[Amps] Step-start; improved circuit.

[David Cutter]

For relay driving I prefer the 555 timer as it does not require a drive 
transistor to the relay, fewer parts to make a good quality timer.
David
G3UNA

----- Original Message ----- 
From: "Paul Kelley N1BUG" <paul.kelley.n1bug at gmail.com>
To: <amps at contesting.com>
Sent: Saturday, March 07, 2009 12:55 PM
Subject: Re: [Amps] Step-start; improved circuit.


> 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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