[Amps] soft start / thermistor

[Jim Thomson]

Date: Sun, 28 Feb 2010 09:58:56 +0200
From: "Alex Eban" <alexeban at gmail.com>
Subject: Re: [Amps] soft start, how to determine if needed?


By the way, an old trick that was used in tube type TV sets was to install a
thermistor in series with the mains. It had a cold resistance of a few tens
of ohms when cold, dropping to an oh or two after about 20 seconds.
The only problem was the at switch off you had to remember to let it cool
before switching on again.
The Sfernice corporation was manufacturing one type especially for TV sets.
(about 7W power dissipation) with values ranging from 27 to 1000 ohms.
Alex 4Z5KS 

## You don't want to install thermistor's  in the pri of a plate xfmr, real bad news.

## they have been used in the primary of a FIL  xfmr  however. The trick is, they
normally run VERY HOT.   If they are installed, below the chassis, and chassis has
pressurized air, or any kind of air flow past a thermistor, it will cool the thermistor,
and it's resistance will NOT  drop down to an ohm or two.  RF Parts sells 
thermistors, rated for step start of FIL xfmr's. 

## what will  work, is either a conventional step start, or a ramped up triac,take 
your pick. 

later... Jim  VE7RF