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Thread: Why LM1085 VReg Overheating ?517 days old

  1. #1
    Junior Member AmandaLi's Avatar
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    Default Why LM1085 VReg Overheating ?

    I have a programmable (using a Arduino MEGA) 32x32 RGB LED matrix:

    that I want to play with. Power for it is 5VDC and up to 4A, so I am using a 5V LM1085 LDO vreg(pdf) which is supplied by a 19.5V, 6.7A laptop power supply.
    The program I am running on it now is only taking 0.5A, but the vreg is getting very hot.
    My question is; why is it overheating at 0.5A when it can easily supply 3A? It is attached to a heat sink with heat transfer paste. I have checked all my connections and they are correct and solid.
    Here is a schematic of what I have:

    It isn't visibly apparent, but there is a 4-way (electrical) junction with: the load, the capacitor, the IC Ground pin, and the negative output of the laptop PS.
    Thanks to anyone that can help

  2. #2
    Prolific Poster towlerg's Avatar
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    Default Re: Why LM1085 VReg Overheating ?

    You are dropping quite a lot of volts, is it possible to reduce the input voltage?

  3. #3
    Member GAMBOA's Avatar
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    Default Re: Why LM1085 VReg Overheating ?

    Power in heat = 14.5 x 0.5=7.25W
    Power for your circuit = 5x0.5 =2.5W
    Total Power = 7.25 + 2.5= 9.75W
    LM1085 can't support that power.
    you need power supply of 6V.


  4. #4
    Member Alan R.'s Avatar
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    Default Re: Why LM1085 VReg Overheating ?

    I have been adding small switcher supplies to all of my power hungry designs. No more hear sinks or fans - just lots of cool power.

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