Simple variable AC supply?

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.



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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.

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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).


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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.

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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".

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

Reply to
Don Lancaster

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...


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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, appropriate?

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) important?

Thanks, Sparky

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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 amplifier..

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 isolation).

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.

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The load he wants to use it with?

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Michael A. Terrell

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

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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.

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Guy Macon

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

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The most simple method is to *use* a variac...

Reply to
Robert Baer

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.

-- bud--

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On Sun, 22 Jul 2007 07:28:29 -0700 in, Don Lancaster wrote,

Have you ever heard of a sine wave?

Reply to
David Harmon

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