Power Supplies - In rush supressors

[Jimmybristol]

Morning,

In history I have used a very simple circuit for inrush limiting, without any fire problems and safety was good (using an MCB on the input (saves changing the fuse every time, depending on your leakage currents - you could also use an RCBO but this can be problematic as if you leakage exceeds the mA (30mA is standard in most circuits) setting it will trip and on motor drives spikes on the supply can cause false trips).

The basic theory goes along the line of this.

You have an inline resistor suitable to limit the current to a level you require, with a timer relay in parallel you set the relay to short the resistor out of the circuit when a certain time has elapsed. This resistor is protected from failure by a thermistor, this thermistor is mains rated, it is normally closed, so opens when the temperature exceeds say 70 degrees (ensuring reduced fire risk).

I have attached an picture I have drawn in word (Using the girlfirends laptop) but I hope that it gets across what I have done before - simple and often you can find the parts used nice and cheap.

Obviously there are some challenges with this circuit, namely the power resistor, but at 1000VA you could make the current significantly lower than the calculations shown.

As ever for information only, if you are not happy working with higher voltages or currents then you should not!! Remember if working on anything that may be charged, on hand in the pocket can save your life.... I take no responsibility or accept liability for anything you use this information for.