Re: Electronic dimmer as Variac

From: Sofie (sofie_at_olypen.com)
Date: 07/05/04 

Date: Mon, 5 Jul 2004 16:38:34 -0700

Dave C:
Cheap consumer grade quadrac, triac/diac and scr based light dimmers change
only the "duty cycle" of the power and NOT the voltage. Unless special
things are done they can not easily be used for induction, transformer and
ballast loads such as motors, fluorescent light fixtures and most devices
that use a power transformer which includes linear supplies and switching
supplies.....
Usually a mostly resistive load such as a soldering iron or incandescent
lamps will be the most appropriate application for cheap light dimmer
circuits. Special fan motor speed controls will cost more and must be
used with caution..... a setting that is too slow can stall the motor and
cause motor overheating and failure.
An auto-transformer based variac the one of the more useful pieces of
equipment on an electronics bench. EBAY regularly has fairly good deals.
In addition, a companion to the variac, an isolation transformer is a
"SAFETY must have" when working on "hot" chassis equipment. 

-- 
Best Regards,
Daniel Sofie
Electronics Supply & Repair
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"Rich Grise" <null@example.net> wrote in message
news:1654012.yluSsjVjQ7@entheos.thunderbird.ops.dsl-verizon.net...
> 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
>

Relevant Pages

  • Re: Electronic dimmer as Variac
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  • Re: Electronic dimmer as Variac
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