Fullscreen HTML5Med Res (??MB)HTML4

By NICHOLAS VINEN High-Current Adaptor For Scopes & DMMs If you want to measure and monitor mains current of up to 30A using your DMM or scope, this is the safe and easy solution. It works just as well with DC and it has significantly better resolution and bandwidth than most clamp meters. I N THE SILICON CHIP laboratory, we often need to hook our digital storage oscilloscope (DSO) up to mainspowered equipment to examine the current waveforms. The two most common ways to do this are with a shunt resistor and differential probe or a clamp meter. But both approaches have drawbacks. A shunt resistor connected in series with one of the mains conductors (eg, Neutral) provides the best bandwidth and resolution but you need a differential probe (which can be expensive), even if you are measuring on the Neutral leg since Neutral is usually a few volts above or below Earth potential. The resistor also limits how much current you can measure depending on its value. For example, a 0.1Ω 10W resistor limits you to measuring around 7A RMS (after de-rating by 50%). This 70  Silicon Chip option can also be quite unsafe as the wiring between the shunt and probe is connected directly to mains. A clamp meter is safer since it doesn’t require any exposed mains wiring. But they tend to have a fairly low output voltage, eg, 1mV/A. This gives you lousy resolution and noise performance with scopes which usually have a maximum sensitivity of 5mV/div. Clamp meters also typically have quite limited bandwidth (eg, 10kHz) which is no good for loads with fast-changing current waveforms such as switchmode supplies. Also, you need to separate out the mains conductors to use a clamp meter since if you just clamp it over the cable, the Active and Neutral currents are of identical magnitude and opposite in direction so the magnetic fields effectively cancel. So you need some kind of special cable or adaptor to measure mains current with a clamp meter. Our solution With our adaptor, you get much higher bandwidth and resolution than a clamp meter (80kHz, 100mV/A) with better safety than a shunt resistor, no need for a differential probe and at a fairly low cost. We use an Allegro ACS712 IC, which like a clamp meter operates on the Hall Effect principle but the whole shebang is within a single chip. One side of the IC contains a 1.2mΩ shunt which can handle a continuous current of at least 30A and pulses up to 100A for 100ms. On the other side is a fully isolated Hall Effect sensor and amplification circuitry. There is no electrical connection between the two halves; sensing is siliconchip.com.au purely based on the magnetic field generated by current passing through the shunt. The chip has an isolation rating of 1500VAC between the two halves so the output can safely be hooked up to a scope or other device even if you are measuring mains current at up to 250VAC. There are three versions of this IC, designed for sensing currents up to ±5A, ±20A and ±30A. They are otherwise identical. For our prototype, we used the 20A version since its output is 100mV/A and this makes it easy to set up our scope to read out directly in amps (by telling it we have a 10:1 current probe). We run it from a 5V supply, giving readings of up to ±25A although linearity is a little degraded at the extremes. The 30A version has an output of 66mV/A and can read up to ±38A. You can use this one if you prefer but then you may need a calculator to interpret the readings. Power comes from a 9V battery because this is much more convenient than a plugpack when setting up a test. We fitted ours with a mains plug and socket for measuring the current drawn by mains devices however it could also have been fitted with DC connectors if that’s what we wanted to measure. The output is a BNC socket, making it easy to hook up to a scope. For connection to a DMM, we use a BNC plug to banana socket adaptor. So that you can’t accidentally leave the unit on and drain the battery (easy to do!), we incorporated an automatic time-out which switches the unit off after about 15 minutes. If you want to use it for a longer period, you just have to remember to periodically press the power button to keep it on. Specifications Accuracy: approximately 2% error Bandwidth: typically 80kHz Range: ±25A* (linear over ±20A) Output: 100mV/A* Noise: ~40mV peak-to-peak (equivalent to ~400mA) Power supply: 9V battery, approximately 20 hours life Resistance: ~2mΩ plus cable resistance Isolation: 2.1kV RMS (suitable for use up to 250V AC) Withstand current: 100A for 100ms Other features: power indicator, auto-off to preserve battery life * With alternative shunt IC, range increases to ±38A (linear over ±33A) with 66mV/A output Circuit description Refer now to the circuit diagram in Fig.1. The power supply is shown at left while the actual current sense portion of the circuit is at lower right. IC3 is the ACS712 shunt monitor IC. In addition to a 100nF power supply bypass capacitor, it has a 1nF filter capacitor from pin 6 to ground. This sets its bandwidth to 80kHz and provides a good compromise between bandwidth and residual noise. The shunt side of the IC, at left, is connected to two terminals of a 4-way terminal barrier, which is then wired to the mains plug and socket. If you increase the value of the filter capacitor at pin 6, the residual noise is reduced but so is the bandwidth. For example, if you use 10nF instead of 1nF, bandwidth drops to 8kHz and noise to ~20mV (200mA) peak-topeak. If you use 100nF then bandwidth drops to 1kHz and noise to ~10mV (100mA) peak-to-peak. If unsure, stick with the recommended value of 1nF. IC3’s output is at pin 7 and sits at half supply (about +2.5V) when there is no current flow. This is buffered by IC4a, half of an LM358 dual low-power op amp. Its is biased into Class-A operation with a 10kΩ resistor from its output pin 1 to ground (The LM358 data sheet explains why this is neces- sary). A 100Ω series resistor prevents instability that may occur due to output cable capacitance and the signal is available at the “+” output of CON2. Ideally, we want 0V across CON2 when no current is flowing, rather than 2.5V, so we generate a half-supply rail at around +2.5V and connect that to the negative output terminal of CON2, so there is no voltage across it in the quiescent condition. This is achieved using a voltage divider consisting of two 10kΩ resistors and 500Ω trimpot VR1. The voltage at 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 August 2012  71 72  Silicon Chip siliconchip.com.au D1 1N5819 47k K A 10M 100nF 47k D2 3.3M 1M 47nF K A 11 10 9 12 RS Rtc Ctc MR D5 14 13 15 1 2 8 Vss O4 O5 O6 O7 7 5 4 6 O9 IC2 4060B O8 14 O10 O12 O13 3 7 IC1c IC1d O14 9 8 13 12 10k 16 Vdd 3.3M 100nF A 10 11 D7 K 6 5 2 1 N IC1b IC1a IC1: 4093B 100nF E A K A B OUT IN N E A K A K A (IN S1) D4 E N 4 3 2 1 IP– IP– IP+ IP+  LED1 K A D3 6.8k OUTPUT SOCKET 1 2 3 4 CON1 470nF VIA CON1, TERMINAL 4 D6 C VIA CON1, TERMINAL 3 INPUT PLUG 4 3 22k E Q1 BC559 GND 5 OUT VIout FILTER 6 7 100nF GND IC3 ACS712 8 Vcc IN REG1 LP2950ACZ-5.0 1nF 10k 100nF K A K 1N5819 A D2-D7: 1N4148 10k VR1 500 +5V 100 F +5V +8.7VSW IC4a 8 6 5 8 1 1 4 10k 7 10k ACS712 4 IC4b IC4: LM358 2 3 100nF E IN B OUT C BC559 GND LP2950ACZ-5.0 100 – CON2 + 100 OUTPUT TO SCOPE OR DMM Fig.1: the full circuit of the Current Adaptor. Connections are shown for measuring the mains current but it can also be used to measure low-voltage AC or DC current. Current flows through IC3’s internal shunt and a proportional voltage appears at its VIout terminal (pin 7). Op amp IC4 buffers this voltage and a half-supply rail to provide differential output voltages at CON2. IC3’s 5V rail is derived from a 9V battery via low-dropout regulator REG1 and switched by transistor Q1, which is controlled by a flipflop formed by IC1a & IC1b. The unit is turned on by a short press from momentary pushbutton S1 and turned off by a long press or after 15 minutes by timer IC2. This prevents the battery from being discharged if the unit is accidentally left on; the timer can be reset with a brief press of S1. ISOLATED HIGH-CURRENT ADAPTOR FOR SCOPES & DMMS 100nF 9V BATTERY 2012 SC  A K POWER S1 +8.7V Parts List: Isolated High-Current Adaptor 1 PCB, code 04108121, 60 x 107mm 1 UB3 jiffy box 1 right-angle PCB-mount tactile pushbutton with blue LED (S1) (Altronics S1181) 1 500Ω mini sealed horizontal trimpot 1 9V battery holder, PCB-mount 1 9V battery (alkaline or lithium recommended) 1 4-way PCB-mount (screw fix) terminal barrier (CON1) (Jaycar HM3162) 1 2-way polarised header, 2.54mm pitch (CON2) 1 2-way polarised header connector, 2.54mm pitch 1 female BNC panel-mount socket (Jaycar PS0658, Altronics P0516) 1 100mm length of light duty figure-8 cable or ribbon cable 3 M2 x 6mm machine screws 2 M3 x 15mm machine screws 4 M3 nuts 2 M3 flat washers VR1’s wiper is filtered with a 100nF capacitor and buffered by op amp IC4b, the other half of the LM358. VR1 is adjusted so there is 0V across CON2 with no current through the shunt. CON2 is normally wired to a BNC socket with the negative pin side to its shell. IC4, the LM358, runs off the +8.7V (nominal) switched rail from the battery so that both outputs have a full 0-5V swing. However, note that once the battery has dropped below 6.5V (when it’s quite flat), the full swing may no longer be available. This could result in low readings towards the end of the battery’s life. To improve performance in this respect, an LMC6482 rail-to-rail op amp can be used in place of the LM358 and this will operate normally with a battery voltage down to 5V. However, the LMC6482 draws slightly more supply current; about 1.5mA compared to 0.5mA for the LM358, so the battery life will be slightly less. Power supply The ACS712 isolated shunt IC (IC3) runs from a regulated 5V rail, drawing about 10mA. This is controlled using momentary pushbutton S1 which also siliconchip.com.au 2 M3 star washers 2 M3 x 10mm tapped Nylon spacers 1 M3 x 15mm tapped Nylon spacer* 3 M3 x 6mm Nylon machine screws 1 sheet of Presspahn insulation, 70 x 30mm* 1 mains extension cord with moulded plug and in-line socket* 2 cord-grip grommets to suit 7.48.2mm cable (Jaycar HP0716, Altronics H4270)* 5 small cable ties* Semiconductors 1 4093 CMOS quad Schmitt trigger NAND gate (IC1) 1 4060 CMOS oscillator/counter (IC2) 1 ACS712ELCTR-20A-T (Element14 1329624) OR 1 ACS712ELCTR-30A-T (Element14 1651975) 1 LM358 dual op amp (IC4) 1 BC559 PNP transistor (Q1) has an integrated blue LED. This LED lights up when the unit is on. When on, pressing S1 briefly resets the autooff timer while holding it down for a second or two turns the unit off. The power on/off control and autooff timer functions are provided by IC1, a 4093B quad CMOS Schmitt trigger NAND gate IC and IC2, a 4060B CMOS oscillator/counter. Both these ICs are permanently powered by the battery but being static CMOS logic, only draw a tiny amount of current, typically <1µA. This is probably lower than the battery’s self-discharge current so it will last many years with the unit switched off. Schottky diode D1 provides reverse polarity protection. NAND gates IC1a and IC1b are configured as an RS-flipflop which controls power to IC3 and IC4. When the unit is off, output pin 3 of IC1a is low and output pin 4 of IC1b is high. Therefore, PNP transistor Q1 has no base drive and so no current can flow through its collector-emitter junction and into the rest of the circuit. The high output from pin 4 in this state also forward biases diode D6, pulling pin 12 of IC2 (MR or master reset) high. This prevents IC2’s oscil- 1 LP2950CZ-5.0 low dropout, low quiescent current 5V regulator (REG1) (Jaycar ZV-1645, Element14 1262363) 1 1N5819 1A Schottky diode (D1) 6 1N4148 small signal diodes (D2-D7) Capacitors 1 100µF 16V electrolytic 1 470nF MKT 7 100nF MKT 1 47nF MKT 1 1nF MKT Resistors (0.25W, 1%) 1 10MΩ 1 22kΩ 2 3.3MΩ 5 10kΩ 1 1MΩ 1 6.8kΩ 2 47kΩ 2 100Ω * For measuring mains current, substitute different parts for DC or low-voltage AC current measurement. Note: the PCB is available from the SILICON CHIP Partshop. lator from running, minimising its power consumption. Less than 1µA flows through the 10MΩ pull-down resistor. When pushbutton S1 is pressed, two 47kΩ resistors, a 100nF capacitor and diode D2 provide a delay to debounce the switch. The delay is around 28ms, whether the button is being pressed or released. Because IC1d has Schmitttrigger inputs (ie, inputs with hysteresis), the resulting slow rise and fall times are not an issue. When S1 is pressed, input pin 12 of NAND gate IC2d goes high and assuming pin 13 is high (more on this later), its output pin 11 goes low. This sets the RS-flipflop, sending pin 3 high and pin 4 low, turning on Q1 and thus the rest of the circuit. Pin 13 of IC1d is driven by IC1c. IC1c’s inputs (pins 8 & 9) are tied together so that it operates as an inverter. It is fed from a further delayed version of the pushbutton signal; the 3.3MΩ resistor and 100nF capacitor form an additional low-pass filter which adds a delay of roughly two seconds. This means that the input to IC1c is still low when pin 12 of IC1d goes high; thus pin 13 of IC1d is also high. August 2012  73 5819 9V BATTERY HOLDER 1nF 100nF 2 CAV 0 3 2 100nF 10k M3 x 15MM NYLON SPAC ER AC S712 (UNDER) 1 100nF IC 1 4093B 1M 3.3M REG1 LP2950AC Z-5 C 47k 4148 D2 3.3M 47k D5 4148 4148 D6 47nF 100 F 22k 470nF 10k 4148 D7 100nF 10k BC 559 04108121 Q1 VR1 + IC 4 LM358 10k 100nF 10M 500 100 S1 D4 4148 4148 100nF D3 IC 2 4060B 6.8k OUT – + 3 IC 3 AC S712 4 !R E G NA D s M M D &OUT sepo cINS rof rNotpadAE tnerru C WARNING: LIVE 230V! 2102 C C urrent Adaptor TOP OF BOARD If S1 is held down, after this two second delay, the second 100nF capacitor charges up, bringing input pins 8 & 9 of IC1c high. IC1c’s output therefore goes low. Since IC1c also drives an input of IC1d, IC1d’s output simultaneously goes high. This condition, with input pin 6 of IC1b low and input pin 1 of IC1a high, resets the RS-flipflop, pulling the base of Q1 high and switching the unit off. When pushbutton S1 is released, pin 12 of IC1d goes low before pins 8 and 9 of IC1c do, due to the different time constants of the two low-pass RC filters. This is important so that the unit stays off when S1 is released. Auto-off timer Alternatively, if pushbutton S1 is only pressed briefly while the unit is on, the 3.3MΩ/100nF RC filter does not have time to charge fully and so the unit does not switch off. But diode D5 will still become forward-biased and this pulls IC2’s MR pin high, resetting the auto-off timer. Once S1 has been pressed, the timer (IC2) runs for about 15 minutes and then switches the unit off. This time is set by the timing capacitor 74  Silicon Chip 04108121 D1 12180140 DANGER! 1 Fig.2: the PCB overlay diagram for the Current Adaptor. IC3, the ACS712 hall-effect shunt monitor is soldered to the underside as shown. A slot in the board prevents surface contamination from forming a leakage path between the high and low voltage sides of the IC. The current to be measured flows between the “IN” and “OUT” terminals of the terminal barrier at bottom and the output voltage appears across the 2-pin polarised header at upper left, just below the 9V battery holder. Pushbutton switch S1 at upper-right provides on/off control, timer reset and power indication via its in-built blue LED. 230VAC C urrent Adaptor for Scopes & DMMs C 2012 UNDERSIDE OF BOARD and resistor (47nF and 1MΩ), which give an oscillator frequency of around 8.5Hz. Output O14 (pin 3) goes high after 213 = 8192 clocks and this gives 8192 ÷ 8.5Hz = 963 seconds or about 15 minutes. When O14 goes high, this forwardbiases diode D7 which charges the 100nF capacitor at pins 8 & 9 of IC1c via a 10kΩ resistor, resetting the RSflipflop and switching the unit off. Regulator When Q1 is on, it supplies the ~8.7V from the battery to REG1, a low-dropout, low quiescent current 5V linear regulator. This draws less power from the battery than a 78L05 would and also allows the unit to continue operating down to a lower battery voltage. The power LED integrated within S1 is powered from the 8.7V rail via two series 1N4148 diodes and a 6.8kΩ resistor to limit the current. The two diodes cause the LED to dim significantly as the battery voltage drops below about 6V, since the LED has a forward voltage of around 3.3V and the two diodes add another 1.2V to this. This gives a low battery indica- tion before the voltage drops too low for the device to function. Construction The unit is built on a PCB coded 04108121 and measuring 60 x 107mm. This is available from the SILICON CHIP Partshop. It’s designed as a singlesided PCB with one wire link although we supply a double-sided PCB with that link already present (as a track on the top layer). IC3, the ACS712, is a surface-mount device (SMD) in an SOIC-8 package and this goes on the underside. There is a slot down the middle of its mounting position, to maximise electrical isolation between the shunt and lowvoltage sides. If you have made your own PCB, you should drill a series of 1.2mm holes between the IC pads where shown and file them into a slot. The first job is to solder this IC in place. It must go in with its pin 1 (indicated with a divot, dot or bevelled edge) towards the bottom of the PCB, as shown in the PCB overlay diagram (Fig.2). The PCB indicates the correct orientation too. Put a small amount of solder on one of the pads with the IC resting siliconchip.com.au Fig.3: the correct cut-out to make sure the cord-grip grommets do grip. Don’t be tempted to simply drill a 16mm hole! The completed PCB, without the two corner mounting posts. We used IC sockets for our prototype but it’s better to solder the ICs to the PCB so they can’t come loose if the unit is dropped. Once the wires have been connected to the screw terminal block, the clear cover is clipped in place (not shown). alongside, heat the solder and slide the IC into place. If it isn’t aligned properly on its pads, reheat the solder and nudge it. Repeat until it is correctly aligned, then solder the rest of the pins. Finally, re-solder the initial pin, to ensure the solder has flowed correctly, making a good joint. Next, fit all the horizontally-mounted resistors, checking their values with a DMM. You can also refer to the resistor colour code table below. Follow with the diodes, being careful to orientate them as shown on the overlay diagram. Make sure that the larger Schottky diode (D1) goes at upper-right as shown. Next, solder the DIP ICs in place. In each case, the pin 1 notch or dot goes towards the top of the board. Don’t get the 4060 and 4093 mixed up. We recommend you solder them directly to the PCB so that they can’t come loose and float around inside the box (rather than using sockets). Fit the MKT capacitors next. There are four different values and they go in the locations shown on the overlay diagram. Then mount transistor Q1 and regulator REG1 which are both in TO-92 plastic packages; check the markings so you don’t get them mixed up. You can then install the single electrolytic capacitor (longer lead toward + symbol) and the polarised pin header, followed by the remaining resistors which go in vertically. Trimpot VR1 can go in next, followed by pushbutton switch S1. You may need to bend the latter’s leads slightly to get them to fit the holes as they are quite delicate and can easily be bent out of shape during transport. That done, use three short M2 machine screws to attach the battery holder to the board, then solder the leads. Table 1: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No.   1   1   1   2   1   5   1   2 Value 10MΩ 3.3MΩ 1MΩ 47kΩ 22kΩ 10kΩ 6.8kΩ 100Ω 4-Band Code (1%) brown black blue brown orange orange green brown brown black green brown yellow violet orange brown red red orange brown brown black orange brown blue grey red brown brown black brown brown Suits 7.4-8.2mm cable 15.9mm 14mm That just leaves the terminal barrier, which is mounted using M3 screws with flat washers under the heads and star washers between the nuts and PCB. Do up the screws tight, check that it is parallel with the edge of the board and then solder the pins, using a hot iron and a generous amount of solder. The PCB assembly can now be completed by attaching three tapped Nylon spacers. As shown in one of the photos, the two M3 x 10mm spacers are attached to the two corner holes adjacent to the terminal strip (ie, on the underside of the PCB) using M3 x 6mm Nylon screws. The M3 x 15mm Nylon spacer goes on the top of the board as shown in Fig.2 and is also attached using an M3 x 6mm Nylon screw. It’s used to help retain a Presspahn isolation barrier. Testing Check that the power supply works by connecting the battery and pressing the pushbutton. The blue LED should light up. Hold down the pushbutton for a couple of seconds and check that it goes off. Then set the trimpot to its mid-position, turn the unit back on and measure the voltage across the polarised pin header. It should be Table 2: Capacitor Codes Value 470nF 100nF 47nF 1nF µF Value IEC Code EIA Code 0.47µF 470n 474 0.1µF 100n 104 .047µF   47n 473 .001µF    1n 102 5-Band Code (1%) brown black black green brown orange orange black yellow brown brown black black yellow brown yellow violet black red brown red red black red brown brown black black red brown blue grey black brown brown brown black black black brown August 2012  75 The unit all wired up and ready to go. Note how the 2-wire ribbon cable for the output signal is clamped by the PCB. There isn’t a lot of room for the output connector next to the battery so we had to trim its central solder pin. You can also see how the Presspahn cover is held in place by the plastic case slots, terminal block and tapped spacer. less than ±250mV. Adjust it as close to zero as you can, using the trimpot, then switch it off again. Preparing the case The next step is to drill a 5mmdiameter hole in the side of the case for the on/off pushbutton. This hole is positioned 22mm down from the top lip of the case (ie, not including the lid) and 47.5mm from the output end (again as measured from the top lip). You can then drop the PCB into the case at an angle, to check that the hole lines up correctly when the PCB snaps into place. If not, enlarge it slightly. Next, make the holes for the output socket(s). We simply drilled a 9mm diameter hole in the middle of the end for the panel-mount BNC socket but you could use a pair of binding posts if you want. Keep in mind that there is only about 11mm of clearance from the battery to the end of the case so whatever you use, it can’t intrude very far. In fact, before installing the BNC socket, we had to cut off most of the central prong since it stuck out too far (you only need a short section to solder to). Remove the PCB and fit the BNC socket. Crimp and solder a 70mm length of light-duty figure-8 cable to the two polarised header pins, then push the pins into the moulded plastic housing. Solder the other end of these leads to the rear of the BNC socket, with the lead from pin 1 on the polarised header (normally indicated on the plastic housing) going to the BNC shield while pin 2 goes to the central pin. Mains leads Two M3 x 10mm tapped Nylon spacers are fitted to one end of the PCB as supports. 76  Silicon Chip If you are not planning on using the adaptor with mains, you can use binding posts or whatever you prefer to make connections to the terminal barrier. However this section will describe the procedure for connecting mains cables. The first step is to cut the extension lead in half. Strip away about 50mm of outer insulation from both ends and then expose 7-8mm of insulation from each Active and Neutral wire and 1520mm for the Earth wires. You will then need to make two holes in the case, at the opposite end to the BNC socket. These are spaced 25mm apart, on either side of the centre of that end and have a diameter of 14mm. Start with a smaller hole (4-5mm say) and then enlarge using a tapered reamer or stepped drill bit. Make sure you don’t make the holes too large since the cordgrip grommets need to be a tight fit. Then profile the holes to the shape shown in Fig.3, using a file. Again, be careful not to make the opening too large. Now place one of the mains leads through one of the cord-grip grommets, with the bare leads towards the narrower end. Squeeze the grommet together hard using large pliers (or if you’re lucky enough to have one, a grommet insertion tool), so that only a short length of the cable’s outer insulation protrudes from that narrow end. Push the grommet into one of the holes and it should snap into place. If it won’t go, enlarge the hole very slightly and then try again. Be careful since once it’s in, it’s very hard to get it out. Do the same with the other cable and grommet into the other hole. Now check that the two mains cords are securely anchored. You must not siliconchip.com.au This close-up view shows how the Presspahn cover is held in place by the plastic case slots, the mains terminal block and the M3 x 15mm tapped spacer. be able to pull the cords out of the case, even if you exert considerable force. That done, connect the two Active wires to the terminals marked “IN” and “OUT” on the PCB. For correct output polarity, “IN” should go to the plug and “OUT” to the socket (current flowing from IN to OUT will give a positive output voltage). Do these up tightly, too. Twist the two Neutral wires together and screw them down tightly to one of the spare the terminals on the PCB (see photo). Do the same for the Earth wires. Make sure both are secure. You can then use several small cable ties to hold the wiring in place. These must be installed to prevent individual leads from moving and contacting other wiring if they come loose. Once these are in place, clip the clear cover on top of the screw terminal block. Presspahn barrier The next step is to fit a Presspahn insulation barrier between the mains terminal block and the low-voltage section of the PCB. This insulation barrier is retained by the adjacent slots in the side of the case and must be trimmed to exactly 63 x 25mm so that it is a tight fit. As shown in the accompanying photo, this barrier is sandwiched between the screw terminal block and the adjacent M3 x 15mm Nylon spacer. If necessary, rotate the spacer slightly so that one of its lobes presses the Presspahn insulation firmly against the screw terminal block. siliconchip.com.au The completed unit with the lid in place. Note how the illuminated on/off pushbutton switch protrudes through a hole in one side of the case. Do not leave the Presspahn barrier out – it makes it impossible for any of the mains wiring to contact the lowvoltage section of the PCB and is an important safety measure. Note that once the lid is in place, the Presspahn barrier is also clamped between the lid and the PCB. A BNC plug-to-banana socket adaptor can be fitted to the BNC output socket if you want to connect a DMM. Final assembly Plug in the polarised header and put the lid on the box. Then use a DMM to make some checks before connecting the device up: (1) Check that the Earth terminals on the mains plug and socket have a very low resistance between them (should read zero or very close to it). (2) Do the same check between the Neutral terminals and then for Active. (3) Check that there is no connection between all three pairs of terminals on the mains plug and then on the socket (ie, many megohms; meter should normally read “0L” or similar). (4) Check that there is no connection between both terminals of the BNC socket and all the mains terminals; again, the meter should read “0L”. Now plug the unit into mains and, without touching anything, switch on and measure the AC voltage between the BNC shield and Earth using a DMM. It should be just a few volts. Do the same check with the BNC centre pin. Only when you have ensured that there is no mains voltage on these two conductors should you connect the BNC output to an oscilloscope. You can then do a functional test by connecting an appliance with a known current to the output. For example, if you use a 1kW bar radiator, its current should be about 2.4A, depending on the actual value of the mains voltage. You can then monitor the current with a DMM or oscilloscope. Check that you get a sensible reading. Assuming all is well, disconnect your test load and check the DC output level of the adaptor. It should be close to zero. If not, disconnect all mains cables, open the unit up, make sure it is switched on and adjust the trimpot again. We found that the offset changed slightly the first time we used the unit to measure a high current, so you need to do the final trimming at this stage to guarantee a low offset. That’s it; using the device is simply a matter of plugging it in and switching it on. Don’t forget to periodically reset the timer if you are undertaking a long SC test or measurement session. August 2012  77