Connecting LM7805's ground to headsink instead of pin 2?

Discussion in 'General Electronics Chat' started by Grayham, May 27, 2010.

  1. Grayham

    Thread Starter Member

    May 18, 2010
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    Can anyone tell me if it is good practice to connect a LM7805's ground to its heatsink instead of connecitng a common 2nd pin for ground?

    I have tested in practice and they both produce same result, but connecting to ground is little easier (as in less wiring) for my simple 12v to 5v regulated voltage drop circuit.

    EDIT: I am not using heatsink paste here, so there should be no issue with insulation. Also ground is connected firmly to the heatsink screw (not actually through the heatsink itself).
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It works, but if and only if the heatsink is firmly connected to circuit ground, and the connection between the heatsink and the TO-220 tab is well made with star washers.

    The possibility of a fault due to a poor connection is great enough that actually connecting pin 2 to a ground trace is worth the effort.
     
    Last edited: May 27, 2010
  3. Grayham

    Thread Starter Member

    May 18, 2010
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    I was told by the guy at jaycar the

    "split lock washers"
    [​IMG]
    are the ones to be used with heatsinks.

    I noticed they also sold the star [​IMG] ones you speak of though at the store.

    Are one type maybe better for dissipating heat, while the other is better for conducting?
     
  4. Ghar

    Active Member

    Mar 8, 2010
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    Has anyone compared transient behaviour or stability when only using the heatsink connection? I would imagine there's some sort of difference though it may be subtle.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    nope...look at the datasheet..
    tab and center pin has a common connection.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The 7805 only has about 5mA to 6mA current flowing to ground from the regulator.

    However, if that connection is not present, your output voltage will be about 2v less than the input voltage; in other words, no regulation.

    You want to make certain that it does not lose it's ground.

    I have found that screws and nuts have an annoying tendency to come loose all by themselves.
     
  7. Ghar

    Active Member

    Mar 8, 2010
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    Just because they're connected doesn't mean they have identical performance, it would depend on how exactly the connection is made. I have no idea on the specifics of this connection which is why I'm wondering.
     
  8. R!f@@

    AAC Fanatic!

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    Read the data sheet
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Have to go with Riff on this one, it is common practice by professionals and hobbyists alike.
     
  10. BMorse

    Senior Member

    Sep 26, 2009
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    I also second R!f@@ and Sarge..... I have actually used the tab before for a ground connection, but I have always soldered it to the ground plane, only if it does not require heat sinking. (I have never used heat sinks that are attached to the PCB, if I do use heat sinks, I would have them upright for better cross ventilation, and to not thermally affect other components on the board, as seen in a pic below)

    [​IMG]

    P.S.
    If you do go with a ground connection through the tab and heat sink, I would use the star washer since you will have more points of contact than a lock washer, plus apply a dab of loctite #290 once you have the assembly together, this will hopefully prevent the nut and washer from inadvertently loosening up especially if the circuit will be moving or possibly get jostled around.

    B. Morse
     
  11. Norfindel

    Active Member

    Mar 6, 2008
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    Any resistance between the gnd pin and gnd will cause trouble. Just use the gnd pin that's why it's there.

    If you use a heatsink, you want to use a thermal interface material. IC/Heatsink heat transfer is nothing short of horrible and unpredictable without it.
     
  12. Ghar

    Active Member

    Mar 8, 2010
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    I read several. There are no specific notes about it.

    I'm not disagreeing, just asking for details.
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
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    For a partial answer, consider the necessity of bypassing the output and input of the regulator through capacitors. If one decided to use the heat sink as the sole attachment for ground to the regulator, then one must also provide the means to connect the sink to the circuit ground, and to somehow connect capacitors from pins 1 and 3 to ground. If a ground trace is not in close proximity to those pins, the the capacitors will have to make a connection to the heat sink. See - http://www.pic101.com/why_is_my_regulator_getting_hot.htm

    Heat sinks are nearly always anodized, so a good connection for a component lead is only really guaranteed by a threaded hole and screw holding a terminal lug. It seems more trouble than is is worth.

    That internal connection between the TO-220 tab and the center pin is there, but the tab is to dissipate heat. With a 78zz regulator, the tab is ground. For 79xx devices, that tab is the input voltage.
     
  14. R!f@@

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    Apr 2, 2009
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    [​IMG]

    Just in case u cannot see the pic, see page 16, last line.
     
    • 7805.PDF
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  15. Ghar

    Active Member

    Mar 8, 2010
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    I saw that line, it doesn't say much... I guess you just can't expect much from datasheets.

    @beenthere; those bypass caps are one of the things I was worried about. Internally it could be a high inductance connection or something, though I admit it's unlikely.
    Personally the mechanical issues are enough for me to not use the case connection.
     
  16. R!f@@

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    It says GND ..what else do you want buddy
     
  17. Ghar

    Active Member

    Mar 8, 2010
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    Yeah I know, I'm picky.
    I was just hoping for some sort of note, sort of like how it talks about the capacitors. I've been thinking about EMC things a lot lately and the lesson there is that almost no connection is what you think it is.
     
  18. R!f@@

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    I think the whole substrate is laid out on the tab
     
  19. Ghar

    Active Member

    Mar 8, 2010
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    That's what I suspect as well but it would be nice to know for sure.
     
  20. R!f@@

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    Yeah....do you a microscope?
     
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