Step down 12v to 5v with resistor and LM7805?

Discussion in 'General Electronics Chat' started by Grayham, Jun 10, 2010.

  1. Grayham

    Thread Starter Member

    May 18, 2010
    My circuit below is a simple car voltage step down to from by car's 12v (or 13.7v) to 5v USB output on the dash.

    Now I want to take some load off the LM7805 so it doesn't heat up as much (or require such a large heatsink).
    According to my circuit simulation I can give it input of 8.6v instead of 13.7 using a 5ohm high power resistor:

    Does anyone know if this is a good idea do include this resistor (R1).
    My main goal is to make the LM7805 not work as hard and disipate so much heat.

    As the dummy 5v bulb load suggests, I will only ever be pulling 1000mA at 5v from the 7805's output, so input voltage will never drop below 8.6v.
  2. Bychon


    Mar 12, 2010
    The resistor is not a problem. It can enhance the protection the 7805 from voltage spikes, too.
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    You could also try a simple switch mode power supply circuit. They are somewhat more complex to build, but they do not need so much cooling as a linear regulator.
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    LT3503 should do what you want if you were inclined to try a Switching Power Supply. You would be able to make a much smaller and more efficient adapter.

    I am going to make a suppose that you already have the 7805 and you just want to make it work.

    You could try to create a current limiter instead of using, a power resistor but upto 5 ohms you have the right idea by keeping it simple.
  5. SgtWookie


    Jul 17, 2007
    A switching power supply for Grayham?

    I am afraid that AAC would run out of space on the servers. :(