[sdiy] 12VDC to 12VAC converter [OT]

Scott Gravenhorst music.maker at gte.net
Tue Sep 4 00:00:51 CEST 2007

Seb Francis <seb at burnit.co.uk> wrote:
>Hi Dave,
>Dave Magnuson wrote:
>> with the 555 timer you can get a triangle-like wave from the timing 
>> cap. I've seen an LFO that did this with an opamp buffer... it would 
>> probably be a little more friendly than the square wave output.
>Yeah I thought about this, but I need 20VA output power so no chance to 
>get that from an opamp.  I was also thinking I could buffer the square 
>wave output from a 555 with a transistor then pass through a simple RC 
>low pass filter (with small R and big C) to get a more rounded wave, but 
>in the end this isn't going to give me AC, just varying DC.

Years ago, I needed a low cost motor speed control for an AC motor.  I found a circuit
like this:

              |        |
            winding    |
              |        |
           +  |      power
       elec  ---    resistor
       cap   ---       |
              |        |
  --/\/\/--|   NPN power transistor

I believe I drove the power transistor base from a 555 astable at 50% duty cycle. 
This circuit can work for low power motors, I was using a phonograph motor.  In my
case, V+ was 160V because the motor was 120vac motor.  I would guess that you would
need 16 VDC for V+ to get a true 12 VAC output.  Running this from a car battery might
produce a voltage on the low side, but it may work depending on what the motor has to do.

The is some power waste though.  When the power transistor conducts, the motor winding
gets current by charging the electrolytic capacitor (I think mine was in the 100 uF
range).  During this part of the cycle, the resistor (which is a power resistor of
some tens of ohms I think) is wasting power, but when the transistor turns off, the
resistor serves as a path through which the capacitor can discharge, again through the
motor winding but this time in the opposite direction.

So this circuit will give you AC at the motor winding.  Again, it's not meant for
large currents, but it might work for you.  The actual values for the components would
need to be calculated for you need.  In your case, you don't care about varying the
frequency, so a fixed or limited range 555 oscillator should work.

For less power waste, I think you need a push-pull inverter.

>I also looked at using some switch mode PSU IC like the SG3525 but they 
>seem to operate at too high frequencies.
>This is the most hopeful thing I've come across:
>Looks like it could be used with a 1:1 transformer to give 12VAC out, 
>but not sure how it will behave driving an inductive load like a motor...
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>Synth-diy at dropmix.xs4all.nl

-- ScottG


-- Scott Gravenhorst
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