What transistor to choose for Relay Switch

Discussion in 'The Projects Forum' started by yuvi1, Aug 4, 2015.

  1. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    Hi all,

    I see many circuits over the internet of a PIC controlled Relay,

    what are the differences and considerations of choosing the right transistor for this kind of circuit ?


    DC Source : 5vots
    Relay : 5volts

    Thanks !
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
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    hi y1,
    For a 5V relay which has a coil resistance of 100R thru 500R, almost any low/medium power general purpose transistor is OK.
    The 2N2222 or 2N3904 are common choices.

    The other consideration is what are the relay contacts being used to switch.?
    That will determine the contact type.
    E
     
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  3. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    Thanks for your fast reply !

    What do you mean "what are the relay contact being used" ?
     
  4. ericgibbs

    Senior Member

    Jan 29, 2010
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    You may be switching a high current, voltage, low level signal etc.
    Relays contact types are available for many different applications, do you have a project in mind.?
     
  5. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    If i want to switch a 220v load up to 200 watts, it depends on the transistor i'll choose ? i thought that the load that i want to switch
    depends on the relay , am i right ?
     
  6. ericgibbs

    Senior Member

    Jan 29, 2010
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    You must get a relay that is contact and insulation specified to work with mains voltages.
    For 200W, assume 2Amps, so get a relay with with at least 3A rated contacts

    Its important that the 5V relay coil has a suppression diode across the coil in order to protect the transistor
     
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  7. yuvi1

    Thread Starter Member

    Feb 1, 2013
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  8. ericgibbs

    Senior Member

    Jan 29, 2010
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    From the datasheet I would say it is NOT suitable, note the maximum voltage. ref image.
     
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  9. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    i don't think it's the same table that i sent in the link but i understand what you wrote ,

    A relay with max switching voltage of 220volts
    and a max switching current of 2A will work with a 400watts load ?
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In the given datasheet is given a maximum resistive load of 60 Watts.
    The 400 Watt load will destroy the relais.

    Bertus
     
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  11. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    Thanks for your answer, Bertus.


    So the relay determines which load i can use ? and the transistor that drives the relay does not affect or determines the load ?
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    A larger relays will also need a bit larger transistor to drive it, as the mass of the switch contacts will be larger.
    This is dependend on the coils resistance in the relays.
    There must be a coil current be given in the relays datasheet.
    Use a transistor that can switch at least 2 X the coil current for safety.
    Also a back EMF diode must be used.

    Bertus
     
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  13. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    so if i understand, coil current and switch current of the relay are not the same thing,

    coil current is the current that's need to drive the relay
    and switch current is the max current of the load that is switched by the relay .
    am i right ?
     
  14. ericgibbs

    Senior Member

    Jan 29, 2010
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    This PDF is the for the type you posted.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,503
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    You could use a device that was specifically designed to interface with a power device with a PIC etc, namely the 2n7000 'Fetlington'.
    Cheap and has the advantage of not loading the μp.
    Max.
     
  16. hp1729

    Well-Known Member

    Nov 23, 2015
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    You need to choose the relay before you choose the transistor.
     
  17. hp1729

    Well-Known Member

    Nov 23, 2015
    1,938
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    FETlington? What a sales pitch!
     
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