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Simple variable AC supply?

Discussion in 'Electronic Design' started by SparkyGuy, Jul 22, 2007.

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  1. SparkyGuy

    SparkyGuy Guest

    Just for grins, I'd like to build a simple variable-voltage AC supply, an
    electronic Variac. I want variable p-p voltage control, not variable
    time-domain control.


  2. Palindrome

    Palindrome Guest

    Do you have similar longings to construct an electronic hammer for
    opening peanuts?

    You possibly already have the makings of such a system, in the shape
    of a high power audio amplifier. You would need to add a mains "audio
    input" signal and a mains transformer to bring the amplifier output up
    to the required maximum output level. Adjusting the volume control will
    give the required output voltage setting.

    Of course, if you want to design and build your own audio amplifier
    (sorry, electronic variac) -feel free.
  3. SparkyGuy

    SparkyGuy Guest

    You possibly already have the makings of such a system, in the shape
    I have all those. Hadn't considered using them for this application... But
    then that's the difference between us (c;

    What would you suggest for the output transformer? What primary and secondary
    specs should I look for to best match the speaker-load outputs to AC
    power-inputs (typically less than 50 vac and a few hundred miliamps).

  4. Palindrome

    Palindrome Guest

    You know the rated maximum rms output power for a particular impedance
    load for your amplifier, so you can work out what the rated maximum rms
    output voltage is...

    You know the maximum rms output voltage you want.

    So you can work out what transformer you need.

    Let's say your amp gives 20v rms out at full power. You want 240v rms.
    So you use a 240 <> 20 volt mains transformer, connected the "wrong way"
    around - with a VA rating equal to the amp power maximum, or determined
    by the load, if less.

    (BTW I'm not sure what you last words meant.. Do you really only need
    50v rms at a few hundred mA, or is your amp only capable of giving 50v
    rms at a few hundred mA - and you want more voltage?)

    Which gives a clue that a mains transformer may not always be needed. If
    your amp produces 50v rms and you only need 50v rms, then you can
    possibly omit a transformer entirely - depending on isolation and
    earthing requirements.
  5. Guest

    Guest Guest

    Oh yeah!

    Years ago my then father-in-law wanted to "play" with servos and I picked up
    an amplifier from Lafayette (now out of business.) I think it was rated
    for 70 watts and I "tested" it by connecting a bell transformer to the input
    and put a 60 watt lamp on the output. You need am amplifier that has high
    Z outputs. I don't know the details, but rather than saying, 8 ohms, 4
    ohm, 16 ohms, it will saying something like "70 volts".
  6. And the problem with a $4 triac light dimmer is...?

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  7. SparkyGuy

    SparkyGuy Guest

    And the problem with a $4 triac light dimmer is...?

    Good only for non-inductive loads? Choppy, non-sine signal may not be good
    for a particular circuit? RF generated by the dimmer could wreak havoc with
    the DUT? Just to mention a few...

  8. SparkyGuy

    SparkyGuy Guest

    Thanks for the help.
    So for a general-use, a 120 v (I'm in the USA) <> 50 v (the amp is rated 200
    watts into 4 ohms) transformer rated for 200 VA would be, roughly,
    I have regular need to power some PCBs from German industrial equipment that
    uses 34 vac as power. Some stuff requires up to 50 vac. Don't usually need to
    provide variable full mains voltage.
    The "audio amplifier as mains power source" is new to my head, so... Is
    impedance matching (for the sake of preserving the amplifier's output stages)

  9. Palindrome

    Palindrome Guest

    If it is rated at 200w rms - then its rms output voltage (maximum) is
    28v. So a 120v 200VA transformer with a 50v secondary would give you
    0-60v ac (approx).
    See above. use a 120v <>50v 200VA transformer. Or use a more powerful

    eg a 1000W rms amplifier into 4 ohms gives over 60v rms at full output.
    No need to pratt around with transformers (other than 1:1 if needed for

    Nope. The output impedance of an audio *power* amplifier is way, way
    lower than the load impedance at all times in normal operation. But I
    would suggest that you use an amp that is protected against speaker
    cable shorts and thermal overload - especially if your power
    requirements are anywhere approaching its rms full output rating.

    You do need to make sure that the amp is capable of sustained running at
    the load level you require.
  10. D from BC

    D from BC Guest

    A generator.
    D from BC

  11. The load he wants to use it with?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  12. amdx

    amdx Guest

    Hi SparkyGuy,
    I posted my first thoughts about how to implement an electronic variable AC
    See alt.binaires.schematics.electronic
  13. Guy Macon

    Guy Macon Guest

    Ignoring for the moment the issue of damage to the amplifier,
    Consider the cases of zero ohms and infinity ohms. If your load
    is zero Ohms (dead short) you will get maximum current through
    the load, but no voltage across it. If your load is infinity
    Ohms (open circuit) you will get maximum voltage accross the
    load but no current through it. Power is voltage times current,
    so both of those cases will give you zero watts of output power.
    Somewhere in between is a load that maximizes power. This is
    usually somewhere close to the lowest ohms speaker that the
    amplifier is rated to drive.

    Keep in mind that audio is "peaky": it spends most of the time
    at a lower-than-peak power level. Your steady signal will
    generate more heat, so be sure to monitor the amplifier
    temperature as you slowly raise the output level.
  14. Palindrome

    Palindrome Guest

    Very definately not for an audio power amplifier. The output impedance
    is very, very low - much much lower than the load impedance will ever
    be. It is basically a constant voltage supply.

    Easily proven, if you have an amplifier running at full power output
    into its designed load impedance and measure the output voltage, then
    double the load impedance, the output *voltage* will hardly alter.

    The process you are describing above is that of matching a load to the
    output impedance of a power source. Do that for an audio power amplifier
    and the result would be a tiny fraction of an ohm.

    The starting point for a power amplifier design is the design output
    power and the design load impedance. From this comes the design output
    voltage swing.

    Or put otherwise, a 1000W rms power amplifier may only be a 200W rms
    power amplifier with a 1000W rms transient capability. Very true. But
    that's marketing for you.
  15. MooseFET

    MooseFET Guest

    There have been many suggestions but I don;t think this idea was one
    of them:

    With an array of transformers with each one having three times the
    turns ratio of the other, you don't need very many steps to cover a
    wide range. The secondaries can be wired in series and the primaries
    connected up by means of triacs. The triacs either wire them as
    adding, shorted or subtracting.

    Starting with a 1V secondary, you need 1, 3, 9, 27, and 81 to get
    zero to 121Vac.
    That would be 5 transformers.

    Since transformers are cheaper than semiconductors, you could use
    transformers with split primaries to get more options per core. You
    could have parallel subtracting, series subtracting, shorted, series
    adding, and parallel adding. This increases the base to 5 so you only
    need 3 transformers to get from 2V to 120V.

    0, 2, 4 < - 0ne transformer

    10-4 .. 10+4 < - Two
    20-4 .. 20+4

    50-20-4 .. 50+20+4 <- Three
    100-20-4 .. 100+20+4
  16. Guy Macon

    Guy Macon Guest

    No it isn't. The only "process" I described is matching the load
    to the rated load found in the specifications. The rest was an
    explanation as to why the impedance matters.
    Again, my suggestion was to match the load to the lowest
    impedance rated load found in the specifications. You are
    arguing against something I never wrote.

    I didn't want to confuse SparkyGuy with details that won't help
    him/her, but for the record:

    Most power amplifiers use negative feedback to obtain an output
    impedance near zero. This confuses some technicians who had
    previously learned to match the load to the output impedance on
    the old tube amps. With a an output impedance near zero, the
    voltage doesn't change much as you change the load, and thus a
    naive technician might calculate that the lower the impedance
    of the load, the higher the current. That is, until he tries to
    calculate a fixed voltage driving a dead short and gets infinite
    power as the answer... In real life, the amplifier will have a
    maximum output current, and if you make the impedance of the
    load too low the amplifier output will most definitely *not*
    remain a constant voltage with a near-zero output impedance.
    It may shut down or catch on fire, or it may clip, or it may
    kick in a limiter (many Carvin amps do this). But one way or
    the other the output voltage *will* drop. So, how to avoid
    hitting the current limit? Same advice as before: match the
    load to the lowest impedance rated load found in the

    If you reply, please check your post to see whether I actually
    wrote what you are disagreeing with. Thanks!
  17. Robert Baer

    Robert Baer Guest

    The most simple method is to *use* a variac...
  18. bud--

    bud-- Guest

    There are also amps, as used for US public address systems, that are
    "voltage matched" instead of "impedance matched". Nominal voltages are
    25 and 70. May be simpler.

    For the use stated, a small Variac would be a lot cheaper.
  19. David Harmon

    David Harmon Guest

    On Sun, 22 Jul 2007 07:28:29 -0700 in, Don
    Have you ever heard of a sine wave?
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