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 Post subject: Stir plate power supply
PostPosted: Mon Apr 06, 2009 6:21 pm 
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Location: Stillwater
I don't have any use for a stir plate but I might as well make one because I have most of the parts and nothing else to make... Plus it's just plain fun to make tornadoes in water. I picked up a 24v 4.7W fan at ax-man, and I have a power supply from one of my lasers that should work if I just change the value of one of the resistors. Can someone tell me if this looks ok, I see that radioshack has a 25.3v 430ma transformer for 6 bucks.

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yey crappy drawings!

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 Post subject: Re: Stir plate power supply
PostPosted: Mon Apr 06, 2009 6:32 pm 
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Doh I forgot a bridge. Anything else missing?

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 Post subject: Re: Stir plate power supply
PostPosted: Tue Apr 07, 2009 9:04 am 
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i have some serious concerns about the nema 5-20 (or 5-15) you're using. it looks like one of the prongs is bent.


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 Post subject: Re: Stir plate power supply
PostPosted: Tue Apr 07, 2009 1:08 pm 
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It'll work, once you put in the aforementioned rectifier. I'd axe the LM317, though- for something like a fan, rectification + a filter cap will do just fine.

For that matter, why mess with making a supply at all? I'd expect Axman to have a 24V supply of some sort that'll cost you less than $7-$8 (which is what the Radio Shack solution will cost, once you add in the rectification circuit and a nice fat power cap). I could be wrong on that, though- 24V is less common as a DC voltage.

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 Post subject: Re: Stir plate power supply
PostPosted: Tue Apr 07, 2009 3:33 pm 
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The 317 is set up to regulate current not voltage, at least I hope it still is, I haven't checked with a meter yet. So I can vary the speed of the fan.

Since it's changing the current, and hopefully always staying at 24v I'm assuming that this is the best way to control a 24v motor? Or is it better to have a constant current and ramp up to 24v? Anyways, the fan won't start spinning if my 25ohm rheostat is too low, I'm assuming that's bad as it would be heating up the coils in the motor?

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 Post subject: Re: Stir plate power supply
PostPosted: Wed Apr 08, 2009 8:57 am 
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I missed the pot.

As far as I know, the fan is going to look pretty ohmic, so you're going to be limiting current by drooping the voltage, right? Bear in mind that that will reduce the torque- fast and by a lot. And you're right, if the motor stalls, your current will be high- possibly high enough to damage the motor.

A better fit for speed control would be some form of PWM. That way, your current stays high, so your torque doesn't drop off, but the torque is being applied only sporadically.

If you want, I can give you a PWM controller IC and a power FET that'll get you where you are trying to be- a knob which will control the speed of your stirring motor.

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 Post subject: Re: Stir plate power supply
PostPosted: Wed Apr 08, 2009 12:19 pm 
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Hmm. I don't think my power supply is the same as what's in the picture I posted. It used to power my laser diode, which should have stayed at the same voltage and only increased the current. I swapped r2 for a value so that it would produce up to 24v instead of 6. Last night I tested the current draw and it goes from 30ma to 200+ beyond the range of my cheap meter. Surprisingly even when the motor was stalled it was only drawing 30-35ma.

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 Post subject: Re: Stir plate power supply
PostPosted: Wed Apr 08, 2009 12:30 pm 
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Here is the actual 317 circuit I'm using
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 Post subject: Re: Stir plate power supply
PostPosted: Wed Apr 08, 2009 10:46 pm 
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It lives!!! and it may be living a little too much. I think this fan is more than just the little bit of overkill that everyone saw tonight. With Mike's new circuit it's spinning maybe 3-4 times as fast.

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 Post subject: Re: Stir plate power supply
PostPosted: Wed Apr 08, 2009 10:50 pm 
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Location: Eagan, MN
Holy crap! You best pulse wave modulate the snot outta that sucka!

(Sorry, was just on HomestarRunner.com. I think I best go the snot to bed.)

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