Elektor amp inrush current limiter question

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In this thread there's a neat little diagram of a current inrush limit very similar to what I was thinking of building.

It references a link to http://mitglied.lycos.de/Promitheus/delay_circuit_for_toroids.htm which is apparently an Elektor magazine circuit.

My question is what is the C1 bypass cap in the circuit diagram for? They spec 330n, but from Mouser I'm having problems finding that, so was thinking of using either .22uf or .47uf X2 caps (if my ancient EE degree hasn't confused nano, pico, and micro).

Actually, when I look at the whole front part of the curcuit it looks a little weird - the RC time constant seems to be R1,2,3 + C2,3, with the rectifier bridge in the middle? Hmm .. I guess that would work OK, but I don't see a voltage divider so I assume you've got to match the relay resistance with R1,2 and 3 to get the right voltage on the cap's.

Thank you,
I tried this circuit. My realay was 12V, 120ohm and I couldn't get it work with sufficient delay time. All I got was less than 2 sec. and the 2 resistors (470 kohm) had to be replaced by 1k ohm to get 8V voltage drop on relay. I don't know what 330nF cap is doing there, but it seems like the circuit is suited for high resistance relays only.
Well, I could run this off a 12v coil in the xformer I was looking at; if I do this, then I'd probably have to switch in the resistors on the rail side of the transformer. It would give me a headache to figure out how much resistance on the main side would affect a capacitor on the low side :D

That was my original idea anyway; too bad, it would have been nice to order these PCB's.
i think the original article spec a 1100R relay for the right voltage drop Elfa (in Sweden) has a very fitting relay with driving 16-42,2V range. but i would still want to put a zener across that relay just to be sure..

if you use a 12V relay the two 470µF would need to be increased like ~4 times to get the same rise time.

I use this circuit for my Aleph5 (2 x 600VA toroids). It is for 230V operation and works all right with the specified values (relais 24volt/1200 ohms).

For my crescendo I use a circuit thats based upon a 555 timer ic, so you can set the delay as you like but you need a seperate 12V supply.


This cap is used as a Resistor (Rc1 =1/2*pi*f*C1) the two 470 K resistors are used to discharge it.
The current for the relay will flow through the capacitor, wich will not get warm as a resistor would do.
So you can calculate C1 by the current requirement of your relay.;)

regards Arne
will the 330n capacitor not be discharged through the 220 resistor? I have a 24V DC relay with a rated coil current of 21,8mA. I assume this current is constant both when the contakt is on and off. Is it correct? With a 330n capacitor, the voltage drop across it will be approximately 230V, not leaving much voltage to the rectifier circuit. Even though it is written that the charge time constant only depends on the the two 470 micro caps, the 220 resistor will also influence? Tau=R*C
Another question about this circuit.


I am thinking of using this circuit and was wondering if using 4 1N4004 diodes would be ok (safe) to construct the bridge rectifier from? If its important I live in the UK, 240V mains here in case you didnt know.

Thank you.
Mark. :)
The 330nF capacitor will make the current 30mA. If you need another current (for the relay), you need to change the size of the capacitor.
Use a resistor of the same coil-resistance instead of the relais, and measure the voltage. Change the capacitor if the voltage is to high/to low.

If you need a larger softstart module _including power supply, schematics and PCB), check my site:
www.vigier.demon.nl --> Projects --> Everything else --> Soft start module.

Grtz, Joris
I should mention that the 110VDC coil relay would be with a 220V input. For 110VAC input, you could use a 48VDC coil relay with a half wave rectifier.

Btw, I just had an idea. This might sound redudant, but instead of using a string of resistors in parallel with the NO relay contact, how about using some low current inrush current limiters? NTC current limit thermistors might be a cool way to get a 'soft start', IAC, with or without a relay. I'm thinking of experimenting using three 120 ohm at 25C 2A in parallel to switch a 15A max load amplifier. If the time constants work out, it could limit inrush to only about 4A for the second or two it takes the filter capacitor bank to charge up and the relay contact to close.
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