[Amps] Step-start; improved circuit.

[Steve Thompson]

> 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