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LM7805CT Heating up

Discussion in 'General Electronics Discussion' started by Supercap2F, Apr 2, 2014.

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  1. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Hi Everyone!

    I have been working on a project that uses a inkjet slider thingy (see photos). It slides the thing that held the inkjet cartridge back and forth when you push a switch. And it has a switch on ether side so when the inkjet cartridge holder thingy comes to the edge it stops the motor to keep it from stalling. Once I have finished this part of it I am going to attached another one just like it sideways on the setup.

    If you do not understand what I just said thats ok. What the problem is is that the LM7805CT voltage regulator is getting very hot after about 10 minutes of use. I do not just mean warm, I mean HOT! I am not pushing any limits that I know of. I'm using a 9VDC power supply (one of those black plug-it-in-to-the-wall things). It's rated at 700mA. I measured it and my meter said it was putting out 12VDC. But that is still lower then the 35VDC max the LM7805CT has. I attached the schematic for the project and a photo of how I have it solder together.

    More photos are available on request. The last photo is of what I plan to do with it.

    Thanks

    Dan:)
     

    Attached Files:

  2. kpatz

    kpatz

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    Feb 24, 2014
    You'll need a heatsink on the 7805. It's a linear regulator meaning it works like a resistor to reduce the voltage, and the extra energy is dissipated as heat. You'll need a decent sized heatsink to keep the 7805 from overheating.

    How much current does your device draw when operating? Let's say it's 500 mA (1/2 amp). If your power supply is putting out 12V, the 7805 has to drop 7 of those volts to get 5V. 7 volts * 0.5 amps = 3.5 watts of heat it has to dissipate.

    Some ways to reduce the heat generated: use a lower voltage adapter, such as one that actually puts out 9V instead of 12, or use a switching regulator instead of a linear one.

    Lastly, your schematic doesn't show decoupling capacitors with the 7805. They're recommended to keep the regulator stable. The datasheets show a 0.33uf cap from input to ground and a 0.1uf cap across output and ground. A larger cap(s) are a good idea in parallel with the smaller ones, such as 10-100uF.
     
  3. Supercap2F

    Supercap2F

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    Mar 22, 2014
    I have a heat sink that is about 3 times as large as the one on it now that I will try.

    I will post some updates when I get every thing hooked up like you said.

    Thanks
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Another really good answer kpatz! I'm glad we have you on board :)
     
  5. Supercap2F

    Supercap2F

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    Mar 22, 2014
    Update time! I found a easer way to do it (no 7805). The only reason I did it that way it the schematic in post #1, is because I did not have the right relays. :p

    Thanks

    Dan
     
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