Shunt or series, current and voltage protection, what is best ?

Discussion in 'General Electronics Chat' started by howdoesthatworkguy, Sep 13, 2007.

  1. howdoesthatworkguy

    Thread Starter Active Member

    Jul 23, 2007
    With the most limited of parts I want to be able to create a circuit that can power my PIC, but also protect it and the other components. I require my circuit to operate as far down as 7v, be able to bad connections for a slipt second without the PIC resetting, current and voltage protection. 7805 was what I was first using but found this limiting and not ideal. I am working with small SMD devices and small is the word :)

    The supply voltage is from 0v to 17v, spikes can go up to 27v, then a zener will clamp it. I had been looking at using a transistor with a zener on the base to control the voltage for the PIC. I don't require much power to run the PIC, most of the other circuit will run off the high voltage rail of 9 to 17v.

    I had seen some use of a mosfet, with the ability to turn off if the polls are reveresed thus protecting the circuit which I thought was good, as well as acting as a current limiting device.

    Any ideas anyone ? I don't want to look at exspensive IC solutions, I feel there must be a way of using a few discrets to do the same job.
  2. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    Try using a LM78L05 regulator. It looks like a transistor. It only supplies 100mA although (can't perform miracles there).
  3. mrmeval

    AAC Fanatic!

    Jun 30, 2006
  4. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    That chip seems too small to supply 500mA (even using the ground PCB track as an heatsink might not be enough). Is it switched?
  5. SgtWookie


    Jul 17, 2007
    Take a look at the LM117 / LM317 series of regulators.

    National Semiconductor has datasheets available on their site.


    The SOT-223 package is small footprint, capable of 1.0A output; needs minimum of two resistors to set the output voltage. Input and output filter caps required depending on length of leads.

    Don't forget that you will need proper heat-sinking in order to dissipate that much power.