Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!sunic!news.funet.fi!polaris.utu.fi!ahahma
From: ahahma@polaris.utu.fi (Arno Hahma)
Newsgroups: sci.electronics
Subject: Re: Big transformers and circuit breakers
Message-ID: <1991Mar23.162927.21031@polaris.utu.fi>
Date: 23 Mar 91 16:29:27 GMT
References: <1991Mar23.024309.12334@spool.cs.wisc.edu>
Organization: University of Turku, Finland
Lines: 35

In article <1991Mar23.024309.12334@spool.cs.wisc.edu> kolstad@jomby.cs.wisc.edu (Joel Kolstad) writes:

>immediately trips (even with no load on the secondary!) since the lack of
>a magenetic field in the toroid causes the primary windings to look like
>a short.

I had a similar problem with a transformer, the 16 A fuse would blow 
unless there was a protection circuit.

I use a relay and a resistor. Connect a 100 ohm resistor in series with
the transformer primary. Connect the relay to short the resistor, i.e. to
let the current flow directly to the transformer. Then use a simple RC-timer
with a transistor (or whatever delay) to connect the relay on, say,
half a second after the power has been switched on. The transformer 
will then have time to "warm up" before the full voltage is connected.

>I know that some people use thermistors in front of (incandesent) lamps to 
>limit inrush current.  Is this a possibility?  (Yes, I realize the source of 

Yes it is, if you find suitable thermistors, that can stand the current the
transformer takes when you have your load on the secondary. Also, the 
thermistor should have a small resistance, as it is warm.

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