Shop OBEX P1 Docs P2 Docs Learn Events
Lm317 — Parallax Forums

Lm317

piguy101piguy101 Posts: 248
edited 2011-06-20 16:04 in General Discussion
I found a LM317 regulator in an odd bag from Radio Shack in my house. I love the thing but it gets incredibly hot and I even have a heat sink on it. I want to know if the LM317 from Radio Shack has thermal overload protection and/or current overload protection. Or is it okay to let this IC run hot?
«1

Comments

  • LeonLeon Posts: 7,620
    edited 2011-06-20 11:19
    It shouldn't run hot unless it is delivering a lot of current and/or you have a high input voltage.

    What are the input and output voltages, and what is the output current?
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 11:21
    The input voltage is 31V and the output voltage is 12V @ 0.25A.
  • ercoerco Posts: 20,261
    edited 2011-06-20 11:21
    It supposedly has thermal overload protection, but it's always wise to add a heat sink if you know it's getting hot. It functions just like a resistor. It will get hotter as more current flows through it, and as it drops more voltage. That's not an efficient use of battery power especially. If possible, drop your supply voltage so that the 317 doesn't have to drop the voltage so much.
  • ercoerco Posts: 20,261
    edited 2011-06-20 11:24
    (31-12V=)19V x .25A=4.75 watts dissipated. That's a lot. Can you reduce your supply voltage?
  • LeonLeon Posts: 7,620
    edited 2011-06-20 11:25
    With 31V going into it I'm not surprised it's getting hot! The poor thing is dissipating about (31 - 12) * 0.25 W = 4.75W.

    You need a *much* larger heat sink, or forget about the LM317 and use a switcher.
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 11:26
    I can't change the supply voltage unless I get another power supply. Here's a thought: Could I use two LM317s? The first to drop the voltage somewhat and the second to drop the voltage to 12V and power the load?
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 11:29
    Here is a page that will explains the basics of power dissipation in a LM317 regulator and how to deal with it.


    edit: Yes, two in series (not parallel) would work It doesn't increase the amount of current you can pull, but it splits the power dissipation.
  • LeonLeon Posts: 7,620
    edited 2011-06-20 11:30
    Use a switcher, as I said previously. You won't even need a heat sink.
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 11:35
    Leon, could you please explain what a switcher is. (When I looked up "switcher" on google all I found was the switchers used in railroads. I also have no idea of what a switcher is.)
  • LeonLeon Posts: 7,620
    edited 2011-06-20 11:40
    http://www.national.com/en/power/simple_switcher/index.html
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 11:42
    If you Google "simple switcher" you will find National Semiconductor's site.

    Here is a video tutorial on the basics:

    EEVblog Linear and Switchmode Power Supply Tutorial
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-06-20 11:50
    Take a look at this regulator, for example:

    It's a pre-built module. You don't have to buy any discrete components. (Be sure to check the datasheet to make sure the specs meet your requirements.)

    -Phil
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 11:58
    I am watching the video posted by RDL2004 with the over enthusiastic but educating guy now. Phil Pilgram, I might try that item from Digikey since I have to buy there all the time. Unfortunately that regulator is rated at 6W and the final circuit would use just under 8W, but I can probably lower it to 6W.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-06-20 12:13
    Here's a bigger one that handles 12W:

    -Phil
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 12:23
    I actually would like a little greater than 12V so I am thinking about:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=102-1713-ND

    Would this product really work at 31V and 500 mA without any external heat sink?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-06-20 12:30
    Yes. It's 93% efficient at its maximum input of 32V, which means the dissipated power is negligible:
    Output = 7.5W, so Input = 7.5/.93 = 8.06W. Dissipated power is 8.06 - 7.5 = 0.56W.

    -Phil
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 12:33
    Well I think this thread is closed and solved. Thank you all. (Now if I can just figure out how to make the thread not say unsolved anymore...)
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 12:34
    It's rated for 32 volts, you are right on the edge. Is your 32 volts actually measured at the regulator input and is it clean DC?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-06-20 12:37
    Good question, Rick. I took the "31V" at face value, but maybe there's more to the story. In any event, I'd be reluctant to call things "solved" until the device was demonstrated to work in the OP's app.

    -Phil
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 12:39
    My power supply is a laptop power supply and it says 31V and I've measured it at 31V. I don't know if it is clean DC though. I could always take the easy way out and use a few diodes in series to lower the voltage a few volts.
  • ercoerco Posts: 20,261
    edited 2011-06-20 12:49
    Diodes also dissipate heat, just like resistors. If you use lot of those, you're right back to dissipating heat like a 317... :)
  • LeonLeon Posts: 7,620
    edited 2011-06-20 12:51
    Buy a different PS. They are quite cheap and you won't need the regulator.
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 12:52
    Will diodes make that much heat? The ones I use are fairly rugged at 1kV and 1A.
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 13:11
    There are too many "ifs" with this power supply to be certain, but if it's a fairly modern laptop supply and weighs less than a pound, it's probably a decently well regulated switchmode power supply and should be okay.

    I would probably use the 317 to pre-regulate down to around 28 volts (or a few of your diodes) and go for it.

    But like Leon said, a more suitable power supply is the best way to go.
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 13:19
    What could the power supply do that could mess it up?
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 13:34
    It's easy to get bogged down in details and refinements before anything is even working. If what you need is 12 or 15 volts at 0.25 Amps from a 31 volt laptop power supply, just run over to Radio Shack, buy another LM317, and run the two of them in series with the best heatsinks you can come up with. It will work and it'll get you started, you can worry about perfection later. Just go for it :)
  • piguy101piguy101 Posts: 248
    edited 2011-06-20 13:39
    I guess that is what I will have to do.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-06-20 14:31
    Or, just get one of these:

    Also check Jameco. They have a lot of inexpensive power supplies that would meet your requirements.

    -Phil
  • ercoerco Posts: 20,261
    edited 2011-06-20 15:18
    Three bucks gets you unregulated 12V@500mA: http://www.goldmine-elec-products.com/prodinfo.asp?number=G9596
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-06-20 15:35
    I have no idea what the OP is actually doing, but linkage doesn't get it working tonight or tomorrow, and please, the shipping is never mentioned. You guys worry too much sometimes :). Make it work, then make it good - and have fun doing it.
Sign In or Register to comment.