Soft Start Inrush Current Limiter- PCB design

[Muzzer]

Wow that is a massive overkill. You can simply get a timer relay and a metal clad resistor from CPC or Farnell to do the job. I used a Finder timer relay - I think it was this one last time https://cpc.farnell.com/finder/80-21...0timer%20relay

Worst thing is, Rod Elliott doesn't seem to understand what causes the periodic / random current surge when the transformer is connected. The effect is called "remanence". During normal operation, there is a fairly large AC magnetising current flowing in the primary due to the primary inductance, resulting in an alternating magnetising flux in the core. This is independent of the load on the output. At the instant when you disconnect the mains voltage, much of the magnetising flux at that instant remains (hence "remanence"). If you tried to dismantle the transformer at this point, you'd find the cores were magnetised and wanting to stay together. The magnitude and polarity of the remanence is random, as the interruption is not synchronised to the voltage waveform. When you now come along and reconnect the mains voltage to the primary, you superimpose a magnetising current / flux on top of the remanent flux. Depending on the instant at which that reconnection happens, you can now end up with a higher flux than you would ever see in normal operation.

The steady state magnetising flux of most transformers is actually very close to the saturation flux of the core, so it doesn't take much additional flux to saturate them, which is what causes the random massive inrush currents. The current during saturation is only limited by the voltage, winding resistance and the impedance of the mains network. And these large surges are a random function of the instantaneous voltages at connection and disconnection of the mains, which is why you only seem them from time to time rather than at every power on. You may have wondered why they only happen every so often.

To implement a resistive soft start on the transformer primary, you need to use a resistor that is small enough in value to drive the magnetising current close to its equilibrium value but high enough to limit the current if there is saturation during the first few cycles. This is why the soft start resistor value needs to be selected according to the specific transformer. For the range 1kW - 3kW, I have found values in the range 3 to 22 Ohms work well and the delay timer only needs to be set to a second or so.

[TStar]

Thanks for the feedback, Rod's design produces a much shorter delay, he says the optimum is around 100ms, just 5 cycles. With the 100K variable resistor my version should produce a range of 0-500ms. The combined resistance of the parallel resistor bank is 50 Ohms. This should only protect against initial inrush, and hopefully won't cause much heating of the resistors. It won't do anything about the problem of remanance though. I guess a solution to that might be a switching circuit which is synchronized to the AC? This would involve zero cross detection. I know there are relays out there which have built-in zero cross detection. It still wouldn't solve the polarity though, it could start half a wave out of sync.

[Muzzer]

Thanks for the feedback, Rod's design produces a much shorter delay, he says the optimum is around 100ms, just 5 cycles. With the 100K variable resistor my version should produce a range of 0-500ms. The combined resistance of the parallel resistor bank is 50 Ohms. This should only protect against initial inrush, and hopefully won't cause much heating of the resistors. It won't do anything about the problem of remanance though. I guess a solution to that might be a switching circuit which is synchronized to the AC? This would involve zero cross detection. I know there are relays out there which have built-in zero cross detection. It still wouldn't solve the polarity though, it could start half a wave out of sync.

Yes, another option might be a means of turning on and off at a defined phase angle, ideally ensuring the appropriate polarity.

A phase angle controller using a triac to gradually open up the duty cycle is the usual method used in industrial soft starts. Both methods are pretty messy / complex, whereas the timer and resistor method is a piece of wind to implement.

[the great waldo]

Hi Muzzer
Any chance of a quick schematic for timer relay and a metal clad resistor wiring ?
Thanks in advance.
Cheers
Andrew

[TStar]

Hi Muzzer
Any chance of a quick schematic for timer relay and a metal clad resistor wiring ?
Thanks in advance.
Cheers
Andrew

I'm not Muzzer but as I understand it, just wire the resistor across the terminals of the timer on the input side ie in parallel. When the timer relay is open the current flows through the resistor, expending inrush current as heat. Then after a short period the timer relay closes, the current will choose the path of least resistance and flow through your main circuit instead. Just have to make sure your resistor is appropriately sized.

It's actually the same thing as my module just using commercial units.