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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