Switching 24vdc From a Low Current (3.3vdc, 3mA) Supply

Discussion in 'The Projects Forum' started by DanR, Mar 3, 2009.

  1. DanR

    Thread Starter New Member

    Mar 3, 2009
    3
    0
    Hi All,

    I have something of a problem with a project and as my electronics knowledge is more than a little rusty I'm hoping someone can point me in the right direction.

    I have an output from a piece of equipment (3.3vdc @ 3mA) which I need to use to switch a 24vdc supply. My initial solution (before I realised just how little current I had to play with) was to use a signal relay, however the current draw is far too high (~70mA).

    It has been suggested that I use a transistor switch but I'm not too sure how to proceed with this, or what kind of device specifications I should be looking for.

    If anyone here is able to shed any light on this it would be much appreciated!

    Thanks

    Dan
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    1. Does the 24V shares a 0V common with the 3.3V equipment or it is an isolated supply?

    2. If isolated, can you join the 0V common of the two without affecting normal operation?

    3. Is it the +Vcc of the 24V you want to switch?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Use a low power BJT transistor to control a relay and control the 24V power supply with the relay.

    Or you can use a low power BJT to drive a MOSFET and control the power supply.

    Depending on your knowledge you can find many ways.
     
  5. DanR

    Thread Starter New Member

    Mar 3, 2009
    3
    0
    Many Thanks for the prompt replies.

    With regards to a shared 0v rail, both supplies are required to remain isolated (just to make things awkward!).

    I've attached a quick diagram showing the current arrangement, the 24vdc supply is a feed into a PLC and the relay is just completing the circuit (between the +ve and -ve rails) to give a high input whenever the instrument gives a trigger output.

    Hope this makes things clearer.

    Thanks

    Dan
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    With the circuit you provided you don't need a common ground. Just boost the output of the instrument with a BJT and drive the relay.
     
  7. DanR

    Thread Starter New Member

    Mar 3, 2009
    3
    0
    Many Thanks for the advice guys.

    I've managed to track down a 3v relay that draws a whopping 1.4mA which negates the need for any additional circuitry.

    Thanks

    Dan
     
Loading...