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Logic Level Relays

Discussion in 'Electronic Basics' started by Jonathan Leppert, Jan 7, 2004.

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  1. Hi,

    Can anyone recommend a good power relay that can be triggered via typical
    low current low voltage logic levels (5 VDC). I need one that can switch a
    12VDC 10 amp circuit and also one that can switch 120 VAC, around 2 amps. I
    am confused about what coil voltage means, etc. so if anyone could give me
    any pointers I would greatly appreciate it. Basically all I need to do is
    switch the source on and off.

  2. The coil voltage is the voltage you apply to the relay coil to operate
    it. If you intend to operate the relay from TTL-ish logic levels,
    then you need a relay with a 5 volt coil that will operate with fairly
    low coil currents - 10 - 20 mA, or so. It is likely that you will
    need to use this relay to operate a higher power relay that will
    actually switch your 12V/10A or 120V/2A.
  3. I'm looking at a tyco made T90 series coil that has maximum contact ratings
    of 30A @ 240 VAC, with a nominal coil voltage of 5 VDC, 185 mA. So am I
    correct in saying I could use a simple transistor to switch this relay on
    directly from a microcontroller?

    Here's the digikey page for the relay ($2.33 ea):

    Thanks for your help.
  4. CFoley1064

    CFoley1064 Guest

    Can anyone recommend a good power relay that can be triggered via typical
    This type of relay used to be fairly common, but I'm not sure anyone makes them
    any more. Driving a relay from a logic output is fairly simple -- you just
    need a transistor, two resistors and a diode to do the job well. (View in
    fixed font as Courier):

    + +
    | |
    1N4002 - C|
    ^ C| RY1
    | C|
    | |
    | |
    Logic Pin | |
    ___ | |/
    o--|___|---o-o-| 2N3904
    4.7 K | |>
    | |
    .-. |
    | | |
    4.7 K| | |
    '-' |
    | |
    | |
    === ===

    This setup will reliably drive 5V relay coils of 50 ohms or more. You can do
    the above with any 5V logic output pin that can source 1mA. Another thing you
    might want to look at is solid state relays. They give you electrical
    isolation from the load with a built in optocoupler, and depending on the type
    chosen, can switch AC loads (triac/inverse-parallel SCR drive) or DC
    (transistor-drive) loads. Make sure you specify one which is made for 5V logic
    levels, and watch the optocoupler drive current. Most SSRs require several mA
    of drive current, and some require up to 20 mA. Many logic outputs can't drive
    that much of a load directly. In that case, just use the transistor setup
    above to drive the optocoupler (you won't need the diode).

    Good luck
  5. Wade Hassler

    Wade Hassler Guest

    I think the catch-diode in that schematic should be across 'R1.'
  6. CFoley1064

    CFoley1064 Guest

    Small error -- sorry. The anode end of the diode is connected to the other
    side of the relay coil/transistor collector rather than the transistor base.

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