[Amps] Step-start; improved circuit.

[TexasRF at aol.com]

Also, there is a series of Dayton timer modules in the W.W. Grainger  
catalog. Most are adjustable over a 10 to 1 time range. 120vac on the input  terminal 
delivers about 119 vac to the output terminal when the time has  elapsed. 
They are rated for a 1 A load so driving a relay or even a modest sized  filament 
transformer is no problem.
 
The modules are encased in plastic, about 2 inches square and maybe 1 inch  
thick with a single mounting hole in the center.
 
I have used many of them for step start as well as warm up timers and have  
never had a failure.
 
Cost is about $20. It doesn't get much easier than this!
 
73,
Gerald K5GW
 
In a message dated 3/7/2009 6:56:13 A.M. Central Standard Time,  
paul.kelley.n1bug at gmail.com writes:

Mark  K5AM wrote:
> Some step-start circuit troubles have been reported  recently.
> 
> The step-start circuit usually seen can sometimes  cause trouble; the
> problem is that the relay coil is part of the RC  timing circuit. This
> means that as the capacitor charges it first  reaches a point where the
> relay armature just begins to move slowly,  and then finally the relay
> closes. Not a snap-action arrangement. This  slow closing can cause
> contact burning and eventual failure.
>  
> To avoid such troubles, I use a step-start circuit with an op  amp
> timer. After a one second delay, full voltage is instantly  applied
> to the relay.
> 
> The circuit is described in a  web article
> "Overload Protection for High Voltage Power  Supplies"
> posted for download at www.zianet.com/k5am. The  article
> also includes ideas for other aspects of high voltage  power
> supply design.

I have to agree whole heartedly. While  many conventional step start 
circuits have worked flawlessly for years,  even decades, I was never 
completely satisfied with any arrangement that  had the relay voltage 
slowly ramping up. It just didn't seem right to  me.

Eventually I came up with an op amp timer very similar to that in  
Mark's excellent article, and have been using it ever since, in  
several HVPS projects. This also makes it easy to customize the step  
start delay for any given situation. I actually used a 1M trimmer  
where Mark has the 680K resistor, so the delay is  adjustable.

73,
Paul  N1BUG
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