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

[Seb Francis]

Steve Lenham wrote:
> - 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.
>

Hi Steve,

Thanks for your idea.  Someone else also mentioned (off-list) modifying 
an old car power amp and feeding with 50Hz input (or possibly some music 
with plenty of bass :)

My main reason not to want to do this is efficiency - a class AB or 
class B (as the TDA2005 is) design isn't going to be anywhere near the 
efficiency of a MOSFET H-Bridge motor driver IC (which also have the 
advantages of built in protection, etc.).  Apart from the fact that I 
don't want to waste battery power, I want to mount the electronics in a 
confined space with little room for heat dissipation.

Of course with an H-Bridge the output is going to be square, but as you 
say this will increase the RMS voltage which will be a good thing.

The motor in question is a shaded-pole type and I've read they will run 
ok from a square wave but be more noisy.  So I'm going to experiment 
with narrowing the positive and negative pulses of the square wave (i.e. 
introduce some dead time when the voltage is 0).  This is basically what 
a modified sinewave output from a cheap inverter looks like, although 
I'm going to try wider pulses to keep the RMS voltage up.

I also need the fan to be turned on and off by humidity level and the 
sensor needs an AC voltage so I will be able to make a second use of the 
50Hz signal.

Anyway, I've probably taken up enough SDIY bandwidth with this topic so 
thank you everyone who replied!  I'll post my final circuit when it's 
done and tested in case it's of interest to anyone.

Cheers,
Seb