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  • 12V relay + diode question

    i'm running a relay, triggered by 2 input voltages. for one input i'm using the 12v ACC line, the alternate input is 12v from the PSU.

    obviously, the std relay i'm using only has one trigger, so i'm using 2 3A/20V diodes to isolate the 2 lines. this setup was working until 2 things happened. 1) i permanently soldered the diodes on, and heated them up too long and burned them up (voltage passed both ways). 2) i wiggled the relay too much and broke it.


    so i've replaced the relay and diodes. the ACC line is hot and hooked up properly. the PSU 12v is hooked up properly and is hot. the diodes are preventing feedback correctly.

    the problem is, if i turn on the ACC, then the PSU, then the ACC off, the relay clicks off. it should stay on cause the PSU is on. basically (tested) the PSU is not switching the relay.

    so... not enough power? its hot at exactly 12v, i'm not sure what else would be the problem. help?!?

  • #2
    Double check the diode on the PSU 12v line. Is the voltage getting through? Also, what voltage does your relay switch at? Remember the diode drops the PSU 12v a little. Could this be the problem?
    2004 4runner

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    • #3
      well, its an automotive 12V relay. the 12v ACC line switches it ok.

      PSU line before the diode is 12.02v, after is 11.90v. on my last relay/diode setup it worked fine. plain and simple, the PSU 12v won't switch it. really, what gives?

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      • #4
        the PSU 12v won't switch it. really, what gives?

        Connect the PSU directly to the relay (without diode) and see if the relay operates. Then atleast you know if your PSU is still working ok with the relay. If it still doesnt work then you know what to check next.

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        • #5
          Your ACC line might be as high as 14V. The relay switch-on voltage might need the full 12V (or real close). It's possible the diode is dropping the voltate too low. Follow Ricky327's advice and see if the PSU 12v rail can power your relay, at all. It's also possible your 12v line has a problem and/or your relay coild draw a ***kload of current. That's pretty much the full range of possibilities.
          2004 4runner

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          • #6
            well, it wont switch it at all. even the 12.02v straight from the PSU won't switch it.

            guess its time to try a different brand relay.

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            • #7
              well, it wont switch it at all. even the 12.02v straight from the PSU won't switch it.

              guess its time to try a different brand relay.


              The 12.02V is the voltage across the relay or with no relay connected? Because 12.02V should be fine for a 12V relay.

              Measure the voltage across the relay with your PSU supplying...it may be much lower than 12V. If this is so then your PSU is having problem supplying enough current to keep the 12V to the relay coil.

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              • #8
                Relays are fairly reliable. It should switch. Is the polarity correct on the diode?
                Fabricator

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                • #9
                  Check the resistance acoss the coil of the relay is zero ohms, those relays coil wires are extremely thin, and do break alot when moved around. if the relay is open, unwrap a couple of turns of the relay and resolder.
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                  • #10
                    maybe i should restate that the 12v ACC line switches it fine. its just the 12v from PSU that won't trigger it.

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