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Re: [Amps] soft start / thermistor

To: Jim Thomson <Jim.thom@telus.net>
Subject: Re: [Amps] soft start / thermistor
From: Roger <sub1@rogerhalstead.com>
Date: Mon, 01 Mar 2010 13:11:43 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>

Jim Thomson wrote:
> Date: Sun, 28 Feb 2010 09:58:56 +0200
> From: "Alex Eban" <alexeban@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. 
>   
In addition, we used to replace those almost as often as the tubes.

Hmmm...I may have given a hint as to how old I am.

73

Roger (K8RI)
> ## what will  work, is either a conventional step start, or a ramped up 
> triac,take 
> your pick. 
>
> later... Jim  VE7RF 
>
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