Transistor Switch or Relay?

Discussion in 'Programmer's Corner' started by wordizlife, Apr 17, 2012.

  1. wordizlife

    Thread Starter New Member

    Sep 29, 2011
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    0
    Hey Guys,
    I am designing a circuit that will require a PIC16F877A to comunicate with another pic and the pc through serial communication. I want to be able to control a certain portion of the circuit and be able to cut power to it. I am currently using a 2n3904 as a switch, the base is connected to my microcontroller pin through a 10k resistor and there is a pull down resistor of 100k also. All the negative connections of that portion of the circuit are connected to the collector of the 2N3904 and the emitter to the circuit ground.

    For some reason the transistor is not working, it says it can handle up to 200 ma of current and my circuit is putting out less then 150 ma. Should I be using a relay for this and controlling this relay with the transistor switch?

    I have attached an image of the schematic, the highlighted blue line are the negative connections of the circuit I want to control.

    Thanks
     
  2. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    please post an image not a bmp in a zip..
    also.
    what are you on about? your circuits so messy i cant see a thing
    nor which transistor youre talking about..
     
  3. wordizlife

    Thread Starter New Member

    Sep 29, 2011
    6
    0
    Sorry about it being messy its just to give you an idea. As I mentionned the highlighted blue line is the connections going to the transistors collector. The switch transistor is the one located at the bottom left of the schematic.
    Here is a link to the image:
    http://imageshack.us/photo/my-images/214/schematics.png/
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    I'm with chrisw: I can't make out a thing on your schematic.

    A good schematic follows a general form to make it comprehensible in a purely visual manor.
     
  5. wordizlife

    Thread Starter New Member

    Sep 29, 2011
    6
    0
    You don't need to get all the details. All you have to understand is that I am trying to control the power to two microcontrollers, a PIC18F2480 and a MCP2551 chip with the use a transistor. I have a 10kohm resistor that goes from my PIC16F877A pin to the base of the transistor and a pull down resistor of 100kohm from the base of the transistor to ground. My emitter is directly to ground and my collector is connected to the load.

    what reasons would cause this not to work? The voltage is successfully switching to 5V at the 877A output but the transistor is not giving sign of life.

    ( I will be cleaning up that schematic to make it clearer because I do agree that it is messy, but for now the image I posted and the explanation should be good enough)
     
  6. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    sorry, and im not intentionally being a pain, but i cant picture the words, your saying you have this and that resistor, thats great, but i cant picture it, and i cant imagine jack so youll have to draw a little sketch? on paint or something, just draw the basics, then we can get somewhere:D
     
  7. wordizlife

    Thread Starter New Member

    Sep 29, 2011
    6
    0
  8. wordizlife

    Thread Starter New Member

    Sep 29, 2011
    6
    0
  9. LucyJW

    New Member

    Apr 20, 2012
    1
    0
    Glad to see you got it fixed in the end, I have a few problems myself but my schematics are very much like your first ones so not sure people will be able to help much until i simplify them.
     
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