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Stop that dangerous kick-back . . . Soft Starter for Power Tools by NICHOLAS VINEN Does your electric saw, router or other large mains-powered hand tool kick like the proverbial mule when you squeeze the trigger? No matter how firmly you hold it, it will still kick and that can be enough to throw you off a carefully lined up cut. This can be bad enough when you are trying to start an accurate cut with a circular saw but it can damage the job if you are using a tool like a large plunge router. But now you can stop that kick with our Soft Starter for power tools. O ur Soft Starter project from core drill bit hard against the wall or to oppose the applied mains voltage April 2012, which tames floor and then press the trigger. The and the resulting surge current can easily be ten times the rated current switch-on current surges pri- resulting torque kick can easily jerk the of the motor with full load. marily in equipment with switch- whole tool out of your hands! And you Elsewhere in this article we show mode supplies, has been very popular. can be injured in the process! some scope grabs depicting these masBut readers started asking “what about Why does it kick? sive currents which luckily die away something similar for power tools?” The reason for that enormous initial to much lower values within less than Many of the smaller mains power tools these days have speed controllers torque is the very high surge current half a second. It is those massive curbuilt into the trigger, so they are very pulled by a universal (series wound, rents which cause the lights to flicker brush) motor when power is first ap- when you switch on a big power tool; controllable when you turn them on. But larger power tools such as circu- plied. Because the motor is not rotat- the mains voltage sags noticeably. lar saws, plunge routers, angle grinders ing, it is not generating any back-EMF The cure and worst of all, large electric Our solution is simple: When drills for concrete core drillFeatures & Specifications you squeeze the trigger switch ing, have a simple trigger or <20A on the power tool, current imthumb switch which applies Inrush current limiting: Minimum load power: ~100W mediately starts to flow but is full power to the motor. Core 10A limited to a reasonable value drilling is particularly danger- Maximum load current: with a big power resistor. Then, ous, as you have to brace the Minimum tool restart interval: 60s recommended 22  Silicon Chip siliconchip.com.au Shown here with two of the hand tools most likely to be used with the Soft Stater, an electric hand saw and plunge router. The unit is housed in the Jiffy Box in front. If used on a building site or other “rough” environments, it could be housed in an aluminium diecast box. after about half a second, we use a relay to short out the resistor and full power is applied to the motor. By that time, the motor is already spinning at high speed so the big peak current is avoided. The basic scheme is shown in the block diagram of Fig.1. In this case though, we have not used a big power resistor, simply because a suitable value with sufficient rating would be large and expensive. Instead we have used two large negative temperature coefficient (NTC) resistors in series with the Neutral side of the load (ie, the power tool). These thermistors have a relatively high initial resistance of about 10Ω each and so they limit the surge current to about 11.5A (230VAC÷20Ω). Now while these thermistors are relatively small, they normally become very hot as their resistance drops. However, we don’t give them a chance to get really hot because they are switched out of the circuit after a short delay. So how do we know when to short out the thermistors? Referring to Fig.1, you will see that there is a current sense resistor in series with the thermistor. This sense resistor has a value of 10 milliohms (0.01Ω) so that the voltage loss across it is quite low. We siliconchip.com.au use this shunt resistor to sense when current starts to flow, immediately after the power tool trigger switch has been pressed. The sense resistor is connected to a comparator, which works by comparing the instantaneous load current to a reference threshold. When you turn the power tool on, it will draw a lot of current at first, well above this threshold. Once this is detected by the comparator, it begins charging a capacitor and after half a second, it operates the relay. From that point on, the tool is effectively connected directly to the 230VAC mains and operates as if the Soft Starter isn’t even there. When the job is finished and you release the trigger switch, the current stops flowing and the circuit resets itself, ready to go again. As long as the tool continues to draw at least 100W (and virtually all do), the relay stays closed. When you switch the tool off, the load current drops to A F1 10A POWER TOOL RELAY1 TRIGGER SWITCH COMPARATOR AND DELAY CURRENT SENSING RESISTOR CURRENT LIMITING THERMISTOR N 0.01  Fig.1: the Soft Starter block diagram. Initially, mains current passes through fuse F1, the power tool motor, a current-limiting thermistor and current sense resistor. A short time after the motor is started, the control circuitry energises Relay1, shorting out the thermistor so the motor gets full power. We actually use two thermistors in series but the principle is the same. July 2012  23 Fig.2: the mains current (yellow) and voltage (green) when starting a 1500W router. The peak current is in excess of 60A, hence the “kick”. Current drops as the motor comes up to speed and it develops more back-EMF, opposing the mains voltage and thus limiting the current. Note the triangular shape of the current waveform which is almost in phase with the mains voltage. Fig.3: with the Soft Starter in circuit, the current at start-up is much lower, initially just 10A peak. This increases slowly over the first 200ms or so as the NTC thermistors warm up, then for the next 400ms the current draw drops as the motor comes up to speed. You can see the slight increase in current as the relay kicks in after 600ms and then the current drops further as the motor approaches full speed. zero and the capacitor discharges. After about half a second, the relay opens and the unit is ready to be used again. Note that if you start and stop the tool multiple times in quick succession, the thermistors won’t have time to cool down properly and the starting current on the second and subsequent starts will be higher than the first and so the tool kick-back will be higher. Even though the thermistors only conduct briefly before being shorted out, they still get quite hot in that short time; quick multiple starts means they getter hotter, their resistance is lower and so the surge currents are higher. So the strategy is clear: to minimise switch-on kick back, don’t stop and start the tool repeatedly in a short time. Wait about ten seconds or so between each cut, or whatever. While this is primarily intended to be used with power tools, there are some other types of load for which may be suitable. For example, it may work with some larger power amplifiers and these could then be switched on using the front panel or remote control rather than having to turn them on and off at the wall, for the Soft Starter to be effective. But there are some caveats. The main restriction is that the load must have a relatively sinusoidal current waveform and draw at least 100W when on. Some devices with switch-mode supplies or with transformers feeding bridge rectifiers will not be suitable. Switch-mode supplies with Active Power Factor Correction (Active PFC) should be OK. The reason is that if the load current is drawn over a narrow part of the mains cycle (ie, near the peaks), the duration of the portion which is above the detection threshold may be too short for the comparator to detect and so the relay will never activate. Active PFC spreads the current out over the full mains waveform, overcoming this issue. However, the only sure way of knowing whether a given device can be successfully used with this Soft Starter is to try it and check that the relay reliably switches in after the load is turned on. If not, the Soft Starter is clearly not suitable for that particular load. 24  Silicon Chip Circuit description Refer now to Fig.5, the circuit diagram. The mains input and output sockets have their active terminals joined via a 10A fuse, protecting both the Soft Starter and the load. The earths are joined, possibly using pin 2 of CON1 as a convenient anchor point. This is vital for safety. The neutral connection is where the soft start action occurs. Initially, the Neutral input (from the mains) and the Neutral connection to the PCB are joined via two series NTC thermistors, TH1 and TH2. Two thermistors provide better in-rush current limiting than one and also reduce the required cool-down time somewhat. Also in series with these thermistors is a 10mΩ (0.01Ω) surface-mount resistor which monitors the load current. Its resistance is so low that it has no effect on the load current and dissipates little power (<1W). When the contacts of RELAY1 close, they short out both thermistors. This has two advantages; the tool gets full power soon after it’s switched on and it allows the thermistors to immediately begin cooling down. The relay is rated at 240VAC/16A, which suits loads up to 4000VA. 15A is the highest continuous current available from “large earth pin” power outlets (10A is the maximum from standard outlets) so we don’t see any problem with the current limitation. The rest of the circuit monitors the voltage across the 10mΩ resistor and turns on RELAY1 when appropriate. It is based around two active devices, quad precision comparator IC1 and PNP transistor Q1. Window comparator IC1a and IC1b are connected so that if the voltage across siliconchip.com.au Fig.4: start-up current of a 1750W circular saw without the Soft Starter. This is quite similar to the 1500W router waveform opposite but the peak current is a little higher. Note how the mains voltage (green, top) sags quite markedly for the first few cycles after switch-on due to the huge initial current. With the Soft Starter, the result is similar to the router (see Fig.2). the 10mΩ shunt exceeds about 3.3mV (ie, a peak load current of 330mA), their common output at pins 1 and 2 goes low. One end of the 10mΩ shunt is connected to ground and the other to pin 6 of IC1b and, via a 1kΩ series resistor, pin 5 of IC1a. Since the current waveform is AC, the voltage at these pins can be above or below ground, so IC1b checks to see whether it goes above +3.3mV while IC1a does the same below -3.3mV. These references voltages are derived from the forward voltage of D3 and D4 (around 0.6V each) using 180kΩ/1kΩ voltage dividers, ie, 0.6V x 1kΩ ÷ (180kΩ + 1kΩ) = 3.3mV. Diodes D3 and D4 are fed from the +12V and -12V rails respectively via 22kΩ current-limiting resistors. Their forward voltages are reasonably stable over a wide range of supply voltages and the expected operating temperature range. The 22kΩ resistors set the current through each to (12V – 0.6V) ÷ 22kΩ = 0.5mA. A small amount of this current flows through the parallel resistors. Now consider the operation of comparator IC1b. The shunt is connected directly to its inverting input while the 3.3mV reference voltage is applied to its pin 7 noninverting input. The open-collector output pin 1 goes low when the voltage at pin 6 exceeds that at pin 7. This will occur when the voltage across the shunt is above +3.3mV. Hysteresis When the shunt voltage is between -3.3mV and +3.3mV, IC1b’s output (pin 1) is pulled up to +12V by a 100kΩ resistor. There is a 10MΩ resistor between this output and the non-inverting input (pin 7) which provides some hysteresis, so that the output does not vacillate when the threshold is crossed. This resistor works as a voltage divider in combination with the resistors connected to pin 7, which provide the +3.3mV reference voltage. When the output is high, the siliconchip.com.au 10MΩ resistor is effectively in parallel with the 22kΩ and 180kΩ resistors at the anode of D3. This allows an extra 12V ÷ (10MΩ+ 100kΩ) = 1.2µA to flow through the 1kΩ resistor, adding 1.2mV to the reference voltage, ie, it becomes +4.5mV. But when the output of IC1b is low (-12V), the 10MΩ resistor sinks a similar amount of current from this point, lowering the reference voltage to around 3.3mV – 1.2mV = 2.1mV. It is the 2.4mV difference between the positivegoing threshold (4.5mV) and the negative-going threshold (2.1mV) which provides the hysteresis. In other words, once the shunt voltage goes above 4.5mV and the comparator output goes low, it must drop below 2.1mV before the comparator output will go high again. The 3.3mV level is just a nominal voltage and does not actually occur in the circuit. The operation of IC1a is similar but since it its inputs must be swapped to allow it to act as the other half of the “window”, the voltage hysteresis is applied to the feedback from the shunt, rather than the reference voltage. The 10MΩ and 1kΩ resistors form a divider which has a virtually identical effect on this sense voltage as described above, ie, it raises or lowers it by 1.2mV depending on the output state. The minimum ±2.1mV thresholds have been selected based on the precision of the LM339A. This has a 2mV maximum input offset voltage with a 5V supply, at 25°C. Unfortunately, the data sheet is coy about just how this varies with supply voltage and temperature but under our operating conditions, it should normally be below 2.1mV. This is why we have chosen the LM339A rather than the more common LM339 variant; if the input offset voltage exceeded the window comparator thresholds, either the relay would switch on with no load or it would never switch off once the load current ceases. (Remember, power is still applied to the Soft Starter even after you’ve let go the tool’s trigger). Time delay When the load current is above the stated threshold and the outputs of IC1a and IC1b are low, this charges a 220nF capacitor via the 2.2MΩ resistor and when the outputs are high, it is discharged in the same manner. Comparators IC3c and IC3d are wired up in parallel and the capacitor voltage is applied to their non-inverting inputs (pins 9 and 11) via a 3.3MΩ resistor. When the relay is off, the outputs of these comparators (pins 13 and 14) are at around +11.4V, since there is little voltage across the relay coil and one diode drop across Q1’s base-emitter junction (~0.6V). The 10MΩ/3.3MΩ feedback voltage divider across the comparators means that when the capacitor is charged beyond 15.8V (ie, its bottom end goes below -3.8V), the voltage at the comparator non-inverting inputs drops below 0V. We confirm this by performing the calculation for this voltage divider, ie, (-3.8V x 10MΩ + 11.4V x 3.3MΩ) ÷ 13.3MΩ = -0.03V. The inverting inputs, pins 8 and 10, are connected to ground so once the capacitor has sufficient charge, the outputs of IC1c and IC1d go low and pull the base of PNP transistor Q1 to -12V. Q1 is an emitter follower and so in this case, it sinks current through the coil of RELAY1, turning it on. July 2012  25   0.01 TH2 SL32 10015 TH1 SL32 10015 –12V A A WARNING VIEWED FROM FRONT E N A OUTPUT SOCKET SOFT STARTER FOR POWER TOOLS SC 22k –0.6V K ZD2 12V 1W K 220F 16V D2 1N4004 Nout 4 Nin 3 2 ALL COMPONENTS AND WIRING IN THIS PROJECT MAY BE AT 230V POTENTIAL IN OPERATION. CONTACT COULD BE FATAL! 2012 K A A 10M 180k –3.3mV D4 1N4148 K 1W 470 10M 1W A 1 CON1 330nF X2 F1 10A E VIEWED FROM FRONT K 1N4004 1N4148 K 12 2 IC1a 4 5 1k A K A D1 1N4004 K A N 230V PLUG A ZD1, ZD2 13 11 10 IC1c 8 9 IC1: LM339AN 3.3M 1k +3.3mV D3 1N4148 220F 16V A K ZD1 12V 1W A 180k +0.6V 22k 1k 6 7 IC1b 3 1 220nF 2.2M 220nF 100k 10M IC1d 14 10M D5 1N4004 K A B –12V E Fig.5: NTC thermistors TH1 and TH2 are connected between the neutral terminals of the input & output mains sockets. A 0.01Ω resistor is used to monitor the neutral current and shortly after it rises, RELAY1 is energised, shorting out the thermistors and allowing the tool to run at full power. The relay is switched off shortly after the tool is, so the unit is ready to go again. B C BC557 C E Q1 BC557 RELAY1 +12V +12V 26  Silicon Chip The voltage at the non-inverting inputs them becomes (-3.8V x 10MΩ + -12V x 3.3MΩ) ÷ 13.3MΩ = -5.8V. This is the hysteresis for this stage and the capacitor must discharge by this additional amount before the relay turns off. This allows the relay to stay on through brief dips in the load current. Diode D5 protects transistor Q1 from any voltage spike created when the relay turns off. Power supply The ±12V rails are derived from the mains Active line via a 330nF X2 series capacitor, 470Ω current-limiting resistor and dual half-wave rectifier formed by diodes D1 & D2. These diodes charge the 220µF capacitors alternately with each mains half-cycle, to provide the positive and negative rails. 12V zener diodes ZD1 and ZD2 limit the voltage across these capacitors to about 11.5V. The 330nF capacitor and 470Ω resistor limit the current and thus dissipation in ZD1 and ZD2 to well below their rated 1W. If you ignore the X2 capacitor and two 1W resistors, this is a traditional AC-to-DC voltage doubler supply. The X2 capacitor has an impedance at 50Hz of around 9.65kΩ which limits the mains current to about 230V ÷ 9.65kΩ = 24mA. It’s a bit more complicated than this calculation implies but that’s a reasonable approximation. We could have used a wirewound resistor of a similar value but it would then dissipate 0.024A2 x 9.65kΩ = 5.5W. The capacitor dissipates virtually no power. The parallel 10MΩ resistor discharges the X2 capacitor once power is removed while the 470Ω series resistor limits the inrush current when power is first applied. For more details on how this type of supply works, see the description in the original Soft Starter article (April 2012). The specified relay has a nominal coil resistance of 1.1kΩ. This means with a 24V supply it will draw around 22mA. As stated earlier, the X2 capacitor limits the supply current to about 24mA; less due to the series 470Ω resistor and other factors. When the relay is turned on, the X2 capacitor and 470Ω resistor form a voltage divider with the coil resistance. The supply rails then drop to about ±6V and the two zener diodes cease siliconchip.com.au 470 1W D2 4004 ZD2 D1 4004 220F 16V 22k 22k CON1 18 0 k D5 1k 220nF 1k Q1 BC557 0.01 3.3M 10M RELAY1 2.2M TH2 SL32 10015 TH1 SL32 10015 4004 COIL 18 0 k 10107121 D4 4148 12V 4148 D3 220F 16V 10M 1W + EARTH OUT 121 7010IN1NEUTRAL ACTIVE Warning: 230VAC! ZD1 12V + 330nF X2 IC1 LM339A 220nF 100k 10M 10M 1k 10107121 COMPONENT SIDE OF BOARD UNDER SIDE OF BOARD Fig.6: use these overlay diagrams and the photograph below as a guide when building the Soft Starter. Just one component, the 0.01Ω SMD resistor, goes on the underside. The diodes, electrolytic capacitors and IC1 must be installed with the orientations shown here. Multiple pads are provided to suit differently sized X2 capacitors. Secure CON1 with a machine screw at each end before soldering its pins. conducting, since most of the input current flows through the relay coil. The relay gets close to the full 24V across its coil initially to turn it on but the 220µF capacitors then partially discharge. The reduced coil voltage is sufficient to keep it energised and the rest of the circuit will run happily with ±6V or less. When the relay turns off, the 220µF capacitors charge back up to their original level. PCB layout While various components in the circuit are shown connected to ground, the main reference point is the “Nin” (Neutral In) terminal of CON1. This is the potential which the shunt sense voltage is relative to. Because this is very low (just a few mV), it’s critical that the ±3.3mV references track this ground potential accurately or the unit won’t work properly. Therefore, the connection between the cathode of D3, the anode of D4 and pin 3 of CON1 is separate from other ground paths. This way, current flowing through ZD1, ZD2, the 220µF capacitors and other components to ground does not interfere with the comparator’s operation. As is typical with a circuit which runs directly from mains, the PCB has a high voltage section at 230VAC and a low voltage section of ±12V (relative to the neutral potential). Since the only components connected to active are the 10MΩ 1W resistor and 330nF X2 capacitor, all other tracks are clear of those pins. There can also be a fairly high voltage across TH1 and TH2 when they are conducting so their terminals are kept clear of other tracks. Construction The Soft Starter for Power Tools is built on a PCB coded 10107121, measuring 59 x 80.5mm. It is a doublesided PCB with tracks on the top side, paralleling the high-current paths on the bottom to improve its currenthandling capability. All components Here’s a view inside the box, fairly close to life-size. You can clearly see the way the wiring is connected to the terminal block on the left end of the PCB – follow this along with the diagram above when wiring it up. If placed inside a metal box, the earth wires must instead be firmly anchored to the box – see text for more details. siliconchip.com.au July 2012  27 Parts list – Power Tool Soft Starter 1 PCB, code 10107121, 59 x 80.5mm (available from SILICON CHIP for $10 + P&P) 1 6-position, 4-way PCB-mount terminal barrier (CON1) (Jaycar HM3162, Altronics P2103) 2 Ametherm SL32 10015 NTC thermistors (Element14 1653459) 1 250VAC 16A SPST relay, 24V DC coil (Element14 1891740 or similar) 1 UB3 jiffy box or 1 diecast IP65 aluminium case (eg, Jaycar HB5046) 4 tapped M3 spacers, 5-6mm long (required only for diecast case) 4 M3 x 15mm Nylon machine screws 4 M3 nuts 4 M3 shakeproof washers 1 chassis-mount M205 safety fuse holder 1 10A M205 fuse 2 M3 x 15mm machine screws and nuts (to attach terminal block to PCB) 2 cord-grip grommets to suit 7.4-8.2mm cable (Jaycar HP0716, Altronics H4270) 1 100mm length brown mains-rated heavy duty (10A) insulated wire 1 50mm length 2.5mm diameter heatshrink tubing 1 short (~1m or so) 10A mains extension cord Semiconductors 1 LM339A quad precision comparator (IC1) (do not substitute LM339) (Element14 9755969) 1 BC557 100mA PNP transistor (Q1) 2 12V 1W zener diodes (ZD1, ZD2) 3 1N4004 1A diodes (D1, D2, D5) 2 1N4148 small signal diodes (D4, D4) Capacitors 2 220µF 16V PCB-mount electrolytics 1 330nF X2 capacitor (Element14 1215460, Altronics R3129) 2 220nF MKT Resistors (0.25W, 5% unless otherwise stated) 3 10MΩ 1 3.3MΩ 1 2.2MΩ 2 180kΩ 1 100kΩ 2 22kΩ 3 1kΩ 1 10MΩ 1W 1 470Ω 1W 1 10mΩ 2W/3W SMD resistor, 6331/2512 package (Element14 1100058) (NB: that is 10 milliohms, not 10 Megohms!) are through-hole types which mount on the top with the exception of the 10mΩ resistor which is an SMD. Refer to the overlay diagram, Fig.6. Start by soldering the chip resistor in place. First, add some solder to one of its two pads using a hot iron. Place the resistor near the pads with its labelled side up, then heat the solder and slide it into place. Remove the iron and check that it is centred over its pads. If not, re-heat the solder and nudge it again. Once it’s in the correct position, solder the other pad. Add a little extra solder to the first one, to re-flow it and ensure a good joint. You can then fit the smaller throughhole resistors, checking each value with a DMM to ensure they go in the right locations. Follow with the seven diodes, orientating them as shown on the overlay diagram. There are three 28  Silicon Chip different types; use the overlay diagram as a guide to which goes where (if you mix them up it won’t work!). Fit the two 1W resistors next, then solder IC1 in place. While used a socket on our prototype (for development reasons) you shouldn’t. Ensure IC1’s pin 1 notch or dot goes towards the bottom left as shown in the overlay diagram. You can then mount Q1, bending its leads with small pliers to suit the pad spacings. Its flat face is orientated as shown. The two MKT capacitors go in next, followed by the electrolytic capacitors, with their longer (positive) leads through the holes marked “+”. There are multiple pads to suit different sized X2 capacitors; solder it in place with one pin in the right-most position and the other through the appropriate left-hand hole. Now you can fit the relay and ther- Fig.7: the correct cut-out to make sure cord-grip grommets do grip! Don’t be tempted to simply drill a 16mm hole! Suits 7.4-8.2mm cable 15.9mm 14mm mistors (pushed as far down as they will go). Attach the terminal barrier using the 15mm M3 machine screws, with a star washer under each head and nut. Do them up tight, make sure it’s straight and then solder the four pins. The PCB assembly is then complete. Housing We housed our prototype in a UB3 jiffy box, which the PCB is designed to fit in. It is pushed down to the bottom of the box, so the taller components will clear the lid. Even though it is a tight fit, to ensure it cannot move around it is fixed to the bottom of the box using Nylon screws (the nuts inside can be Nylon or metal). If this unit is to be used on construction sites or in other rough situations where it’s likely to be knocked around a bit, it should be housed in a larger, sturdier ABS plastic or (preferably) a diecast aluminium case. If you want to do this, fit four tapped spacers to the mounting holes on the PCB and then drill four corresponding holes in the box. If the box is plastic, be sure to use Nylon spacers and screws (metal is OK on the inside) so that you don’t breech the insulation barrier. If you use a diecast aluminium box, the two mains earth wires must have crimp eyelet connectors fitted (use a ratcheting crimping tool), both terminated on a machine screw through the case which is fitted with star washers and two nuts. This earths the case so that an internal wiring fault can’t create a lethal situation. Whichever housing you use, the first step is to drill three holes; two 14mm holes for the cordgrip grommets which the mains cables pass through and one 11-12mm hole for the chassis-mount fuse holder. The fuse holder can go alongside the entry for the mains supply lead. Use needle files to expand the grommet holes to the correct profile (see Fig.7). The requirements for fuse holders varies but they also often require the hole to be profiled; refer to the supplier or manufacturer data for the correct shape. siliconchip.com.au Solder a short length of brown mains-rated wire to one of the fuseholder terminals and heatshrink the joint. Fit the fuseholder to the box and position the completed PCB inside it. You can then cut the extension cord in half and strip a 50mm length of the outer insulation from both free ends. Also strip back 6-8mm of insulation from each of the three inner wires of the two cables. Feed the cables through cordgrip grommets, squeeze the grommet halves together and push them into place through the holes you made earlier. If you are lucky enough to have a tool for inserting cordgrip grommets use that, otherwise some sturdy pliers will do. The grommets are hard to take out once they’re in so check that you have fed through an appropriate length of cable so that the individual wires will reach the terminals on the PCB. Keep in mind that the brown (active) wire from the plug end of the cable must reach the fuseholder. Slip some heatshrink tubing over that Active wire (plug end) and solder it to the free tab on the fuseholder. Slip the tubing down and shrink it over the joint. Secure the five remaining wires into the PCB terminal barrier as shown in the photo on page 27. Make sure there are no stray copper strands and that the terminal screws are done up very tightly so nothing can come loose. As mentioned earlier, if you are using a metal box (eg, diecast aluminium) you will need to make the earth connections to a chassis earth point rather than on the PCB. Testing Because the X2 capacitor limits the circuit current, it can be quite safely tested from mains – but don’t put your fingers anywhere near the PCB. o Here’s the complete project, ready to use. There are no controls on the box . . . because there are no controls! If used in a rough environment, we’d suggest a diecast box – even if a little larger (eg, Jaycar cat HB5046). First, check your wiring. Then put the lid on the box and install a fuse. Use a DMM to check for continuity between the Earth terminals of the plug and socket. The resistance must be low (<1Ω). Do the same check with the two Active terminals and two Neutrals. The resistance between the two Actives should also be low (<1Ω) while between the two Neutrals should be around 20-30Ω (the cold resistance of the NTC thermistors). Also measure the resistance between each combination of Active, Earth and Neutral on each plug. You should get >10MΩ resistance between Earth/Neutral and Earth/Active at both plug and socket. The resistance between Active and Neutral should be around 10MΩ at each end (it may read lower initially due to the capacitors charging). Connect a 100W or greater 230V lamp (eg, a portable PAR38 floodlight – incandescent, not LED!) to the output socket. While keeping your eye on the Resistor Colour Codes No. Value 4-Band Code (1%) o 4a 10MΩ brown black blue brown o 1 3.3MΩ orange orange green brown o 1 2.2MΩ red red green brown o 2 180kΩ brown grey yellow brown o 1 100kΩ brown black yellow brown o 2 22kΩ red red orange brown o 3 1kΩ brown black red brown o 1b 470Ω yellow violet brown brown a 1 of the 10MΩ is 1W b1W siliconchip.com.au 5-Band Code (1%) brown black black green brown orange orange black yellow brown red red black yellow brown brown grey black orange brown brown black black orange brown red red black red brown brown black black brown brown yellow violet black black brown lamp, plug the power cord into the wall outlet and switch it on. Check that the lamp switches on properly – for all intents and purposes, it should appear pretty normal in brightness. But about one second after this, you should hear the relay click and the lamp will get slightly brighter. Switch the lamp off and check that the relay clicks off after about a second. If it doesn’t work, switch off at the wall, unplug both ends, open the box and remove the PCB. Check for components which are swapped or incorrectly orientated. If you don’t see any component problems, check the solder joints and ensure that there are no breaks or short circuits between the tracks or pads. (Kit suppliers tell us that around 50% of problems with kits are mistakes in component placement. Most other problems are bad solder joints [or components not soldered in!]). Assuming all is well, you can then do a full test with a power tool to check that it is working as expected. Remember that if you start the tool multiple times in quick succession, the second and later starts will not have as effective current limiting due to the thermistors heating up. SC Capacitor Codes Value µF Value IEC Code EIA Code 330nF* 0.33µF   330n   334 220nF 0.22µF   220n  224 * must be X2 type July 2012  29