View Full Version : Power Supplies - In rush supressors

20-05-2011, 04:25 PM
Has anyone any experinece in using inrush supressors with toroidal transformers. I have a 1000VA transformer and at switch on it can make the lights dip. Does anyone know how to rate the supressor for the load etc.

20-05-2011, 05:15 PM
Just out of interest does it matter that the lights dim when you switch it on? My 500VA for the stepper motors does the same, so does the milling machine. Neither have ever tripped the breaker so I don't really see a problem.

20-05-2011, 06:10 PM
Does to me. I'd also like to understand how to use supressors.

20-05-2011, 06:15 PM
Does to me. I'd also like to understand how to use supressors.

Ok, what type of suppressor? Active or passive, do you have a link to an example?
You can probably calculate it from the inductance of the transformer, since that gives the energy that the transformer 'stores' (E=0.5*L*I^2) and the energy stored in the the capacitors. Then decide how long you're prepared to wait for it to turn on and if it's passive choose one with a suitable resistance for that time constant...

20-05-2011, 06:43 PM
Passive I guess, something like http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5167861

21-05-2011, 08:38 AM
I hate playing with mains voltage levels. Even if you survive death by electrocution, the ever present potential for fire is the last thing on your mind as you are hurled backwards, heart racing, body trembling. Rod Elliott, one of the few people on the net who I trust for advice on mains related projects, has this to say about thermistors in soft-start circuits:

Many people like the idea of using NTC (negative temperature coefficient) thermistors for inrush limiting, with many claiming that no additional circuitry is needed. In a word, DON'T.

He goes on to explain what you should do... http://sound.westhost.com/project39.htm


21-05-2011, 12:13 PM

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.


21-05-2011, 02:36 PM
Thanks for the replies guys, I think I have enough information to proceed.

03-06-2011, 09:46 PM
I've built the circuit shown in the link from ecat. This circuit works well but I had to change the value of R1 from 27K to around 80K to get a 100ms delay on my relay.