Transistor as relay?

Discussion in 'The Projects Forum' started by riddler, May 28, 2009.

  1. riddler

    Thread Starter New Member

    May 28, 2009
    4
    0
    Hi Guys..

    Sorry if this is a dumb question, but realy struggling to find answer.

    Right here we go!!
    a few years ago, for an alarm system, i needed a small component to convert the dc 12volt (low current)positive signal from a vehicle door contact,
    to a dc 0 volt signal,
    A guy at a local component store (no longer there) sold me, from what i can remember a Mosfet transister, and showed me how with just this single component
    will do what i needed, and it did..

    Problem i have is i cannot remember the exact component or how he wired it.
    and i need to do the same thing again,

    so if there is anyone who can help,

    please help me.


    Thanks in advance Julez....
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Can you tell us more about what you are trying to do? A door switch closes (conducts current) when the door is open, and opens when it closes.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    What do you want to switch to 0V with the MOSFET?

    What are its current and voltage requirements?
     
  4. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    If the load of the circuit is very small, you may use either small TO-92 package mosfet or transistor to do it as the attached circuit. For some mosfet, there is a very small leakage between the 'G' and the 'S' leads, the resistor R1 can therefore be omitted
     
  5. riddler

    Thread Starter New Member

    May 28, 2009
    4
    0
    Thank you tkng211
    the pupose for this is slightly different from the first time i needed it.

    i have a faulty ecu on a old car that i have and the ecu should be outputting 0volt (ign-on)on pin39 to swich external relay.

    what i am trying to do is on the inside of ecu, use one of the ign-on 12v+
    inputs to create a 0volt that i can feed to pin39 and switch this relay on..
    i do not know the current draw on pin39, if i need to test and post it please let me know..

    if this is not going to work i could just use an additional external relay to do chang-over (12V+ to 0v)for me, but was trying to keep fix out of sight..

    I know this poberbly not the best soulution for a fix but to have the ecu repaired / replaced i am looking at alot of money, and car not worth it..


    Many thanks
    Julez
     
  6. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    Since the cheap mosfet 2N7000 can only handle 200mA, You'd better measure the current flowing from pin 39 to ground (the current passing through the mosfet when the output of pin 39 is '0' ) by connecting an ammeter between pin 39 and 'ground'. I guess the current could be 500mA max. for a relay working at 12V. It can be as small as 100mA or even less. After knowing the value, will suggest you which transistor to be used and the corresponding value of the resistor. The cost of all the parts should be less than a dollar excluding the shipping cost.
     
  7. riddler

    Thread Starter New Member

    May 28, 2009
    4
    0
    Thanks again tkng211

    I have just measured between relay input and negative on battery.

    meter set at mA and measured 170mA to energise/activate relay.

    Thank you for your time helping me..

    Regards Julez..
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A 2N7000, while rated for 200mA, would be pretty stressed running at that current level continuously.

    Julez, are you in the USA? If so, your local Radio Shack might have some IRF510 power MOSFETs in stock. These are in a TO-220 package, and are rated for up to 5.6A, 100v.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I don't think that that's ever advisable. A floating gate is an invitation to erratic and unstable operation.
     
  10. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    Thank you for your advice. I totally agree that a floating gate of a mosfet in a circuit is not stable.
    As Julez posted: A guy at a local component store (no longer there) sold me, from what i can remember a Mosfet transister, and showed me how with just this single component will do what i needed, and it did..
    I just wanna explain to him why the single component might sometimes do the job. Hope Julez can learn something from you too.

    Julez, as SgtWookie suggested, you can buy some N mosfet with current rating> 1A ( e.g. IRF510) in TO-220 package for this application. Even there's an internal diode connected across the 'D' and 'S' to protect the MOSFET, it's better to connect a flywheel diode across the relay coil to play safe.

    You should also thank all who have given you the valuable advice on this thread. Hope you are happy with this modification in your car.

    Cheers!
     
  11. riddler

    Thread Starter New Member

    May 28, 2009
    4
    0
    Thanks guys
    keep up the great advice.

    Regards Julez....
     
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