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Home > Archive > Electrical Engineering > July 2007 > Simple variable AC supply?
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Simple variable AC supply?
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| SparkyGuy 2007-07-22, 9:25 am |
| 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.
Suggestions?
Sparky
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| Palindrome 2007-07-22, 9:25 am |
| SparkyGuy wrote:
> 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.
>
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.
--
Sue
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| SparkyGuy 2007-07-22, 9:25 am |
| > 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.
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).
Thanks,
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| Palindrome 2007-07-22, 9:25 am |
| SparkyGuy wrote:
>
>
> 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).
>
> Thanks,
>
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.
--
Sue
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| Don Lancaster 2007-07-22, 1:25 pm |
| SparkyGuy wrote:
> 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.
>
> Suggestions?
>
> Sparky
>
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: http://www.tinaja.com/whtnu.xml email: don@tinaja.com
Please visit my GURU's LAIR web site at http://www.tinaja.com
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| SparkyGuy 2007-07-22, 1:25 pm |
| > 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...
Sparky
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| SparkyGuy 2007-07-22, 1:25 pm |
| Thanks for the help.
> 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.
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?
> (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?)
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.
> 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.
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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| Palindrome 2007-07-22, 1:25 pm |
| SparkyGuy wrote:
> 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?
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).
>
>
>
>
> 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.
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).
>
>
>
>
> 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?
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.
--
Sue
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| Michael A. Terrell 2007-07-22, 1:25 pm |
| Don Lancaster wrote:
>
> SparkyGuy wrote:
>
> And the problem with a $4 triac light dimmer is...?
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
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"SparkyGuy" <SparkyGuy@mumcrank.ck> wrote in message
news:0001HW.C2C8691E00427F52F01826C8@news.sf.sbcglobal.net...
> 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.
>
> Suggestions?
>
> Sparky
>
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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>
> 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.
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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| Guy Macon 2007-07-22, 5:25 pm |
|
SparkyGuy wrote:
>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?
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.
--
Guy Macon
<http://www.guymacon.com/>
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| MooseFET 2007-07-22, 5:25 pm |
| On Jul 22, 1:53 am, SparkyGuy <Sparky...@mumcrank.ck> wrote:
> 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.
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
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| Robert Baer 2007-07-23, 3:25 am |
| SparkyGuy wrote:
> 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.
>
> Suggestions?
>
> Sparky
>
The most simple method is to *use* a variac...
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| nni/gilmer@nni.com wrote:
>
> 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".
>
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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| David Harmon 2007-07-25, 3:25 am |
| On Sun, 22 Jul 2007 07:28:29 -0700 in sci.electronics.design, Don
Lancaster <don@tinaja.com> wrote,
>And the problem with a $4 triac light dimmer is...?
Have you ever heard of a sine wave?
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