LM317 Help

system · May 24, 2011, 4:51pm

Hello again

I will be making an Arduino fan controller which will regulate the voltage on the fan with the help of the LM317 Voltage Regulator, now I have made a schematic and I have a question:

The input voltage will be 12V from a PC PSU, will the LM317 be able to give 12V output?

Grumpy_Mike · May 24, 2011, 4:58pm

The input voltage will be 12V from a PC PSU, will the LM317 be able to give 12V output?

No.

You need a voltage greater than 12V say 14V to get 12V out. Also you have no input or output capacitors and have you allowed for the Vssat voltage between collector and emitter when the transistor is on?
You also need resistors in the base of the transistors to limit current flow.

retrolefty · May 24, 2011, 5:00pm

The input voltage will be 12V from a PC PSU, will the LM317 be able to give 12V output?

No, the chip has a regulator drop out voltage specification. It's dependent on how much current is being drawn from the regulator and is about 2 volts at 1 amp drop, so plan on around a 10vdc max output for the regulator if being supplied with +12vdc.

Also be sure to add series current limiting resistors going to the base of those switching transistors.

Lefty

system · May 24, 2011, 5:05pm

Thanks for the replies guys

Any idea on how I would get more than 12V with a PC PSU?

Grumpy_Mike · May 24, 2011, 5:09pm

I will be making an Arduino fan controller which will regulate the voltage on the fan

I am assuming the fan is 12V, the simple answer is not to use a regulator circuit like this but to use a FET driver in the continuous (non switched) mode.

system · May 24, 2011, 5:14pm

Any idea on how I would get more than 12V with a PC PSU?

I assume PC supplies still have -12V outputs.
-12 to +12 is 24 volts.

retrolefty · May 24, 2011, 5:23pm

AWOL:

Any idea on how I would get more than 12V with a PC PSU?

I assume PC supplies still have -12V outputs.
-12 to +12 is 24 volts.

Not really usable that way in a common grounded system.

Lefty

system · May 24, 2011, 5:26pm

Depends what you declare "ground" to be.

retrolefty · May 24, 2011, 5:29pm

AWOL:
Depends what you declare "ground" to be.

Correct, we would need to know if he is using the PC supply for any other loads, or desires to.

Lefty

system · May 24, 2011, 5:48pm

I'll be using the same PSU for: Pumps (Water Cooling), Micro Controllers, Fans

system · May 24, 2011, 7:25pm

Mike has the best solution; use a single power FET and control it with a PWM output from Arduino. And it (could) handle a lot more current than an LM317T.

Nothing against the LM317T. I once bought a barrel of 14,000 of them for $55 from a junkyard near IBM I'll send 5 to anyone who'll send a SASE... I still have about 500 left.

UPDATE: Sorry! I'm always criticizing Acronyms...

Self-Addressed-Stamped-Envelope (sent to me, will return 5 LM317t's..)

Terry King
152 Colby Road
West Topsham, VT 05086

But: 3 weeks till I get there from the Middle East...

system · May 25, 2011, 1:10pm

Can you give me more info on using FET's with Arduino? Schematic(s), Maybe even some example code

Grumpy_Mike · May 25, 2011, 1:48pm

Maybe even some example code

analogWrite(pin, value);

Schematic(s)

use the PWM output filtered and into the gate
http://www.thebox.myzen.co.uk/Tutorial/PWM.html

system · May 25, 2011, 2:21pm

So I use Frequency dependent potential divider with low pass filter connected to a FET, and the divider is hooked up to Arduino PWM output pin? And if so, which FET should I use?

Grumpy_Mike · May 25, 2011, 4:43pm

Frequency dependent potential divider with low pass filter

close:-
Frequency dependent potential divider AS low pass filter.

which FET should I use?

Any one that will take at least 200mA (most will) and is a logic level FET.
The FET will get hot because you are burning the excess voltage off inside it when it is not full on. The power dissipated will be with half the voltage dropped across it at what ever current the motor runs with only half the voltage across it. You might need a heat sink.

system · May 25, 2011, 5:48pm

@Grumpy_Mike

Thanks!

I hope I'm not too annoying with all these questions :P, maybe something like this? http://www.nteinc.com/specs/2900to2999/pdf/nte2987.pdf

Could you also tell me what exactly am I look for? N-Channel MOSFETS?

Grumpy_Mike · May 25, 2011, 6:11pm

maybe something like this?

Yes very meaty, that would do.

system · May 25, 2011, 6:42pm

Alright!

Now, if only I could find that in Eagle

retrolefty · May 25, 2011, 7:58pm

Rogy:
@Grumpy_Mike

Thanks!

I hope I'm not too annoying with all these questions :P, maybe something like this? http://www.nteinc.com/specs/2900to2999/pdf/nte2987.pdf

Could you also tell me what exactly am I look for? N-Channel MOSFETS?

Mouser shows that at $7 a pop: http://www.mouser.com/ProductDetail/NTE/NTE2987/?qs=sGAEpiMZZMutXGli8Ay4kFfMPLZ9xW8yKOlRS6m%252bwN0%3D

Sparkfun sells a logic level N-channel power mosfet for a buck: N-Channel MOSFET 60V 30A - COM-10213 - SparkFun Electronics

Lefty

Grumpy_Mike · May 25, 2011, 8:19pm

I said it was meaty.