> 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.
>
> 73,
>
> Mark, K5AM
>
I do a similar thing, but with a RC feeding the gate of a MOSFET
with the relay in the drain. The turn on characteristics provide
enough of a 'snap' action to activate the relay cleanly.
Delightfully simple, and if you use a TO220 packaged FET you can
get away without a pcb if you want.
Steve
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