Re: Electronic dimmer as Variac
From: Rich Grise (null_at_example.net)
Date: 07/05/04
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Date: Mon, 05 Jul 2004 21:47:01 GMT
on Monday 05 July 2004 09:15 am, John Popelish 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
> well. SMPS usually rectify the line waveform and apply that directly
> to an energy storage capacitor that charges up to the peak waveform
> voltage each half cycle (and does that during a small part of the
> cycle, near the peak).
>
> When your dimmer fires, the rectifier and cap will look like a near
> short circuit to a voltage that is something less than the line
> voltage at that instant (whatever voltage remains on the cap from the
> previous half cycle). And the moment the line voltage starts to go
> down, the rectifier will turn off, isolating the cap from the dimmer,
> causing the dimmer current to fall below its holding current, so it
> will turn off. In other words, you will probably not get the smooth
> control you are needing (any setting that includes the peak of each
> half cycle will produce the same result) and and may damage the dimmer
> and/or the rectifier and/or the capacitor with the big pulses of
> current that occur as the dimmer switches on.
>
> Sometimes you need the right tool.
>
I tried to use an SSR once to power a SMPS. I think I went through
about 3 SSRs before I figured out that you can't do that. And that
was just on/off!
-- Cheers! Rich
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