24V DC relay circuit

Discussion in 'General Electronics Chat' started by corrado33, Oct 27, 2009.

  1. corrado33

    Thread Starter New Member

    Oct 22, 2009
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    Hi guys. I have a quick question. I'm building a project with a 24V DC power supply, however I want there to be an ON/OFF switch (obviously). But I want to do this with a VERY small switch and relay. However, I can't seem to find a good way to implement this.

    I need to use a very small contact switch to control a relay which controls power to 4 motors at 3 amps a piece and some other small things, but I only have 24V DC power. Most if not all small switches can't handle 24V DC. How can I get around this? Could I build a small voltage converter? Maybe using a small MOSFET transistor? Since the relay would only draw very little amps could a MOSFET handle a large voltage drop? Say to 5v or less?

    Thanks!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What are the possible switches you can use?

    How small you want them and why?
     
  3. corrado33

    Thread Starter New Member

    Oct 22, 2009
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    I have about a very small area to fit my switch in. I haven't looked up the switches yet because I'm not sure if the circuit is possible. Basically I have a door that I'm shutting and it's going to activate this switch when it's closed. The door is about half an inch think (or tall, however you want to look at it.) So the switch could be at most an inch tall. Basically I'm looking for a tiny switch like this... http://www.radioshack.com/product/index.jsp?productId=3190382 But if I can't get it to work I can DEAL with bigger ones.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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  5. corrado33

    Thread Starter New Member

    Oct 22, 2009
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    Ok so if the relay coil pulls let's say 50-100mA, is that too much for small switches?


    Oh and I liked the magnetic switches, but they're a bit big.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Give us an example of what you consider a "small" switch. It does not sound like it should be a problem.
     
  7. corrado33

    Thread Starter New Member

    Oct 22, 2009
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    Here is one way that you could use that low voltage, low current switch with a Darlington transistor to drive that relay, and stay within the switch specifications.

    [​IMG]

    R1 limits current to <1mA. D2 is a 5.1v Zener diode; however with the very low current flow, it's breakdown voltage will be less than 5v.

    S1 is that 5v 1mA switch you found. When it closes, it feeds about .7mA to the base of the Darlington transistor, which turns on the relay.
     
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