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

 

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.

 

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.

 

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.

 

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.

 

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.

e.g.

24vdc across coil.
0 A across coil

All the other 24v relays have 2mA

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.

 

All the other 24v relays have 2mA

That seems an extremely low coil current. I'm curious as to the make/model?

 

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.

 

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

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.