Quick question - sharing heat sinks LM317

Discussion in 'General Electronics Chat' started by chrischrischris, Sep 26, 2012.

  1. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Hi.
    Could anyone please answer a quick question about heat sinks.

    Can 2 different LM317 voltage regulators (TO-220 case), running 2 different voltages share a common heatsink given the metal part of the case is active?
     
    Last edited: Sep 26, 2012
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The tab is common with Vout, so the only way they could share a heat sink is if one or both is electrically insulated from the sink (with a mica washer or similar.)
     
  3. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Thanks tracecom. I suspected it couldn't simply be connected. Don't have any mica washers, so I'll just use 2 separate heat sinks.

    On another query, I thought I'd be smart and use a 24V DC supply to create 3 voltages that need (namely 5V, 12V and 24V). I need these for different circuits consecutively. However the 12V draws 0.7A (i.e. 8.4W to dissipate). Is this wasting power? Should I just use a separate 12V supply?
     
    Last edited: Sep 26, 2012
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Or another transformer tap? (You can get multitap 30v transformers from DickSmith and Jaycar).

    Re the insulating washer you can use a large to3 washer if you have one, they can be easily cut to size with a pair of scissors.
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    You can save a lot of hassle by getting the LM317 in a plastic-coated version; I think the package is called the TO-220FP. From ST Microelectronics, it's the LM317P. See Digi-Key part 497-6707-5-ND.
     
  6. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Given the heat it is puting out I think I'll stear away from this idea. I didn't realize it would get so hot. I calculated 8.3W dissipation - very inefficient.

    I checked the multitap transformers - too expensive. Cheaper using my powerpack.
     
  7. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    John, I don't understand about the T0-220FP. I can't see how it can help my cause. It was a TO-220 that I'm already testing.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The P versions are plastic encapsulated so the tab doesn't need insulating. That's the only difference.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You can get a small power supply block off ebay for about $5 that puts out 12V and 5V at about 2A for use with external hard drives. Can't build one for that price.

    http://www.ebay.com/itm/Wonderful-1...=Laptop_Adapters_Chargers&hash=item27c9174631



    and for 24V:

    http://www.ebay.com/itm/90W-Power-S...=Laptop_Adapters_Chargers&hash=item1e72bbd66d
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
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    Yes, if you use the plastic-encapsulated part, all you need to do is run a screw through it to hold it to the heat sink. No fiddling with lots of hardware. And you can have as many LM317's on the same heat sink as you want.
     
  11. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    That's a really cheap neat little unit. I do however currently have a 12V 1A supply that I could use. I was just toying with using the 24V supply that I also need by using the LM317. I did calculations however on the 8.4W heat dissipation and found I needed quite a big chunk of aluminium to get rid of the heat. I therefore now "assume" this must be a very inefficient way of supply the 12V due to so much heat loss. Is that correct?

    John P - oh, I see the difference - plastic case. However again as mentioned above to bountyhunter, I'm now steering away due to what I think is probably inefficiency.

    Thanks everyone for the suggestions however.
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    YES. Wastes too much power.
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Do you have any old plugpacks? You can pull them apart and get the small 12v transformer out, and use that for your low voltage LM317 to save the 8W. Most housholds have boxes of old plugpacks from chargers and old appliances etc.

    Of course you could just buy a small second transformer but you sounded price-sensitive, and anyway, recycling is good for everyone. :)
     
  14. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I have re read this thread to try to understand this post. Why would you take a plugpack apart to get the 12V transformer. Why not just use the plugpack?
     
  15. chrischrischris

    Thread Starter Member

    Feb 18, 2012
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    Tradecom your right, I wouldn't need to pull it to pieces. In the end I decided to use an existing 12V plug pack that I had.

    My original thought was that since in my circuits I need 24V, I would use the same supply to create both the 5V and 12V supplies that I also need. The 5V - no probs - low power, therefore low power loss. The 12V however was high power, so too high power loss.
     
  16. Rleo6965

    New Member

    Jan 23, 2012
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    And don't forget applying thermal paste between the transistor, mica and heatsink for better heat transfer to heatsink.
     
  17. chrissyp

    Active Member

    Aug 25, 2008
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    Hi
    You could drop your 24v to 12v using an LM2575-12 buck converter which are much more efficient(about 80°/°) than an LM317, therefore no where near as much heat to dissipate
     
  18. chrischrischris

    Thread Starter Member

    Feb 18, 2012
    283
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    Now that sounds very tempting. These units seem to come in both a 5V as well as 12V which is what I need. Cost is around $3 for the 12V and $4.30 for the 5V plus a 100uF, 330uF and IN5819 (whatever that is). Great idea, but maybe ending up too expensive. Thanks for the suggestion however. Good to know a better way of handling it.
     
  19. chrissyp

    Active Member

    Aug 25, 2008
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    Hi
    A 1N5819 is a schottky diode
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    I bought every 1N5819 Tanner's had (local mom/pop electronics shop), they are one of those parts every bench needs. I also have a plentiful supply of plastic shoulder washers, for TO3 and TO220 case styles. I use kapton tape to isolate the package from the metal heat sink, want pictures? I'm about to do a lot of others for another user.
     
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