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Want Adjustable AC Voltage Circuit

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elektrodude

New Member
:)Hi All

I'm newcomer here and I want to make the circuit that can adjust the stepped down 12VAC like DC adjustable voltage circuit by using LM317, I neither want a variac nor transformer secondry tapings...PLz help me to do that ,note that Im using step down transformer for getting 12VAC from main with 1 or 0.5 Amp current....

Thaking you all in anticipation...

[email protected]
 

crutschow

Well-Known Member
Most Helpful Member
Don't think there's anyway you can do variable AC power from a transformer without using a variable transformer.
 

tcmtech

Banned
Most Helpful Member
two LM317's back to back will do it. Each covers one half of the sine wave. Just make sure you have blocking diodes and reverse diodes to let the current flow in the right one at the right time. But then jump over it on the reverse side of the cycle.
 

crutschow

Well-Known Member
Most Helpful Member
two LM317's back to back will do it. Each covers one half of the sine wave. Just make sure you have blocking diodes and reverse diodes to let the current flow in the right one at the right time. But then jump over it on the reverse side of the cycle.
That will just clip the sinewave giving many harmonics. Not sure if that's acceptable for his appplication.

If so then maybe an SCR type light dimmer circuit would also work.
 

Hero999

Banned
Sorry but a variac is the only sensible way to do it - just connect a 12V mains transformer to the secondary.
 

dougy83

Well-Known Member
Simple voltage divider buffer

What about dividing the voltage using a pot then buffering it. I've attached an example schematic, if you have any questions please ask.

Note that the maximum output voltage will be less than the input voltage due to the voltage drop of the diodes, the transistors and the output voltage swing of the opamp. The input voltage may be increased to overcome this, however watch out for the maximum voltage specs of the opamp (and other components, of course). Transistors may need to be darlington (depending on your choice of opamp) and will need heatsinking.

-Doug
 

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medelec35

New Member
Actually there is a good way of controlling AC Voltage.

If you connect an ac load to ac part of bridge rectifier.

Then if + and - of bridge is shorted together, then full ac waveform would appear across load.

If short is removed then no current will flow though load.

What you need to do is switch FET on at each zero crossing,then switch FET off after a period of time between 4 and 9 ms on each cycle.
Microcontrollers e.g. 8 pin 12F series are ideal for the job.

Since FET is connected to + and -, then it sees only DC.

Note: If using an inductive type load, then you must connect a suitable AC rated capacitor (e.g) type in some types of fluorescent lights, or AC motor absorption caps in parallel with the load. This type of circuit will work on mains, but FET and cap must be suitably rated. Most of all be careful! Mains can kill. You must be fully competent to work with mains if not get help form some one who is!
 

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Leftyretro

New Member
Well I too suggest that a Variac is the best way to go. However if you are set against it then you need a AC amplifier with adjustable gain being feed with a constant amplitude fixed 60hz oscillator stage.

A audio amplifier of suitable power rating would meet this requirement. It's volume control would set the output voltage desired.

Lefty
 

Externet

Active Member
Use a subwoofer audio amplifier if you are talking about 60Hz.

Feed a couple of volts of 60 Hz to its input trough a potentiometer that will set the gain to the speaker output level you want.
 

tcmtech

Banned
Most Helpful Member
Rube Goldburg smiles down upon you from heaven! :)

Variac it and be done! :p

The time spent researching this could have been spent working minimim wage at Mcdonalds then that money put towards buying a Variac on ebay for far less than new price. ;)

Just my thoughts.
 
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