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Thread: Question about hooking up LED's as Indicator of power.

  1. #1
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    Question about hooking up LED's as Indicator of power.

    I have an issue at the moment and not sure how to fix it.

    I am using some standard automotive 30/40 amp relays.

    I have some 12 volt LED's.

    I have some small switches I am using to switch the relays on and off with.

    I want to indicate the relays are on with the LEDs.

    I hooked the LED's to the output of the switch for the positive and the negative goes to ground.

    What I believe is happening is that when the switch is activated the power is going through the relay coil to ground and the LED doesn't see any of it. I have tested the LED's and they light up when powered separately.

    I BELIEVE what I need to do is wire the LED in series with the relay instead. To limit the power the LED is carrying I believe I have to wire a resistor in parallel with it but have no idea the size of resistor I need.

    Since I am using this as an indicator on if the relay is supposed to be switched or not I need to have it on the coil.

    Any suggestions?

  2. #2
    Raw Wave SNOtwistR's Avatar
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    sounds like you have the led polarity reversed long + short - , are you energizing the relay correctly with the switch? SNO

  3. #3
    FLAC Mickz's Avatar
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    If using 12v LEDS then they already incorporate a dropping resistor in series with the LED (as you said).

    A relay cannot pull power through a switch and stop a LED across it's coil from seeing 12 volts UNLESS you have a resistance (accidental or on purpose) in series with the 12v feeding the switch.

    I assume you are using 30/40A contact rated relays to switch 12v at current? Why don't you put the LED on the switched output to really show that the relay coil is not only supplied with 12v but is also actually working (switching)?
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    This switch and LED circuit is setup to plug into a harness I made. I unplug the switch/led circuit and hook ground up directly to the ground and +12 to the wire normally going to the relay and my LED's are lighting. I did this to test that my LED's were properly wired. And since the switches and LED's will be mounted into a removable console piece I wanted them to plugin to make things simpler to disassemble.

    I COULD go to the output of the relay but I am trying to insure the control circuit is enabled. I have the control circuits fused in the dash and I have no other way to insure the circuits are energized other than the click of the relays.

    And yes Mickz I am using standard Automotive 12 volt contact relays.

    I will know the relays are actually working when the computer comes up and the screen comes on.
    The RAP lead only powers the screen at this time. Which allows the computer to run a few more minutes after the key is turned off. And yes, if I sit in the vehicle the computer should shut down before the RAP turns off....

    Mostly at this point trying to figure out why the LED is not lighting and it doesn't make much sense to me. I did find a site where someone was trying to do the same thing and in his case the LED blew right away. A poster answered him and said he needed to limit the amperage through the LED and he needed to wire it in parallel with the relay coil. Suggested a 600ohm resistor but that a 1k resistor would work great too. I figured I should be able to just wire them in and they should work.

  5. #5
    FLAC Mickz's Avatar
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    Ok understand what you are doing, however I'm not sure what that guy is talking about, but taking a typical LED which requires 20ma to illuminate. At 12 volts a 1k resistor in series with a 20ma LED that IS NOT 12v rated (no inbuilt resistor) is fine.

    A 12v LED (with inbuilt resistor) must light if it is connected between ground (if it really is ground) and the switch AND there really is 12v on the switch. I assume you measured across the 12v led when its powered but not lighting for some reason.

    Anyway best of luck, I'm sure you'll get it sorted
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    Thanks Mickz, I thought I would share what is going on to see if it makes sense to anyone else...

    I have a total of 4 switches but I figure whatever happens with this one should work with the others...

    1 switch is an on/off/on spdt switch for Ign. This allows me to let ignition work as normal, turn it off, or manually turn it on even if the key is not on.
    second switch is also on/off/on spdt but it is for RAP. This is also setup to work with RAP as normal, off or manually on. At this time I only use RAP for monitor as already mentioned.

    The other switches are simple on/off and are for ODBII and Audio out so the computer can play over the air on the radio through an auxilary input. (It actually is the factory XM input but you can splice into the audio wires and make it into an Auxilary input that works instead of the XM. I have a DPDT relay setup to default to the XM to the radio, with it on the XM goes to the input on the computer and the computer goes to the radio.)

    Interesting part is none of the LEDs light up but when I turn off the ignition these other two LEDs flash momentarily. Makes me wonder if that is a sign of the coil "collapsing" when power is removed. It is not an issue for me if they flash like that but just an interesting artifact. (Both of these on/off switches are on the powered side of the ignition relay so they SHOULD light. )

    going to have to trouble shoot this more this weekend.. Now watch it be something stupid like I forgot to connect the ground or something.

    Rodney

  7. #7
    FLAC Mickz's Avatar
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    Yes back EMF from the collapsing field in the coils will do that, I always put a diode across the coils (even when switching transistors or FETS are not involved) as it stops clicks and glitches in system. Like you said, sounds like a floating or incorrect earth connection somewhere, its so easy to get caught out sometimes with vehicle wiring. Look forward to seeing exactly what it was.
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    I was under the understanding that Automotive relays already had a diode across the coil but as this demonstrates that is not likely.

    Rodney

  9. #9
    FLAC Mickz's Avatar
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    Yes, some do but a hell of a lot don't these days. Any that do will have the positive and negative terminals marked, but in some cases this is also misleading, connect it up back to front with a series resistor that limits current to a few hundred mils, normally they can still close and only in one direction. If not it should also flow more current in the reverse direction if it's got a diode.
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