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Soft SoftStarter Starter – tames Are you alarmed by the juicy “splattt” from your mains power point when you plug in something like a large plasma TV set? Do you sometimes burn out light and power point switches because of the surge currents at switch-on? Or perhaps you occasionally trip circuit breakers because of appliance switch-on surge currents. This is a very common problem but there is a simple cure: our SoftStarter. It tames those nasty surge currents while having no effect on appliance performance. By NICHOLAS VINEN T his project was triggered by a number of readers experiencing problems with switch-on surge currents. The first was a school teacher who wanted to switch on banks of laptop computers in a language laboratory. Each time he attempted to do so it would trip out the mains circuit breakers. The breakers would trip out even though the total power drain of the laptops was far less than the breaker’s rated current. Eventually he found that the only way to switch on without tripping the breakers was to switch on the laptops in groups of three or four. The second instance was a reader who fitted a large number of 10W compact fluorescent lamps to a large chandelier – he was trying to toe the government line by not using those nasty (but attractive candle style) incandescent lamps. He found that each time he switched on the chandelier, it tripped the 10A breaker. We have a similar problem in the SILICON CHIP offices with computer workstations comprising two monitors and a desktop PC. Each combination has around 1.15µF of capacitance at the mains plug and it can draw in well excess of 100A when switched on! Worse, one of our staff members measured the input capacitance of his current model Panasonic 50-inch plasma TV at 1.3µF, between Active & Neutral (with its mains switch off). Add in the capacitance of a DVD player and VCR used to feed the Plasma set and you can start to see there is a major problem. All of the above problems relate to appliances which have switch-mode power supplies. In essence, these look and behave like a large capacitor being switched across the 230VAC mains supply. No wonder you get a big splat from the power switch. Fig.1 shows the essentials of a switch-mode power supply. There is typically a 470nF capacitor connected directly between the Active and Neutral leads followed by a bridge rectifier feeding a 470µF 400V electrolytic capacitor to develop around 325V before the switch-mode circuitry itself. No wonder these circuits generate such big surge currents. We did a simulation of this circuit to get a handle on how big these currents can be. Fig.2 tells the story. Depending on the moment of actual switch-on, the peak current can easily be more than 200A and this is backed up by some scope measurements which tell the same story. + A RSOURCE ~ N Mains Supply E GPO 470nF 250VAC X2 ~ 470F 400V Switchmode Circuitry RESR .34 RLOAD DC Output(s) – Fig.1: the configuration of a typical switch-mode power supply. An X2 capacitor (typically 100-470nF) is connected between Active and Neutral to reduce the amount of switching noise that couples from the switching circuitry back into the mains leads. The 230VAC is then rectified and filtered to produce around 325V DC and this is converted to lower regulated DC voltages by the switch-mode module. Also shown is typical capacitor bank ESR (equivalent series resistance) and the mains source impedance due to cabling etc, both of which affect the unit’s peak current draw at start-up. 28  Silicon Chip siliconchip.com.au the surge current menace! Here’s the SoftStarter in the form we believe will be the most popular – in line with a 4-way powerboard which means four different devices (computer, monitor, modem and CFL desk lamp for example) all can have their switch-mode supplies “tamed”. Note that some switch-mode power supplies have active power factor correction (active PFC) which involves extra circuitry. This reduces the in-rush current but there is still an initial surge as the storage capacitor(s) charge. And while no switch-mode circuitry is involved, a similar surge current problem can occur when large transformers are followed by bridge rectifiers and large capacitors. Think about the reader who built a very large power amplifier with a 1kVA toroid power transformer. Switching it on could also trip a circuit breaker or cause the room lights to momentarily flicker. The SoftStarter solution We actually tried several different approaches before coming up with the SoftStarter. Perhaps the simplest and most obvious approach is just to wire a high current NTC (negative temperature coefficient) thermistor in series with the 230VAC mains supply, eg, inside a power board. WARNING! This Soft Start circuit is powered directly from the 230VAC mains and operates at lethal voltages. DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS PLUGGED INTO A MAINS OUTLET OR CONNECTED TO MAINS WIRING and do not operate the circuit outside its plastic case or without the lid screwed onto the case. These devices initially have a fairly high resistance which drops quickly as they heat up. The high initial resistance limits the in-rush current and after a shortt period, this drops enough to allow normal current to flow into the load after the initial surge. The problem is that they run really hot – up to 228°C or higher! This is unavoidable since they rely on the heat to Switchmode Supply Power-on Simulation (RSOURCE = 0.5, RLOAD = 100) with 10 NTC Switchmode Supply Power-on Simulation (RSOURCE = 0.5, RLOAD = 100) +100 50 20 0 10 -100 -200 1 -300 200 +200 Potential (Volts) Potential (Volts) 100 Mains At Socket Capacitor Bank Mains Current Mains Current (Amps) - logarithmic 200 +200 100 Mains At Socket Capacitor Bank Mains Current +100 50 20 0 10 -100 -200 1 Mains Current (Amps) - logarithmic +300 +300 -300 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Time (milliseconds) Time (milliseconds) Fig.2: SPICE simulation of Fig.1. Mains source impedances are set to 0.5Ω and the load resistance is 100Ω. Inrush current peaks at over 200A, limited by the mains source impedance, bridge rectifier impedance and capacitor bank ESR. The capacitor bank charges almost completely in the first half-cycle. The high current distorts the mains waveform both during the initial in-rush and at the voltage peaks where some “flat-topping” is visible. Fig.3: SPICE simulation with the same circuit as shown in Fig.1 but with a 10Ω 15A NTC thermistor connected in series between the mains socket and suppression capacitor/ bridge rectifier. The capacitor bank charges more slowly, over several cycles and peak current is reduced to around 30A (close to our measurements). Note how the bridge conducts for a longer period, even after the capacitor bank has charged. siliconchip.com.au April 2012  29 ACTIVE OUT  ACTIVE IN RLY1 S 4162A (10A) OR JQX-105F-24 (20A) 1 TH1 SL32 10015 2 +24V 150nF* 250VAC X2 (FOR 10A RELAY) 10M 1W 470 1W *OR 330nF 250V AC X2 (FOR 20A RELAY) D1 1N4004 A EARTH D5 1N4004 D2 1N4004 1M A K D4 1N4004 A A ZD1 24V 1W 220F 35V B C Q1 BC547 E C B 47F 16V 4 10M Q2 BC547 E 10M 3 CON1 A K K D3 1N4004 A NEUTRAL K K K NOTE: ALL CIRCUITRY AND COMPONENTS IN THIS PROJECT MAY BE AT MAINS POTENTIAL. CONTACT COULD BE FATAL! 0V BC547 SC 2012 SOFT STARTER 1N4004 A K ZD1 A B K E C Fig.4: the complete circuit diagram of the SoftStarter. NTC thermistor TH1 limits inrush current and after about two seconds, it is shorted out by relay RLY1 for minimal heat generation and power loss. NPN transistors Q1 & Q2 drive the relay coil and their switch-on is delayed by the 47µF capacitor. The +24V rail is derived from mains using an X2 series capacitor, bridge rectifier and zener diode. lower their resistance and allow enough current to flow. Plainly, they run too hot to be installed inside a plastic power board; they would melt the plastic! Apart from that, it’s a waste of power. Depending on the load current, dissipation could be in excess of 5W. Our solution is simple – we use a relay to short out the thermistor after a few seconds. The voltage drop across the relay is very low and so there’s virtually no power loss apart from that required to keep the relay energised. In the case of our SoftStarter, this is less than half a watt. The proof that it works is in Fig.6. This shows the same computer set-up as in Fig.5 being switched on with the SoftStarter connected in series. The inrush current is now limited to around 25A. Note that the current waveform is much smoother and lacks the big initial spike. Note also that the power supply capacitors charge over many more mains cycles than they would without the SoftStarter connected. A number of scope screen grabs in this article reinforce the story: without the SoftStarter you get big in-rush currents and splats from the power switch. Those splats, by the way, are not just annoying: each one is responsible for just a little more of the switch contacts melting and wearing away. Fig.5: current for a computer workstation over the first few mains cycles after power is applied The initial draw of 103.6A is due to the initial charging of the capacitor banks in the switchmode supplies. The second half-cycle peak is much lower. Fig.6: the same situation as Fig.5 but with the SoftStarter in use. Maximum current draw is much lower at 25.3A for the first half-cycle and 14.1A for the second. The capacitor banks charge more gradually, over five full mains cycles or so (100ms). 30  Silicon Chip siliconchip.com.au Here are the two versions of the SoftStarter – on the left the PCB is attached to the base of a standard electrical junction box (in this case an Arlec 9071 but it could be a Clipsal, HPM etc). This version has the 20A relay but again, it could be the 10A relay. On the right is the same board (with 10A relay) placed inside a standard UB3 Jiffy box, as shown in the photo at the start of this article. With the SoftStarter everything is sweetness and light and there is no drama at switch-on. Two versions The SoftStarter can be built in two different ways. First, its PCB can be housed inside a UB3 jiffy box in-line with a standard power board, extension lead or equipment mains lead. It also fits into a standard electrical junction box so that it can be permanently wired into, say, a lighting circuit. It can handle loads of up to 10A or 2300W. That’s the maximum load rating of a typical residential power point (or GPO – which stands for General Purpose Outlet). Circuit description Refer now to Fig.4, the complete circuit diagram. Incom- Fig.7: current flow for a 300VA toroidal transformer charging a large capacitor bank through a bridge rectifier, at switch-on. Peak current draw is 24A on the first cycle and 14A on the second. It could be much higher with a larger transformer. siliconchip.com.au ing mains power is wired to the ACTIVE IN and NEUTRAL terminals while the load is connected to the ACTIVE OUT and NEUTRAL terminals. NTC thermistor TH1 is permanently connected between the incoming Active line and the load. This is an SL32 10015 thermistor has a nominal resistance at 25°C of 10Ω, falling to 0.048Ω at 228°C, which is its sustained body operating temperature with a load current of 15A. That is its rated maximum steady state current and it takes around four minutes to reach operating temperature under full load conditions. In our application, this will never happen as it’s shorted out after about two seconds by the contacts of relay RLY1. NTC thermistors have a few advantages over power resistors in this role. Fig.8: the toroidal transformer based power supply, this time with the SoftStarter connected up. The inrush is much lower with a peak of 14A on the first cycle and 11A on the second. Current is drawn over a larger portion of the mains cycle. April 2012  31 TH1 SL32 10015 SILICON CHIP © 2012 SoftStarter RLY1 S4162A D3 4004 4004 BC547 Q1 10M 10M 1M 24V ZD1 D4 470 1W 10M 1W (330nF X2) BC547 Q2 47F + NEUTRAL 150nF X2 EARTH 4004 ACTIVE IN D5 ACTIVE OUT (JQX-105F-24) 220F + CON1 35V WARNING: 230V AC! D1 4004 D1-D5 1N4004 4004 D2 Fig.9 the component overlay for the SoftStarter with a straight-on shot of the PCB at right for comparison. Take care with the mains wiring and NEVER operate the SoftStarter with the lid off the case – it bites! Firstly, they are rated to handle the very high (~250W) initial dissipation. Secondly, their natural drop in resistance as they heat up provides a gradual increase in current. Finally, they are much more compact than a typical power resistor of equivalent current rating. There are no timer ICs or oscillators in this circuit. Instead, the relay time delay of two seconds is provided by the low-pass filter formed by the 1MΩ resistor and 47µF capacitor, in combination with the base-emitter voltages of NPN transistors Q1 & Q2. At switch-on, the 220µF capacitor is initially charged to 24V and the 47µF capacitor starts out discharged. After a couple of seconds, when the charge across the 47µF capacitor reaches about 1.5V, the Darlington formed by NPN transistors Q1 and Q2 turns on and energises the relay. Its contacts short out the NTC thermistor, applying the full 230VAC to whatever is being switched. After that, the full load current passes through the relay until such time as incoming mains power is switched off. After a second or so, the 220µF capacitor discharges and the relay switches off. Diode D5 protects Q1 & Q2 from the resulting inductive voltage spike. After switch-off, the 47µF capacitor discharges via its parallel 10MΩ resistor (also via Q1’s base-emitter junction and the 1MΩ resistor). After about 30 seconds it’s sufficiently discharged for the unit to be switched back on again with close to the normal two-second delay. If it’s switched back on earlier, the delay will be shorter but should still be sufficient. Power supply The 24V rail is derived from the 230VAC mains using a capacitor/zener regulated supply. Diodes D1-D4 form a bridge rectifier feeding the 220µF filter capacitor and 24V zener diode ZD1 limits the voltage across this capacitor to around 24V. If we simply connected the full 230VAC mains to the input of the rectifier, it and the zener diode would burn 32  Silicon Chip out in spectacular fashion due to the virtually unlimited current flow. This is similar to the problem we are trying to avoid with the SoftStarter! We need limit this current to a safe level. The obvious way to do this is to use a resistor but then that resistor would have about 200V across it and its dissipation would be high, making the circuit very inefficient. So instead of using resistance we use the reactance of a capacitor to limit the current. We simply choose one with an impedance of around 20kΩ at 50Hz. The formula for capacitor impedance is:          1            (2 π f C) so for a 150nF capacitor at 50Hz we get 21.2kΩ. This gives a much higher efficiency; over 50%. This process is illustrated in Fig.6, the output of SPICE simulation of the power supply circuit (using a 220nF capacitor but the principle is the same). The dashed green trace shows the voltage across the X2 capacitor and the difference between it and the mains voltage waveform (red trace) is the voltage across the rectifier, which is limited to around ±25V due to the zener diode. The dashed mauve trace shows the current flowing through this X2 capacitor while the dotted blue trace shows siliconchip.com.au ***NEW KITS*** ULTRA-SONIC PARKING RADAR This kit comes with all parts required and includes cables and connectors. The driver's K318 10W WEATHER-PROOF display shows distance (max 2.5M) via a 7 FLOODLIGHT KIT segment display, left & right LED bar-graphs This kit comes complete with 1 X 10W LED, and audible alarm. The distance displayed is 1 X 10W LED driver kit, 1 X Weatherproof, surprisingly accurate and has a 100mm diecast aluminium housing resolution. Paint and moisture don't seem to bother the sensors and the radar will work with 1, 2, 3 or 4 sensors. 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[TX300] $27ea + + SC_APR_12 Parts List – SoftStarter 1 PCB, code 10104121, 58 x 76mm (available from SILICON CHIP for $10 + P&P) 1 6-position, 4-way PCB-mount terminal barrier (CON1) (Jaycar HM3162, Altronics P2103) 2 M3 x 15mm machine screws with flat washers, star washers and nuts 1 Ametherm SL32 10015 NTC thermistor (Element14 1653459) 1 10A 24VDC coil SPDT relay (Altronics S4162A or equivalent) or 1 JQX-105F-24 20A SPDT relay, 24V DC coil (Futurlec JQX-105F-24 or equivalent) 1 UB3 jiffy box or mains junction box (eg Arlec 9071) Semiconductors 2 BC547 100mA NPN transistors (Q1, Q2) 1 24V 1W zener diode (ZD1) 5 1N4004 1A diodes (D1-D5) Capacitors 1 220µF 35V/50V electrolytic 1 47µF 16V electrolytic 1 150nF X2* (Element14 1215452) (for 10A relay) or 1 330nF X2* (Element14 1200831) (for 20A relay) (* X2 capacitors will have their value printed on them) Resistors (0.25W, 5%) 1 10MΩ 1W (code: brown black blue gold) 2 10MΩ (code: brown black blue gold) 1 1MΩ (code: brown black green gold) 1 470Ω 1W (code: yellow violet brown gold) Additional parts for Jiffy box version 2 cord-grip grommets to suit 7.4-8.2mm cable (Jaycar HP0716, Altronics H4270) 1 short length 2.5mm diameter heatshrink tubing 1 power board 1 small cable tie Additional parts for junction box version: 4 No.4 x 9mm self-tapping screws the product of this current with the mains voltage, ie, the instantaneous power. This power figure is positive when the current and voltage are in phase and this represents power drawn from the mains while when it is negative, the current and voltage are out of phase and it represents current flowing back into mains. As you can see, power tends to be drawn from the mains when the X2 capacitor is charging, ie, when the voltage across it is increasing in absolute terms. It is returned to the mains when this capacitor is discharging. There is also the additional current flow which is that consumed by the circuit being driven which is on top of the capacitor charge/discharge currents. The actual power consumed is the difference between that flowing into and out of the circuit. As you can see from the figure, the area under the curve representing the power drawn from mains is slightly larger than that returned and the simulation gives the difference in this case as 421mW. This is the real power drawn by the circuit. 34  Silicon Chip A straight-on pic of the alternative mounting, the mains junction box. This is actually on the baseplate; the box fits over the top when the baseplate is mounted (eg, to a joist). The apparent power is calculated by multiplying the RMS current by the RMS voltage (ie, 230V). The RMS current is 15.6mA; therefore the apparent power is 3.59VA. This gives a power factor of 0.421 / 3.59 = 0.12. This may seem low but given how little actual power the circuit draws, it isn’t a problem. If we re-run the calculations using a 150nF capacitor, we get a real power of 210mW, an RMS current of 10.7mA, an apparent power of 2.46W and a power factor of 0.085. This agrees almost exactly with our measurements. The 10MΩ resistor has negligible effect on the operation of the circuit and simply serves to discharge the X2 capacitor once the unit is unplugged (so you won’t get a shock if you open up the box). The 470Ω resistor limits the inrush current when the X2 capacitor is initially charged to a maximum of 0.5A. Both of these resistors are 1W types since these are generally rated for use with mains voltages. An important aspect to note is that while 24V zener diode ZD1 limits the voltage across the filter capacitor (220µF) to 24V initially, once the relay is actually energised, the voltage will drop to around 15-16V and ZD1 no longer conducts. The reason for this is that the voltage divider formed by the reactance of the X2 capacitor, the 470Ω series resistor and the relay coil resistance (around 1600Ω) limits the filter capacitor voltage to around 15.8V. This is enough to keep the relay reliably energised but reduces the power consumption of the circuit. Relay & X2 capacitors One of two specified relays can be used: one is rated siliconchip.com.au Capacitor/Zener Mains Power Supply (SPICE Simulation) 20 Power In Power Out 100 10 0 0 -100 -10 -200 -20 -300 Power (W) 200 Potential (V) 30 Mains 230VAC Input X2 Capacitor Charge Current Draw Power Draw Current (mA) 300 -20 0 5 10 15 Time (milliseconds) 20 Fig.10: SPICE simulation output showing how the X2 capacitor/zener power supply works. The X2 capacitor charges and discharges with each mains half-cycle, dropping the 325V DC peak voltage from mains to 24V. The extra energy from the higher voltage is stored in the capacitor and returned to the grid later in the half-cycle. to switch 10A – it can be an Altronics S4162A or JQC21FF-024. The other is physically larger and is rated at 20A (7200VA) and has type number JQX-105F-24. We have specified a 150nF X2 capacitor for use with the 10A-rated relay and a 330nF X2 capacitor for the 20A-rated relay because its coil resistance is lower, at 660Ω. Construction The SoftStarter is built on a 58 x 76mm PCB, coded 10104121. It is double-sided with plated through-holes, so the top layer can carry some of the load current. Start by fitting the three smaller resistors. Use the colour code table or a DMM to check their values. Follow with the five standard diodes and the zener diode, orientated as shown on the overlay diagram (Fig.9). All diodes have their cathode stripes facing either the right side or bottom of the PCB. You can then fit the two 1W resistors, again using referring to the colour codes table or a DMM. Crank the leads of the two BC547 transistors to suit the PCB mounting holes, using small pliers, then solder them in place. Follow with the small and then larger electrolytic capacitors. In both cases, the longer positive lead goes in towards the right side of the board. The X2 capacitor and relay go in next. Use 150nF for the 10A relay or a 330nF for the 20A relay. You may need to turn up your soldering iron temperature to solder the relay as it connects to a large copper area. Then fit the thermistor, making sure it is pushed down as far as it will go before soldering its leads. It will also need a hot iron. Attach the terminal barrier using two M3 x 15mm machine screws. Place flat washers under the heads and star washers between the nuts and PCB, then tighten them down. Check the terminal barrier is parallel to the edge of the PCB and then solder its pins, again with a hot iron. Housing As already noted, the SoftStarter PCB can be installed in either a UB3 jiffy box in-line with a standard 4-way 230VAC power board or extension cord, or in a standard junction box if the device is to be permanently wired into a circuit. We will deal with installation in a UB3 jiffy box first. Originally we designed the PCB to snap into the moulded side rails of the UB3 box but the thermistor is quite tall and interfered with the lid, so we have made the final board narrower and it simply sits in the bottom of the case. It can be glued in place after it has been wired up and tested, so it can’t move and put stress on the wiring. Start by drilling a hole centred in each end of the box, 4-5mm at first, then enlarge them to 14mm using a tapered reamer or stepped drill bit. It’s better to make the holes slightly too small and enlarge them later if necessary since if they are too big, the cord-grip grommets will be loose and you will have to get a new box and start again. The holes can then be elongated with a file in one direction, making a 14 x 15.9mm opening (flat sides, rounded ends), to prevent the grommets from rotating. The correct profile is shown on page 244 of the Altronics 2011-2012 catalog (Type B). Now cut the power board cord. We cut ours about 23cm from the power board so that the SoftStarter unit sits close to the board. Strip 75mm of the outer insulation, then expose 7mm of copper from the Active and Neutral and Earth wires. At the other (plug) end, strip 130mm of the INPUT NEUTRAL WIRE 24V MAINS OUTPUT LEAD 4004 INPUT EARTH WIRE OUTPUT ACTIVE WIRE + CORD CLAMP GROMMET INPUT ACTIVE WIRE + MAINS INPUT LEAD SILICON CHIP © 2012 SoftStarter NOTE: ALL CIRCUITRY AND COMPONENTS IN THIS PROJECT MAY BE AT MAINS POTENTIAL. CONTACT COULD BE FATAL! NYLON CABLE TIE OUTPUT EARTH WIRE OUTPUT NEUTRAL WIRE CORD CLAMP GROMMET Fig.11: here’s how to wire it inside the UB3 Jiffy box. We placed it in line with a standard 4-way powerboard – at about $2.50 each they’re the cheapest way to get a mains plug, cord and (four!) sockets. siliconchip.com.au April 2012  35 outer insulation, then the inner wires the same as before. Place one of the cables inside a cord-grip grommet, with the narrower part towards the exposed wires and a small amount of the outer insulation protruding beyond the grommet. If you’re lucky enough to have a grommet insertion tool you can use that but otherwise, squeeze it together hard with a large pair of pliers and then push it into one of the holes in the jiffy box. This requires quite a bit of brute force and co-ordination but if you do it right, the grommet will go in and it won’t be possible to pull it out. If it won’t fit, enlarge the hole slightly and try again. Give the cords a firm tug to check they are anchored properly – you must not be able to pull them out or move them. Now twist the exposed strands of the Active and Neutral wires and screw them into the appropriate locations on the terminal barrier. Refer to the wiring diagram of Fig.11. The two Neutral wires go into the location marked “N” and should be twisted together. The Active wire from the power board goes to the terminal at the opposite end (“ACTIVE OUT”) while the Active wire from the plug goes next to that (“ACTIVE IN”). Twist the two earth wires together tightly and attach them to the terminal marked E. In each case, ensure that the screw is done up tightly and that there are no exposed or stray copper strands. You can then place cable ties to hold the Active and Earth wiring in place (see photo). Secure the PCB into the bottom of the box using hot melt glue or silicone sealant and fit the lid. Junction box We also designed the board to fit in an Arlec 9071 junction box (other brands such as Clipsal and HPM are very similar). The PCB’s four mounting holes line up with those in the base of the junction box and the rounded corners leave enough room to access the other mounting holes, so you can screw it to a ceiling joist or whatever. The 230VAC mains wires can enter the box lid from the side, using one or two of the knock-out sections. Note that if it is to be installed in permanent wiring, the task should be done by a licensed electrician or suitably qualified person. Check the wiring Going back to the version in a UB3 Jiffy box, before powering up, it’s a good idea to do some basic tests. Measure the resistance between the incoming and outgoing Active wires – it should be close to 10Ω which is the cold resistance of the NTC thermistor. If it is much lower than this, you may have a short circuit somewhere. Also check the resistance between each Active line and the Neutral line. The reading should be around 15MΩ. Again, if it is low, check carefully for shorts. Finally, check for continuity (ie, 0Ω) between the Earths of the in-going and out-going power cord. Then apply power (it isn’t necessary to attach a load). After about two seconds you should hear the click as the relay turns on. Remove power and the relay will click again within a second or so, as it releases. Assuming all is well, repeat the test with a load and this should confirm that it is working properly. For best results, once you have switched off power to the SoftStarter, wait sc at least 30 seconds before turning it back on. 36  Silicon Chip Why is the 50Hz AC E veryone knows that the 50Hz AC mains waveform is a sinewave, right? Well, in theory it is a sinewave but in practice it is distorted because the peaks have been clipped off. For years now our scope screen grabs have shown this but we have not dwelled on the reasons why. Recently though, we have had emails from readers who have sent photos of their scope screens showing the classic flat-topping of the mains waveform. And they want to know why this is happening. You can blame this gross distortion of the mains waveform on two factors: gas discharge lighting and switch-mode power supplies. Gas discharge lighting refers to all lighting systems which use an electric current through a gas to generate light. It applies to all high and low-pressure sodium lamps, mercury vapour lamps and fluorescent lights. In each of these cases, the gas discharge draws current from the AC mains supply only when the actual voltage across the lamp exceeds about 100V. So the current is only drawn from the peaks of the waveform and this inevitably loads down or clips off the peaks. In recent years the situation has become much worse for the electricity generators and distributors with the widespread use of switch-mode power supplies in virtually all electronic appliances. It more or less started with the advent of PCs and their adoption of the more efficient switch-mode rather than conventional mains transformer-driven power supplies which are much heavier, bulkier and more expensive. Switch-mode power supplies were naturally also used in laptop supplies, then TV sets, DVD players etc. Now they are used in virtually all electronic equipment with the sole exception of high performance audio amplifiers (such as our own Ultra-LD amplifier series). Naturally all those large power-hungry Plasma TVs (albeit these days not quite so power-hungry) and large screen LCD TV sets use switch-mode supplies. The reason why switch-mode power supplies are such a problem is that they all essentially consist of a bridge rectifier and a big capacitor, followed by the switch-mode circuitry itself. It is the bridge rectifier and big capacitor which is the problem because current only flows into the capacitor at the peaks of the 50Hz mains sinewave. All of the power drawn by the appliance is drawn from the mains during the peaks of the waveform – not at the other times (unless they are fitted with active power factor correction and relatively few are). Have a look at the simulation of Fig.2 on the second page of the SoftStarter article. This set of curves depicts what happens: large pulse currents which coincide with the peaks of the mains waveform. The simulation is for a 100Ω load which will draw a nominal 529 watts from 230VAC mains. But the current drawn from the mains is not a nice sinusoidal 2.3A but is a pulse waveform with peaks of about 15A! No wonder the peaks of the waveform are being clipped off. siliconchip.com.au