Big, Fat Electronics question for all y'all.
 

Here's my latest question, it's an electronics question. Not my strong suit, I could hold my own back in junior high school, but I think that had rather more to do with me being put in a class with riders of the short-ish, if not completely truncated, bus.
Zippy was onto something when he sussed out my yeast ranching ambitions. I've been trying to build a DIY magnetic plate stirrer for yeast culturing. It seems that every dick and his dog has made one that works and then put up a youtube video of it stirring smugly.

I've got a couple of DC fans from inside a junked 'puter.
1) a 12V 0.4amp
2) a 12V 0.3amp

I also got a 10k Ohm audio taper pot, 250V DC .25 Watt

When I powered it up with my 12V .3amp power supply the pot started smoking. What's up with that?

I thought I had all my shit in one sock, but it seems like I didn't.

It's a three terminal pot and I had it wired correctly: line-in load-out.

Any electronic adepts out there? Any magnetic plate stirrers at Ye Olde Pawne Shoppe?

C'mon! No electronics love for the ol' footster?

It's a cold, cold world.

http://dallas.craigslist.org/ndf/ele/3635696392.html

85 bucks

several more like it on craigslist

Yeah, that's $95 more than I have right now.

Is this your payback for my unhelpfulness with your cable problem?
:p:

Remember I still have to buy a nanolariat and those you can't DIY.

no.... I'd forgotten that. Maybe I'll make one and send it to you with your other unshipped goods.

Can I ship you some flint (not you, flint) so you can ship it to foot?

I have quite a stack of outgoing goods myself.

The rating of the wall wart is probably nominal; there's little to say it can't give you more current if you didn't ask for it. The smoke means you were turning the potentiometer into a small heating element, which would make sense if say suddenly you were running several amps through it. A quarter watt is not much at all (12V at .3A is 3.6 watts.)

I always find DC sort of boggles the mind.

But it seems like for your purposes you could set aside the 12V wall wart and the (now-smoke-faerie-free) potentiometer and just cut the plug off the charger for a cell phone that you don't have anymore. Probably 5V. Probably spins the fan substantially slower, maybe slow enough (might overheat? who knows). Or run the two fans in series. As far as I can see the premise of a DIY yeast-stirring contraption is to just make whatever and call it good enough.

ETA: A really handy source for low-RPM cheapo motors is thrift-store citrus juicers. They come with gearing to get it down in the 10RPM range (grandma doesn't want her OJ on the wall, after all.)

Ya what he said ,
Oh and use This circuit ;)

Thanks gvidas. I meant to say I had the option of either of those fans, not both at the same time. I'm going to retrace my steps. I've powered it the way you suggest.

Thanks for the diagram Zip. I'm gonna print it out and hang it over my bench!

That schematic is also a layout. It shows where every wire is located.

However missing the is the hook to hang it from the ceiling. 3M makes a excellent solution that best attached to a battery. Then the entire circuit can spin even in the wind.

I printed that out for a guy at the shop , he started pointing out the flaws right away , i just looked at him , same dude I had try to put this back togather
http://farm9.staticflickr.com/8011/7...7cc68678_n.jpg
Photo258 by zippyt, on Flickr

Same concept applies to light bulbs. A 100 watt bulb (on 120 volts) should draw 0.83 amps (W = VI). Reality is much different. That bulb will probably first demand 8.0 amps. Most don't know that since numbers provided for light bulbs forget to mention it.

Your motor is similar. It demands more power (current) to startup.

Apparently your intent is to make a speed controller. So get a 12 volt supply (wall wart) with more like one amp. Or do something more interesting. A maybe 15 or 18 volt supply (wall wart) powered through a transistor. That transistor driven (controlled) by a 555 timer.

A 555 timer is one of the world's most famous integrated circuits (created by Signetics). Designed more than 40 years ago. And still sold even in Radio Shack.

A 555 pulses the transistor. The transistor then pulses a motor. Longer pulses cause the motor to spin faster. Pulses (not continuous) power means 15 or 20 volts cause no motor damage. And (depending on the motor) can even make startup easier without smoke.

I'm guessing the pot is rated up to 250 V-DC and should work on less. IMHO devices only draw the amps that they need, if so, I wouldn't worry about that aspect.
But I have not a clue about a yeast stirrer. A cheap batterer charger might work.

I ignored numbers for that audio tapered pot. 10k and 0.25 watts to drive a 12 volt 0.3 amp fan? Let's say the pot reduces 12 volts to 11. Then the pot must be turned 0.1 degrees. Or 0.0003 of a turn. That cannot happen.

Suppose the pot is turned a tiny bit more. At 0.1% of a full circle turn, it is now heating to well over its .25 watt limit (almost 1 watt).

An adjustable pot must be in the tens of ohms range (not xK ohm range) to control a 0.3 amp fan. And somewhere at or above 4 watts so that it does not burn.

Relevant equations are V=IR (R=pot resistance, I=fan current, V=number of volts below 12).
And W=VI (those same V and I. And W= watts heating the pot).