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Open Collector Relay Control

Discussion in 'Electronic Basics' started by Hank Brandenburg, Aug 27, 2006.

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  1. I have assembled an electronic timer kit, #148 from Hobby Engineering
    ( I plan to
    control a 20-amp 120VAC circuit with this timer. The relay I've chosen
    to do this with is part # 653-G7L-2AB-DC12 from Mouser.

    The relay specs say 1.9W is consumed by the relay for 12VDC, so I'm
    expecting to need at least 160 mA to energize the relay.

    The timer circuit uses open-collector output with a BC547 transistor
    for control, and I'm sure this transistor will not handle the current
    load required by the relay.

    I'm looking for suggestions on how to accomplish this - perhaps cascade
    the control circuit with a larger transistor?

    Any ideas are much appreciated.
  2. default

    default Guest

    Replace Q7 with a MPSA13 or equivalent. Plenty of gain and power
    handling capability, and cheap.
  3. Chris

    Chris Guest

    Yup. Replacing the BC547 with a darlingon NPN transistor like the
    MPSA13 is an easy, one component fix.

    The pinouts of the two transistors are different, though. You might
    want to turn the flat side of the MPSA13 around, and install it
    "backwards". That will result in the correct pinout on the board

    If you install them with the flat of the TO-92 pointing the same way,
    it won't work. Look at the drawings on the data sheets and see for

    Good luck
  4. Jamie

    Jamie Guest

    change the transistor to a 2N2222 and put a little add on heat shink..
    the 2N2222 will do 800 ma. it's 1.8 watts how ever, i think the
    transistor is most likely in saturation and that means you will not
    be seeing that heat at the transistor.
  5. ehsjr

    ehsjr Guest

    Use a BC337 or 338 as a direct replacement - it has the same
    pinout. It will handle up to .8 amps and can dissipate more
    power than the MPSA13 (which has the wrong pinout) recommended
    earlier. It also does not suffer from the > 1.2 v Vce that
    is unavoidable with darlingtons. It has about the same
    capability of the 2N2222 mentioned earlier, but the 2N2222 also
    has the wrong pinout.

  6. Chris

    Chris Guest

    Hi, Ed. Good choice -- that will be a closer replacement, and it's pin

    I was wondering, though -- the 4.7K pullup will limit base current to a
    little less than 1mA. If the OP decides to go with your better choice,
    he might want to replace that resistor (R20) with a 470 ohm 1/4 watt
    resistor, to ensure enough base current to saturate the BC337. He is
    driving a 160mA load.

  7. default

    default Guest

    As long as the PIC can drive Q6 it should work. Since it is easy to
    just change the program - I figured limited drive from the processor
    is the reason two stages were used in the first place -and I'm too
    lazy to try to find out . . .
  8. ehsjr

    ehsjr Guest

    Good point. I didn't look at the schematic, just the xsistor
    specs. The 470 you mention would be much better. It's not a
    good relay driver circuit as it stands, and on top of that
    the BC547 is too wimpy for many relays, so I hope he decides
    to replace it with one of the three options he's been given.

  9. Thanks to all for great feedback on this. I'm going to try the
    BC337/338 with 470 ohm resistor; The parts should be here by the
    weekend and I'll let all know how it comes out.

    Thanks for helping out a novice!
  10. ehsjr

    ehsjr Guest

    Good - getting rid of that wimpy 547 is the right thing
    to do. Don't forget to put a diode across the relay coil
    backwards (banded end connected to the + side).

  11. Just a quick update. I replaced with a BC337 and 370 ohm resistor and
    the circuit power the relay as desired.

    Thanks to all for the help with this!
  12. Chris

    Chris Guest

    OK, Hank. Glad to help.

    Just as a simple reality check, try touching the case of your new
    transistor after the relay has been on for a few seconds -- make sure
    it's not more than a little warm.

    By the way, please bottom post if you'd like to post again. Since
    you're a Google person, please read Google Groups Help Topic "What's
    good 'netiquette' when posting to Usenet?"

    Good luck
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