Re: Electronic dimmer as Variac

From: John Popelish (jpopelish_at_rica.net)
Date: 07/05/04 

Date: Mon, 05 Jul 2004 12:15:12 -0400

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. 

-- 
John Popelish

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  • Re: Electronic dimmer as Variac
    ... A dimmer needs a minimum current load to keep it ... > conducting the remainder of each half cycle after it is fired. ... > voltage each half cycle (and does that during a small part of the ... > voltage at that instant (whatever voltage remains on the cap from the ...
    (sci.electronics.basics)