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Old 07-12-2010, 10:54 PM   #1
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Default LM317T vs rheostat....stir plate build (cheap!)

looking to build a stir plate and i see people use either a rheostat or the LM317T fan controller. It seems people say the LM317T is better but is it worth the extra cost and hassel? My next question is if the LM317T is worth it www.sritrplates.com has a DIY kit which has everything minus the fan box and magnet for $20 shipped. Price wise it seems like this may be very close to the price to buy the stuff individually. would you agree or is there cheaper places to but the parts?



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Old 07-12-2010, 11:53 PM   #2
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you can use a cheep pot to control the 317 that will put out an amp or more depending on voltage in - voltage out, and heat sinking. around the south san francisco bay ther are several e-junk stores to get stuff cheap, that would be the route I would go. (lm317 is a voltage regulator, not a fan controller but that is a rant for another time)
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Old 07-13-2010, 12:04 AM   #3
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The only real difference is that the LM317 is designed to safely dissipate the heat and a cheap potentiometer is not. (Although as lincoln suggested, if you use a 12 V supply you're right on the edge of needed a heat sink.) Get a pot that's rated for 3 W or so and there's no real difference - the LM317 will probably give you slightly better regulation, but for this application, who cares?

$20 sounds like a ripoff, but I guess compared to what you'd pay to buy components individually at Radio Shack it probably isn't. You can probably do better buying the stuff online, so it comes down to a question of how much your time is worth. I put together a quick build writeup when I made mine.

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Old 07-13-2010, 02:00 PM   #4
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so i got an old 5.7V phone charger from the GF. Would it be just as good to hook that up straight to a toggle switch and then to a 3A rheostat and be done with it? It would cost be about $6.

a10t2 - you said the potentiometer would give be just as good but yet on your build you went with the LM317 and heat sink which cost about $24 without the stir bar. Was it worth it or would going the cheap easy route produced the same results

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Old 07-13-2010, 02:51 PM   #5
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so i got an old 5.7V phone charger from the GF. Would it be just as good to hook that up straight to a toggle switch and then to a 3A rheostat and be done with it? It would cost be about $6.
that will work fine,

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a10t2 - you said the potentiometer would give be just as good but yet on your build you went with the LM317 and heat sink which cost about $24 without the stir bar. Was it worth it or would going the cheap easy route produced the same results
it really depends on how big your fan is; you also don't have to buy a ready made heat sink, any piece of plate aluminum will work to some degree, or if using a metal box just use that.

if you turn the thing on and the 317 gets hot after 20 min to an hour you need more heat sink / airflow, if it just gets warm no worries.
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Old 07-13-2010, 06:10 PM   #6
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a10t2 - you said the potentiometer would give be just as good but yet on your build you went with the LM317 and heat sink which cost about $24 without the stir bar. Was it worth it or would going the cheap easy route produced the same results
Using an LM317 would give you better regulation, but for this application a potentiometer is probably fine. I just had all the parts gathering dust in a drawer - it wasn't going to cost me more than a few dollars either way. Not sure where you're getting $24 though. The only kit I see on that site is $14.95.

Also, 5.7 V may not be enough. I have to run mine at about 9 V to get good circulation in a gallon starter. Might as well try it and see.
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Old 07-13-2010, 06:48 PM   #7
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(lm317 is a voltage regulator, not a fan controller but that is a rant for another time)
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I assume you want to use a 555? I thought about building the controller that way, but the 317 was just too simple and cheap.

Using a big rheostat is kind of the brute force way of doing it. It works, but it ain't pretty. Using a 317 gives you more direct control, as you can set it up for whatever voltage range you want, rather than depending on the load from the fan, which will vary.

As for price, it can be done really cheap. I keep a bunch of 317s around, they were <$1 each. Add in some caps and some resistors at pennies each, and a scavenged pot, the entire controller didn't cost me more than a couple of bucks.
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Old 07-13-2010, 07:26 PM   #8
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I assume you want to use a 555? I thought about building the controller that way, but the 317 was just too simple and cheap.
PWM would definitely be the elegant way to do it. I just didn't want to run a 250 mA fan with a 3904, and didn't have any FETs handy.
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Old 07-13-2010, 07:38 PM   #9
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PWM would definitely be the elegant way to do it. I just didn't want to run a 250 mA fan with a 3904, and didn't have any FETs handy.
I'm using a 19V power supply, so I was going to have to do some sort of voltage regulation anyway. Might as well just use the 317.

I do have a question, though. Being that the fan is an inductive load, I used a snubber diode. I see that you didn't mention it on your site. Is it not needed?
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Old 07-13-2010, 08:36 PM   #10
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I do have a question, though. Being that the fan is an inductive load, I used a snubber diode. I see that you didn't mention it on your site. Is it not needed?
It certainly couldn't hurt, I just didn't think of it. IIRC the data sheet for the 317 mentions that it's optional for low-power applications.

Actually, the fan motor is going to take a couple seconds to stop spinning once the current is removed, so wouldn't the back voltage be much less than in something like a solenoid where the cutout is nearly instantaneous? If that's the case and you're using an ordinary silicon diode with a ~2V drop, it might not even help.


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