Do you have similar longings to construct an electronic hammer for
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.
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).
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
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.
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.
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".
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.
There have been many suggestions but I don;t think this idea was one
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
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