Using a PNP common Collector to drive a relay

Discussion in 'The Projects Forum' started by cork_ie, May 10, 2013.

  1. cork_ie

    Thread Starter Member

    Oct 8, 2011
    348
    58
    Hi all,
    I need a little help with this circuit.
    Basically I want to ditch some electronics within an existing touchpad that got wet and just use the touch pad contacts to drive a transistor to drive a remote mounted relay. space inside the touchpad is very limited so it has to be simple.Relay should only operate when touchpad is pressed.

    Specs are as follows,
    1) Relay is 75 Ohm 12V permanently fed by 12V+ and grounded by touchpad
    2) I just want simple 2 wires going to touchpad i.e. pad to ground transistor base when pressed which will ground the relay.
    I have decided to use a TIP32C PNP as the driver transistor, but am having some trouble biasing it and it tends to lock on.
    I think this is due to the emitter going to ground when the transistor turns on.
    Any ideas anyone?
     
    Last edited: May 10, 2013
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    Add a 47k resistor from the emitter to the base.

    Check the resistance of the keypad. They can be really bad about current carrying ability and you need about 2 milliamps.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,972
    744
    Put the relay in the collector side and emitter to posative, with a 10-47K resistor from the base to emitter.
     
  4. cork_ie

    Thread Starter Member

    Oct 8, 2011
    348
    58
    Thanks to both of you,I would never have got there without some guidance.
    Unfortunately the relay is mounted a million miles away from the plug for the remote, so it wasn't an option.
    I tore my hair out on this one, I tried at 47K between B&E and worked down trying to saturate the TIP32C , in the end it worked OK at 4.7K, but as soon as the voltage rose with the engine running, it started to lock on. Eventually 3.3K worked and a 2μF between E and C (Gnd) seemed to iron out all the problems.
    I am enjoying a nice meal of fish this evening for my efforts.

    Thanks again.
     
  5. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    :eek:

    Well, that explains why my resistor value was so far off! You didn't tell us it was a long wire in a noisy environment. (Grrr. I hate to miss that badly.)

    Anyway, good fishing. You can't get a better supper than the one you caught yourself.
     
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