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

Seb Francis seb at burnit.co.uk
Wed Sep 12 20:24:31 CEST 2007

P.S. This is what it looks like all finished:
The final schemo (with humidity sensing control):

Now there's no chance my wife will forget to turn the fan on and steam 
the rest of the vehicle from the shower!

Seb Francis wrote:
> 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
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