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SoftStarter: Taming The Surge Current Menace!

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 some-times 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

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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.

This 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.

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Here's the SoftStarter in the form we believe will be the most popular - in line with a 4-way power- board which means four different devices (computer, monitor, modem and CFL desk lamp for example) all can have their switch-mode supplies "tamed".

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.

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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.

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.

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.


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.

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