[sdiy] 12VDC to 12VAC converter solution...

[Seb Francis]

Hi,

Thanks everyone for your help with this, I've now got a good solution 
working ...
http://burnit.co.uk/sdiy/stuff/shower.png

Went for the MOSFET H-bridge in the end (L6203) which gives a 12VAC RMS 
square wave output.
The motor (a shower fan) seems to be quite happy running on a square 
wave: it doesn't seem particularly noisy (no more than one would expect 
for a fairly powerful domestic shower fan) and the motor's not getting hot.

I'm glad I didn't go for the audio power amp solution as even the very 
efficient L6203 is getting quite hot driving this fan (it needs a decent 
heatsink to be comfortable to touch).

Note that there's no reason I'm using a 556 as opposed to a 555 other 
than the fact that I blew up my only 7555 and had to switch to using an 
NE556.  In fact the 7555 blew up immediately on connecting the fan so I 
guess the delicate CMOS architecture didn't like all those nasty voltage 
spikes from the motor on the power rails!  The 556 seems quite happy .. 
had it running continuously for about 30 minutes now.

Now I just have to add the humidity sensing circuitry .. should be quite 
easy as I already have 50Hz AC to run through the resistive humidity 
sensor, and the ENABLE input of the L6203 can be used to switch the fan 
on and off.  So just the addition of a couple of opamps should do it .. 
1 to rectify and 1 to compare the current through the sensor .

Seb