Switching power to PIC micro

Discussion in 'General Electronics Chat' started by orko007, Nov 13, 2009.

  1. orko007

    Thread Starter New Member

    Nov 13, 2009
    I need some guidance on how to accomplish this task. I am fine when it comes to programming but on the electrical connections I need some help.

    Here is the quick background. I am using a PIC 16F684 inside of another device which has a 3.0v supply (two AA batteries). The problem is that there is no switched 3.0v power source inside the device so even when it is off the PIC is still drawing power. The device does have a 1.5V switched power source that I though I could use to switch power to the PIC using a NPN transistor. The issue is that when the device is off several of the connections from the I/O pins of the PIC are connected to 0V and the PIC still powers on, I believe this is called parasitic power.
    So I am wondering what I can use to switch the 3V to the PIC on the high side, so I do not have the parasitic power problem, using the available 1.5v switched source in the device.
    I have been reading all day and it seems like a small signal logic level Mosfet could possibly be used but I'm still a little confused if that is what I need.
    Also looking to keep it as simple as possible.

    hopefully this is understandable and sorry for my lack of electronic circuit knowledge. That is why I am here :)

  2. BMorse

    AAC Fanatic!

    Sep 26, 2009
    Is the original device just going into sleep mode or is it switched off with a switch? Possible use of a MOSFET on the high side would probably work... possibly an IRF540 or similar, but the switching power is a little on the low side and Im sure it wont even turn on the Mosfet, especially with only 3 volts of power..... so if the original device is just going to sleep, put the uc in low power mode or sleep....

    My .02
    Last edited: Nov 13, 2009
  3. orko007

    Thread Starter New Member

    Nov 13, 2009
    The main device that the PIC is installed into just sleeps when turned off, there is no physical switch, that would be too easy :) Guess I sound have mentioned that in the original post.

    From what i have read it sounds like mosfet on the high side is what I need but trying to figure out which will work and the wiring of it is has me blind.
  4. SgtWookie


    Jul 17, 2007
    3v really isn't enough to fully turn on the gate of a MOSFET. Many of them will be in the threshold level, but that's pretty high resistance.

    It may help to set all of the PICs' pins to INPUT mode (high impedance) before putting it to sleep, and then changing them back to OUTPUT pins when it wakes up.

    It would help a great deal if you could post a schematic of the complete circuit.