Originally Posted by Kirch3333
There was a post about this earlier in the thread (or maybe it was another stir plate thread). If you have a wire connected to the center post of the rheostat it will apparently cause this. You only need connections on the two outside posts.
I am not an electrician, but this is incorrect. Connecting potentiometer using 2 outside posts means removing adjustable resistance and running with full resistance this potentiometer is rated at.
The problem may be with your potentiometer Amp rating - yours may be too low for the job so check that first - at 12VDC and average fan consumption you probably want at least 2A rating and preferably 5A (but those are pricy) to avoid heat build up and premature failure.
Having said that, this simplistic design with using potentiometer to adjust power while works OK for very low powered applications
, has a major flow since potentiometer has to absorb any energy it redirects/adjusts and transform it into heat - and that's exactly what u are seeing.
In real world applications you would want to use a semiconductor e.g. transistor, teristor etc. in tandem with potentiometer where potentiometer handles small amt of Amps (low powered circuit) and therefore can have much lower Amp rating and semiconductor handles high powered circuit (fan).
And after typing all this, I find electric diagram I was describing right here on this board!http://www.homebrewtalk.com/f51/new-stir-plate-115080/
That LM317T thingy in that diagram is what controls high-powered fan circuit and potentiometer Amp rating can now be much lower.
I'd add a heat sink to LM317t in that application.
And since we are on this topic, but I did not see anyone mention it (ok, I did not read this entire epic thread!), I'd add one more bit to stir plate design in order to let the fan expel air from enclosed box. Alternatively cut fan blades off.
This would allow for a couple of things:
1. reduce fan power consumption
2. reduce heat generated inside the enclosure and increase lifespan of a fan and components
2. allow fan to spin faster since now it is pushing free air as opposed to dead air mass trapped inside enclosure.
The way to achieve air circulation would be by
- cutting out an opening by tracing fan diameter in a top cover of enclosure,
then adding a new top plate (plexiglass with neon lights?
on stand offs just above the fan to allow air to escape to the sides of the fan
- alternatively drilling a few exhaust holes right above the fan, but then you'd need to make sure those are not blocked by the flask sitting on top of them
- adding air intake hole(s) to the perimeter sides of enclosure
Now I am all fired up about building my own stir plate - was putting it off for a while but with my increased mash efficiency (around 85%) I am bound to do this sooner rather than later.