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Driving multiple relays

Discussion in 'Electronic Design' started by Dh, May 17, 2007.

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  1. Dh

    Dh Guest

    Hi guys,
    I've got to drive some eight relays (12v, 200 ohm coil, 300mW
    sensitivity). I'm planning to use the old 2n2222A to drive all of
    them(in parallel). is it disastorous? would u suggest a better
    transistor for this application? is it ok to use single transistor or
    i should go with separate transistor for each relay??
    Please reply soon..
  2. Fred Bloggs

    Fred Bloggs Guest

    There is such a thing as applying arithmetic. 12V and 200 ohms means
    12/0.2=60mA per relay, and then 8x this would 480mA. But then 300mW at
    12V means a current of 300/12=25mA per relay for 8x or 200mA total. So
    no one can answer your question until you get your information correct
    and consistent.
  3. D from BC

    D from BC Guest

    This might be kinda silly but..How about using one more relay to drive
    a bunch a relays?
    D from BC
  4. ehsjr

    ehsjr Guest

    Should be ok if you use 2 2N2222A transistors, each
    driving 4 relays. Your circuit would need to be designed
    to provide approximately 50 mA to the base of the
    transistors whether you use one or two. If you want to
    cut down on the base drive current and use a single
    transistor, a power darlington like a TIP120 would work.

  5. Dh

    Dh Guest

    There is such a thing as applying arithmetic. 12V and 200 ohms means
    Hi Fred,
    thanx for the reply.
    ur arithmetic is correct, but the data i've given is correct as i
    suppose sensitivity means minimum power required to turn the relay ON.
    so, 25mA current would be sufficient, the 60mA is upper limit for the
    current which is useful when we want to connect a resistance in series
    with the coil for short circuit protection.
    please correct me if anything is wrong..
  6. Dh

    Dh Guest

    thanx for the reply Ed
    why 50mA?? the maximum current i need will be (12/200 * 8) = 480mA as
    pointed out by Fred, in that case my Ic of the transistor required is
    480mA and to drive this transistor (hfe=75) into saturation i need
    minimum base current of (480/75) = 6.4 mA or as a thumb rule atleast
    double of it which makes base current = 12.8 mA.
    Please tell me how u arrived at 50mA..
  7. Dh

    Dh Guest

    Hi guys,
    one more addition, i'm going to use AVR microcontroller to drive the
    transister, the pin current capacity is 40mA max.
  8. Winfield

    Winfield Guest

    Lose the transistor and use a single MOSFET for all eight relays.
    For example, Fairchild's RFP3055LE is a sixty-cent TO-220 through-
    hole logic-level mosfet that will switch 480mA with only 50mW of
    dissipation across its 0.1 ohm Ron, with 5-volt logic gate drive.
    And, it sports the long-famous '3055 workhorse part number.

    If you must use a TO-92 transistor, use one of Zetex' high-current
    high-gain Super E-line types, like a ztx851, which will dissipate
    under 50mW at 480mA with 10mA (beta=50) base drive.
  9. MooseFET

    MooseFET Guest

    I'd suggest the supertex TN0604N3 if he had to use a TO-92. It would
    be about as much current as I'd trust one for. The on resistance is
    about 1 Ohm so you have about 1/4W to get rid of.
  10. It's a shame theere aren't more powerful TO-92 mosfets. It's
    not for a lack of small enough dies, because we see many good
    new candidates in coming out small surface-mount packages.
    It's a lack of interest in bringing out any new TO-92 parts.
    We do still get new TO-220 through-hole parts, but there have
    almost no new TO-92 parts introduced for quite a few years now.
  11. Jasen

    Jasen Guest

    you want saturation or else you'll cook the transistor.
    for saturation treat Hfe as approx 10

  12. ehsjr

    ehsjr Guest

    Better rule of thumb: base current = Ic/10 for
    guaranteed saturation. BTW, 50 mA Ib for saturation
    with 500 mA IC is on the datasheet.

    But in this circuit, what do you care? If you have
    plenty of current available, that's fine, if you're
    limited, use a darlington.

  13. MooseFET

    MooseFET Guest

    Today, I mostly use surface mount for the small signal stuff. There
    have been very few times that I wanted a power device in the 0.25 Watt
    range. I can come up with a long list of things I'd like to see
    before I see a new transistor in a TO-92.

    A reference with less than 50nV/sqrt(Hz) noise

    An LT1247 that really does work at 1MHz without getting red hot.

    A PA7540 in low power CMOS.

    A fast low noise op-amp that will work up to 35V

    A line of analog filters you can program like a PROM to be the filter
    you need.

    A fast comparitor that can take large input swings.

    Linear voltage regulatators with sense pins.

    LDOs that are stable for all capacitive loads.

    HC4046 VCOs that remain linear at the extremes.

    Strong alloy "brass" parts that aren't magnetic.

    1N5314 in a SMD package.

    While we are at it: diamond heat sinks.

    I think you will agree that many things are more important than new
    TO-92 parts.
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