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Electronic dimmer as Variac

Discussion in 'Electronic Basics' started by DaveC, Jul 5, 2004.

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  1. DaveC

    DaveC Guest

    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?

  2. 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.
  3. Ben Bradley

    Ben Bradley Guest

    My immediate response is don't do this. The SMPS will pull large
    amounts of current as the voltage approaches peak (rectifier from
    power line charging main capacitor), and it may exceed the peak
    current rating of the dimmer, even accounting for the fact that light
    filaments pull a large amount of current when cold (at least they heat
    up rather quickly and pull less current).
    My advice is (with all the caps dischharged) double-check all the
    rectifiers and other 'main' semiconductors (such as the main flyback
    transistor), then stand back and give it full power.
    But then I'm posting from SED and don't do much repair. If you
    don't want me steering you wrong, don't crosspost outside the *.repair
  4. Rich Grise

    Rich Grise Guest

    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!
  5. Sofie

    Sofie Guest

    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
    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.
  6. feasible?

    Read the instructions that came with the dimmer. It probably says that
    you're only supposed to use it with a resistive load such as a light
    bulb. It'll probably burnout if used on a PS.
  7. Likely not, use the light bulb trick instead - I prefer a Toaster in series
    mysel just to save the fuses.

    A SMPS will, if it is a decent design, have a threshold where the supply
    will start - so the first 1/3 of the input voltage range it will be off
    anyway and ramping up the voltage does not buy you anything other than
    saving a fuse: It will start suddently by itself.

    A SMPS is usually a capacitive load, the dimmer is designed for an Ohmic
    load i.e. a light bulb, and the TRIAC in the dimmer will not appreciate
    piring directly into a capacitor that will look like a dead short. They you
    can stick an inductor in series - and you might get at resonant overvoltage
    blowing the supply/dimmer away.

    IOW: It creates more trouble than the problem you started with.
  8. El Meda

    El Meda Guest

    "Frithiof Andreas Jensen"
    And if the OP uses the dimmer to control a light bulb, and connect the
    SMPS in parallel with the bulb? Will it work?
  9. DaveC

    DaveC Guest

    OK, OK, I think I get the idea. I had no plans to try it unless others here
    thought it might work. Guess not...

    So it's a light bulb (or toaster) in series.

    Thanks for lending me your minds,
  10. A divider made of two light sockets wired in series may be helpful,
    also. Connect the supply across the bulb on the neutral side of the
    divider. If the two bulbs are the same wattage, the voltage is
    limited to no more than half the line even if the supply does not draw
    current. It takes little time to screw in various bulb combinations.
  11. Ian Stirling

    Ian Stirling Guest

    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.
  12. It might just work if you use one resistive load (Say, a 25 to 40 watt
    light bulb) as a minimum load and put a second resistor (say, a 40 to
    100 watt light bulb) in series with the input of the supply as a
    second load. That one would function as part of the low pass filter
    you describe.
  13. Ian Stirling

    Ian Stirling Guest

    I assume that the first one is to keep the triac conducting.
    If it's going to a rectifier/cap, then how does it matter?
    When the current goes to zero, the rectifier turns off anywya.
  14. The phase timing circuit in many dimmers only functions properly if
    there is a resistive load on the dimmer. The timing current passes
    through the load.
  15. The simple answer is that if the dimmer sees mostly a resistive load, it
    will work reasonably well and survive. This *can* include things like
    shaded pole motor-based fans (but probably not other types of induction
    motors or universal motors). However, if it's driving a lot of capacitance or
    inducatance, at the very least it will behave strangely and more likely
    will self destruct or damage the equipment. There's also usually a minimum
    load below which it won't do anything predictable. In short, get a proper
    Variac. You know my motto: "You can never have too many Variacs!". :)

    --- sam | Sci.Electronics.Repair FAQ Home Page:
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    Important: Anything sent to the email address in the message header is ignored.
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  16. James Sweet

    James Sweet Guest

    This won't work, a light dimmer doesn't vary the voltage, it varies the duty
    cycle, so if you plug a switching supply into one you'll fry it.
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