Mcu outputs voltage but no current.

Discussion in 'Embedded Systems and Microcontrollers' started by Mcaudle870, Jul 13, 2016.

  1. Mcaudle870

    Thread Starter New Member

    Aug 13, 2015
    I'm troubleshooting a proprietary elevator with very limited schematics and I have a 24vdc relay that will not pick. It has 24vdc across the coil and the coil is good, but the relay will not pick. All the other 24v relays have 2mA, but this particular relay has zero.

    My question is, is it possible for an mcu (or transistor) to fail in a way that I'm getting voltage but no current? I understand this is a tough question to answer with no schematics, but I'm wondering if anyone has ran across this before.

    Thanks in advance
  2. Papabravo


    Feb 24, 2006
    Literally speaking the answer is no, but there may be circumstance you cannot see that are causing the problem. It is current through the coil that activates the mechanical part of the relay. The nature of the contacts being normally open or normally closed should not affect the behavior of the coil. How the coil is wired with respect to the switching device may be confusing you and convincing you that what you think you are seeing is what is actually happening. Does the transistor switch connect one side of the relay coil to GND or to +24V? The answer to that question will help to determine the problem.
  3. crutschow


    Mar 14, 2008
    If you indeed measured 24v directly across the coil (not just coil to ground) and it is not pulling in then the relay must be bad (perhaps welded contacts).
    If there is voltage across a good coil then there also has to be current.
  4. Mcaudle870

    Thread Starter New Member

    Aug 13, 2015
    24vdc across coil.
    0 A across coil

    When a good relay is swapped out it will still not work but then when I put it back to its original socket it works just fine.

    the schematic only shows a 24vdc feed with a 0vdc output.
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    The fact that you see 24 volts across the coil means that it is open circuit. You have voltage across. No current through.

    Try continuity test across the coil when machine is off. In my experience the relay coil, when looked at using continuity test, will show as small resistor. The multimeter that I used (Fluke 27/FM, Fluke 77) would show a few hundred ohm, there might be a continuity sound at first, but then it turns off and you have ohm measurement on the screen. If multimeter shows large resistance, then the coil has probably failed and you need new relay. Resistance measurement can probably verify the continuity reading, but I don't think I ever did it since I was building new machines and had new relays.
    BobaMosfet likes this.
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    Any partial schematic might help. I can tell you a story that happened not once to me, but twice. This piece of computer hardware had an AC line fuse, but no voltage after the fuse. When i removed the fuse, the resistance of the fuse indicated that the fuse was good. Totally, perplexed because the circuit was simple, I measured the voltage across the fuse. It read 120 VAC indicating the fuse was open.

    It actually turned out the the fuse was intermittent. One a load was placed on it, the fuse would open.

    In your posts, your mixing Amps and volts confusing me even more.


    Swapping the other way may help too: The "acting bad" one into a "working spot".

    Looking at the voltage across the driver transistor can help too.

    Fluke actually makes a meter that you can actually make the input Z lower to gard against "stray" voltages which is more common with AC circuits.
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    That seems an extremely low coil current. I'm curious as to the make/model?
  8. BR-549

    Distinguished Member

    Sep 22, 2013
    Hard to say without a print. One can always pull the coil and measure it, closely inspect it, and test it's operation with a 24 V supply.
    Do this with a good coil first, and you have a reference.
  9. BobaMosfet

    Senior Member

    Jul 1, 2009
    How are you measuring current and voltage. For 2mA, your 'good' relays have to have a series resistance limiting to that current. If the mcu can't drive the current requirement (no resistance in series with the relay), you might not get a good result. Beyond that, voltage with no current is always an open circuit.
    Last edited: Jul 14, 2016