Re: LF: What NTC to use with an transformer

[G0MRF@aol.com]

In a message dated 1/30/04 10:05:34 PM GMT Standard Time, knaap159@zonnet.nl writes:

> Hello all,
>
> I am starting with an powersupply in order to have something to feed an amp.
> I have some nice transformers here, one of them being 50V/1000VA.
> To limit the current when switching on such a large powersupply,
> I'm thinking of using an NTC in the primaire part.
> Question however is, what value should I take??????
> Their nominal value seems to be given at 25 degrees C.
> I'm thinking of 1Ohm, and an Imax of 9 Amp.
> Will that be good enough??????
>
> Dick, pa4vhf


Hello Dick.

A simple and very reliable alternative is a slow start circuit with a resistor and a relay.

Circuit:

A resistor of about 30 to 150 Ohms is wired in series with the mains live and the transformer primary.

An 'open' relay contact is wired across the resistor.

The relay coil is wired across the DC side of your supply.  

Method of operation:

As the mains is switched on, the inrush current on the primary is limited by the resistor.
The voltage slowly builds up on the secondary side and the big smoothing capacitor charges at a sensible rate.  As the voltage on the capacitor reaches a point where the relay coil energises the relay pulls in and the resistor is shorted out.

I've used this technique on a 50V solid state amp and with a 3-500Z tube which uses a 3500V supply.  Both have been very reliable over many years.

73 es GL

David