes to the appliance’s external metalwork or chassis. inverter circuit generates a high voltage (up to 500V DC) and this can give you a nasty shock. In particular, note that the trip current protection circuit only works for connections between the “+” and “–” high-voltage terminals. It’s there to provide protection against accidental contact with the output terminals, mainly when the unit is installed in its case. Conversely, any contact between the circuit ground (or any other low- voltage point on the circuit) and high DC voltage on the “+” output will not cause the circuit to shut down. So take care and apply power only when your other hands are safely away from the PCB. To test the unit, you will need to first solder short lengths (eg, 10mm) of red and black mains rated wire to the “+” and “–” high-voltage PC stakes (the output sockets are not installed at this stage). The test and calibration Issues Getting Dog-Eared? Keep your copies of SILICON CHIP safe, secure & always available with these handy binders REAL VALUE AT $16.95 * PLUS P & P Order now from www.siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for overseas prices. siliconchip.com.au April 2015  39 to the top of the switch threads and tighten them down to hold the assembly together. The four LEDs can now be pushed into their front panel holes and their leads soldered. In addition, you will need to solder the high-voltage output leads to the banana socket terminals. Preparing the case This is the view inside the completed Appliance Insulation Tester. The battery is clipped into a holder that’s attached to the lefthand side of the case 15mm up from the bottom and between two sets of internal ribs. procedure is as follows: (1) Connect a multimeter using clip leads across these “+” and “–“ highvoltage leads and set S2 to the 500V position. (2) Apply power (keep your hands away from the leads) and check the high-voltage output. Assuming you get a reading, carefully adjust VR1 (using an insulated screwdriver) for a reading of 500V DC on the multimeter (3) Set S2 to its 250V position and check that the reading is 250V. (4) Switch off and connect a 1MΩ VR25 or VR37 resistor across the highvoltage terminal wires. (5) Set S2 for 500V, apply power and adjust VR2 anticlockwise until at least the third LED (from the left) in the LED bargraph lights. (6) Adjust VR2 clockwise until the second LED (the 1MΩ indicator) just lights, then set S2 to 250V and check that the unit shows the same 1MΩ reading on the LED bargraph. That’s it – the calibration procedure is complete. Final assembly Now for the final assembly. First, insert LEDs1-4 into their PCB holes, 40  Silicon Chip noting that LED4 is green and that all LEDs mount with their anode lead towards the lower edge of the PCB. If the LEDs all have a clear body, you can usually determine which is the green LED by using the diode test feature on your multimeter. The LEDs may only glow dimly using this test but that’s all that’s needed to reveal the colours. Don’t solder the LEDs at this stage but just leave them sitting in place on the PCB. Next, wind a single nut all the way down onto each switch mounting thread. Once these are in place, fit the red and black shrouded banana sockets to the front-panel PCB and secure them with the supplied nuts. Do the nuts up tightly, then fit the front panel over the three switches and push it down so that the LED bargraph goes into its rectangular hole. Note that the corners of this rectangular hole may need to be “squared off” using a file so that the bargraph will fit. Now adjust the three previouslyfitted switch nuts so the LED bargraph display sits flush with the top of the front panel. Check also that the panel is parallel to the PCB, then fit nuts Only a small amount of work is required on the case. The first step is to attach a mounting clip for the 9V battery to the inner lefthand side. That’s done by drilling a 3mm-diameter hole some 15mm up from the outside bottom of the box and between two sets of ribs (see photo). The mounting clip can then be attached using an M3 x 10mm screw and nut. In addition, the internal ribs on the case ends must be cut down, as they prevent the front panel from sitting directly onto the four corner pillars. This can be done using side cutters or a sharp hobby knife. The 9V battery can then be clipped into its holder and the completed PCB and front panel assembly lowered into position and secured using the supplied screws. That done, switch it on and check that the output voltages (250V and 500V DC) are correct. Finally, press S3 to check that the Trip function works correctly. If it does, LED2 should light to indicate that this has occurred. Testing appliances When testing appliances, the condition of the mains plug, lead and earth connection (where used) will need to be checked. Make sure that mains wires are not frayed, repaired with insulation tape, broken or exposed. For earthed appliances, check the resistance between the Earth pin on the mains plug and any exposed metal. There should be less than 1Ω resistance when measuring with a multimeter set to the low ohms range. The accompanying photos show how the Appliance Insulation Tester is used to test a mains appliance. One probe is used to simultaneously connect to both the Active and Neutral pins of the mains plug, while the other probe connects to any exposed metal parts on the appliance. The appliance’s power switch must be on. Note that some metal parts may be painted or anodised and so contact with bare metal will not be made with siliconchip.com.au POWER SWITCH SET TO ON SIGNAL HOUND USB-based spectrum analyzers and RF recorders. SA44B: • Up to 4.4GHz • USB 2.0 interface • AM/FM/SSB/CW demod SA12B: This jigsaw gave an insulation resistance measurement of >16MΩ on the 500V test range, indicating that it is safe to use. • • • Up to 12.4GHz plus all the advanced features of the SA44B AM/FM/SSB/CW demod USB 2.0 interface BB60C: An insulation resistance reading of around 4MΩ was the result when testing this old soldering iron. This indicates some leakage but it’s still safe to use. By contrast, any appliance with an insulation resistance of 1MΩ or less is unsafe. • Up to 6GHZ • Simultaneously monitor two stations or stream the entire FM radio band to disc. • • Facility for GPS time-stamp of recorded RF streams USB 3.0 Interface Vendor and Third-Party Software Available. Ideal tool for lab and test bench use, engineering students, ham radio enthusiasts and hobbyists. Tracking generators also available. the probe. The way around this is to scrape away any coating (without causing too much unsightly damage) so that a proper connection is made to the metal. The Active and Neutral mains plug connection can be made using a large clip attached to the probe. Alternatively, the appliance could be plugged into an extension cord mains socket which has its Active and Neutral leads brought out, connected together and terminated in a crimp eyelet for easy connection to the tester – see photos. siliconchip.com.au Note that normally a 500V insulation resistance test should be made but when an appliance test fails because of internal over-voltage protection (eg, if MOVs are fitted), then a 250V test can be made instead. Any appliance that has a measured insulation resistance of 1MΩ or less is unsafe. Note that this does not apply to portable RCDs that have a functional earth or for mineral insulated metal sheath heating elements (for more information refer to the latest AS/NZS SC 3760 standards). Virtins Technology USB based DSO’s and Signal Generators. Bitscope Digital and Analog USB test and measurement. Silvertone Electronics 1/8 Fitzhardinge St Wagga Wagga NSW 2650 Ph: (02) 6931 8252 contact<at>silvertone.com.au April 2015  41