Re: Electronic dimmer as Variac
From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 07/06/04
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Date: Tue, 06 Jul 2004 16:36:22 GMT
In sci.electronics.design John Popelish <jpopelish@rica.net> wrote:
> DaveC wrote:
>>
>> I want to turn up a switching power supply slowly after repairing it, so as
>> to see if anything else is damaged. I have no Variac, and I could use the
>> series light bulb trick, but I already have a light dimmer I'm using as a
>> variable-temperature control for my soldering iron that could do double-duty
>> in this application.
>>
>> I know that the SMPS, when running correctly, will be a load pulse at 75 kHx.
>> Has anyone tried using a dimmer as an "electronic variac"? Is it feasible?
>
> It is very doubtful. A dimmer needs a minimum current load to keep it
> conducting the remainder of each half cycle after it is fired.
> Resistive loads are ideal, and average the resultant waveform quite
A lot of the nastiness can be reduced by putting an appropriate resistor
in series with the dimmer.
It's got to be high peak wattage so that it can take series connection
between the (effectively) peak mains voltage, and zero of an uncharged cap.
If you pick the R so that the RC filter formed is some 5ms or so, then
that'll probably be around the right number.
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