transistor to turn on relay

Discussion in 'The Projects Forum' started by Abdo, Mar 4, 2007.

  1. Abdo

    Thread Starter New Member

    Mar 1, 2007
    7
    0
    Two seperate voltages 5vdc and 12vdc.
    I have a 5volt supply, very low current. I'm using that 5 volts to turn on a npn transistor. I wanted to use that transistor to turn on a 12 relay, but I know It wont work because my 12vdc supply for the relay is not the same common as the 5vdc. How can I do it? In other words how can I turn on a 12vdc relay using a rated power source of 5vdc about 50ma. The rated input currect on the relay is much higher than 50ma, I dont remember exactly how much maybe 200ma. The 5volt supply can turn on the trans but not the 12vdc relay. Thank you as always.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can use a FET like a VN10LP that is rated for full saturation with 5 volts on the gate. Use the FET to switch currrent through the 12 volt relay coil.
     
  3. Abdo

    Thread Starter New Member

    Mar 1, 2007
    7
    0
    Im a bit confused, just started reading about simple electronics. I can connect my 5v to gate and that common to source. My 12v will connect to my relay and the 12v common will connect to the Drain??? Would help if I saw a schematic. Thanks for the reply
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    What is the max current you can take from the 5V supply? It sounds as if you need a photo isolator if you cannot tie the grounds together. You can easily drive a photo isolator with very low current and let the 12V supply provide all the current to drive the other side of the isolator, the transistor and the relay.
     
  5. kender

    Senior Member

    Jan 17, 2007
    263
    0
    Here's a schematic for a MOSFET relay driver. Two components that haven't been mentioned before:

    - Back EMF (aka kickback) protection diode. You need it, because when you turn the current through the relay winding off, it can develop high voltage. Diode shorts this voltage out and protects the rest of your circuitry. Ideally, you would use a Schottly diode, but silicon (like 1N4841 or 1N4001 depending on the size of your relay) is okay in most cases.

    - Pulldown resistor. It guarantees that every time your whole device turns on, the relay will be in the off state. The value of the resistor should be on the order of 100 kOhm.
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I'm confused - why can the two grounds be tied together?:confused:
     
  7. kender

    Senior Member

    Jan 17, 2007
    263
    0
    I'm even more confused. Which grounds are you talking about? I can't find a place in this thread, where anyone would mention 2 separate grounds.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    From the first post:
     
  9. kender

    Senior Member

    Jan 17, 2007
    263
    0
    I see.

    If you have 2 different commons, you need to use an optocoupler (usually made with a photo-transistor) to control a 12V relay coil. Alternatively, you could choose a different relay with a +5V winding. Alternatively, you could use a solid-state relay (SSR).

    Although, I would still join thingmaker's question: can you "simply" connect the commons of the +12V and +5V supplies together? It would also help if you give us some background about your project. Where do the 2 suplpy voltages come from? What are you controlling with the relay? Block diagram of your system would, probably, help the most.
     
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