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

[Steve Lenham]

> I need to produce a 12VAC 20VA 50Hz supply from a 12VDC input.  It's to 
> run a 12VAC motor from a car battery, so the AC voltage doesn't have to be 
> particularly sine-like or regulated.
>
> Any ideas of a simple circuit or IC that will do this with reasonable 
> efficiency?  I thought about a 555 timer with power transistor to boost 
> the output but not sure a square wave will be good for the motor ...

Hi Seb,

One suggestion to add to those already given:

- Use your 555 to generate a 50Hz squarewave.

- Brutally lowpass filter it, either with cascaded passive RCs or something 
more sophisticated.

- Use the resulting pseudo-sinewave to drive a bridge-tied-load audio power 
amplifier like the TDA2005 running from the 12VDC supply. Connect the motor 
across the outputs.

Advantages are that the power amp is cheap, easy to find, current limited, 
thermally protected and has plenty of drive capability. Disadvantage is 
that, as already noted, with just 12Vdc available the sinewave will be only 
around 20V peak-to-peak (~7.5Vac), but that is true for any solution not 
using either a boost converter or a step-up transformer. Omitting the 
filtering and using the squarewave directly will increase the voltage at the 
expense of waveshape.

In general, audio power amps can be quite useful in other situations where 
an unsophisticated but well-protected power driver is required.

Cheers,

Steve L.