Which pins do I measure current for a LM7805C

Discussion in 'General Electronics Chat' started by atrumblood, Aug 9, 2012.

  1. atrumblood

    Thread Starter Member

    May 13, 2012
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    2
    Hello all,

    I have been trying to determine which pins on a lm7805c I would use to measure input and output current usage.

    The regulator has recently been getting very hot.

    I am using it to regulate the voltage from a 12V 500mA wall adapter. The load I am trying to power with the regulator says it needs 800mA @ 5V. The regulator should be able to supply up to 1A if I read the datasheet correctly.

    The concerning thing other than being very hot is that when I try to measure the current the regulator is using, my meter won't display the Amps it displays OF (over flow). My meter should be able to read up to 10A safely.


    The pin configurations of the regulator are (1:input)(2:GND)(3:eek:utput)

    I have been placing my meter between the +12V and the (1:input) pin.

    Am I measuring in the wrong spot ?


    LINK TO DATASHEET LM7805C

    PS: I have been using this setup for a while now without issues. The fact that I can only supply 500mA of current for the load just means it charges a little slower. The overheating is a new development
     
    Last edited: Aug 9, 2012
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    current must be measured in series.. So you disconnect the output of the LM and place the meter in series at that point. (essentially break the output lead of your circuit and reconnect each end with the meter)..make sure you have the leads in the right jacks on the meter.. Most you need to switch one lead to the 10A fused jack

    And a 1A rating is sort of meaningless.. Its really how many watts is it dissipating (based on the current and the voltage drop across the regulator). And 1A also doesn't mean it will run there without a heatsink.
     
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  3. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    Ok. So without a load the output measures about 6mA. With the load I read ~250mA. Seems to be quite a bit off from what the load says it should pull. Now would I be correct in assuming I would connect the meter in series with the input and +12v to read the current the regulator is pulling? Or does the output represent that usage?
     
  4. #12

    Expert

    Nov 30, 2010
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    Both of them.

    The waste current from a 7805 is microamps.

    or do you want to get microscopic about this?
     
  5. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    No I just want to figure out why the regulator is getting so hot. When I measure in series with the input and 12v I get (OF). This is with the Jack in the 10A slot. I have tried 2 lm7805c chips so far. Any ideas on that?
     
  6. #12

    Expert

    Nov 30, 2010
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    The heat is caused by the voltage across the chip (about 7 volts) times the output current~about 250 ma. That's 1.75 watts.

    That's also asuming you installed the local capacitors to keep the chip from oscillating.
    Did you?
     
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  7. atrumblood

    Thread Starter Member

    May 13, 2012
    59
    2
    Yeah I installed them. Those are also heating up quite a bit. Seems odd that with the same load it wasn't heating up this hot before much too hot to touch. Maybe there is something wrong with the load?
     
  8. #12

    Expert

    Nov 30, 2010
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    Maybe the capacitors are getting hot because you put them REALLY close to the chip...like you're suposed to. That would mean the capacitors are not bad, just too friendly with something that is hot.

    Obviously something changed, but was it the load, or was it your awareness of the heat that is being produced?
     
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  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    get a heatsink on it and move on.
     
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