Automotive relays diodes shorting?

I bought some 12 volt 30 amp Automotive relay for controlling large 12 volt loads.
Here is the circuit setup along with the relays information. The issue now is that sometime (about 40% of the time) the relay seems to short out because the voltage across the coil drops to 4 volts. The resistors do protect from a full short (I tested it without the resistors the short was about 3.5 amps) I checked about 15 times for a wire shorting and can’t find anything. My only guess is that the diode is shorting or breaking down. It is the correct polarity since the relay does function normally sometimes. I added the diode to prevent flyback from the relay could the diode be to small? I am using a


---------relay information------------------
Coil voltage: 12 volts
Pull-in voltage 6 volts
coil resistance 66 ohms
nominal current 160mA
drop-out voltage 3.6 volts

Thank you, your help is appreciated.


What's the point of the resistors in series with the coil?

The coil it's self was using too much current and was heating up. I had another topic on this forum about the voltage of the relay,174285.0.html The relay does work best with the resistors.

a 12 volt coil overheats on 12 volts? Somethings gone dicky here. I see your not sure about it being a 12 volt relay. I'd recommend using 9v instead. Anything above the pull-in voltage should work.

The resistors are not the problem. I believe the diode is somehow shorting is this possible?

There are only two reasons a diode could fail for 1) exceeding PIV 2) exceeding max forward current

It's a 1000 volt diode, so Cause No. 1 is out. It's a 1.5 amp diode, so Cause No. 2 seems "remote".

arduinoPi: I believe the diode is somehow shorting is this possible?

Have you removed the diode, tested it, and found signs of failure? [You cannot test it in circuit.]

arduinoPi: The resistors are not the problem. I believe the diode is somehow shorting is this possible?

Since you already know the answer, why ask here?

The diode is soldered in I can desolder it and will try that. I didn't know if diodes could short for any reason

I would have put that 33 Ohm resistor in the 12 volt line, certainly in a car setup where every metal part is also part of the circuit.

If you're going to remove the diode, have a look to see if you've still got your short with it removed besides testing the diode.

One thought is that you've heat-damaged the diodes soldering them (they are much more sensitive to overheating than many devices as the die is mounted directly on the thick copper wire. Solder with a hot iron quickly, don't cut the leads too short either.

The only other thought is that those high voltage rectifier diodes might be pretty slow to switch - use a schottky diode instead?

Hi, hand drawn circuit does show a diode and if it is fitted across the coil in that orientation good. However once you have soldered the diode the relay coil can only be powered up in one configuration only. Positive supply MUST only be connected to the coil terminal with the CATHODE of the diode connected to it. Negative or ground to the coil terminal with the ANODE of the diode connected to it.

The CATHODE is the end of the component with the WHITE band around it.

If its fitted the wrong way around the current will go through the diode and not the coil, if your 12V supply is big enough it would cook the diode, ie let the smoke out, the diode may eventually go from short to open and you relay work again.

Your power supply, what is it? Battery, Proper power supply or just a transformer and rectifier?


The power supply is a 12 volt lead acid battery.

A 3watt 60 ohm resistor or 3 100ohm resistors should do the job.


Dump the resistors and use a transistor to switch the relay on.

I just ran a test with the relay. I just connect it directly to a small lead acid battery (1.2 amp an hour) with out a diode. it turns on fine but I also had my multimeter on range 1000 volts DC on the 12 coil terminals of the relay. When ever the relay first turns on it goes over the multimeter 1000 volt range. I added a diode it and it does the same thing the relay has no flyback voltage but the diode shorts out about half of the time. The diode polarity is correct. I am also not using resistors as in my first design.

Any high-voltage event would result from the coil's being de-energised (turning off), the then collapsing magnetic field's resulting a counter-emf, but not from its being energised.

arduinoPi: ...but the diode shorts out about half of the time.

So, you have removed the diode and, using the diode test function on your multimeter, confirmed that it is "shorted", or what? Please explain.

If you don't [u]know[/u] how to test a diode then ask. A picture of your multimeter could be helpful in providing an answer.

Where did you get those diodes? I'm beginning to suspect a bad batch or something....

Also have you tried fast rectifiers?

How to test a diode?
I have a cheapie harbor Freight MM.

My problem is this. Auto lighting controlled by a RC-5102 12vdc relay with a 1N4005 diode across the coil terminals. The relay does not work.

When testing across the coil terminals, MM set on highest Ohms, I get 1.7 Ohms with probes connected either way (Red to either side).

If I remove the diode from the relay, the MM gives me 119 Ohms across the coil, and when 12vdc is applied the coil pulls in just fine and the NO and NC terminals act normally.

Is this indicating a blown diode? I would expect a bad diode to pass nothing.

I hoped someone could tell me how to test a diode with this MM.

Diodes typically fail in one of two ways - shorted or open. The shorted condition happens when you hit it with a spike over its PIV rating. Open typically happens when you exceed the current capacity of the diode, however most of the ones I have seen blown "open" show physical signs of damage - like burn marks where the magic smoke came out. The fact that you are seeing a low reading with your multimeter trying to measure resistance with the probes either way is suspicious since it reads higher with the diode removed. What do you get if you just measure across the diode in both directions - it should not be the same when you reverse the probes - diodes are also non-linear and you will often get different readings depending on what range the meter is on (which determines how much current flows). Does your meter have a diode test position? (shows as a diode on the selector switch).

Which Harbor Fright meter do you have? Even the cheap $6 one shows a diode test function on the selector switch. Also, you should not be measuring the resistance on the highest range - use one that is close to what you expect to see - for example, either the 2000 or 200 ohm range on the cheap one would be more appropriate. You may get higher than expected readings across a diode with some meters that do not provide enough voltage on the leads to forward bias the diode.

Hi, How are you powering the relay?

If you have a diode across the coil, you can [u]ONLY CONNECT IT ONE WAY[/u] . Positive to the coil terminal with the Cathode of the diode connected, and Negative to the terminal with the Anode of the diode connected. If you connect it the other way it will not operate, and will appear as if you have a short circuit coil. If your power supply is able to supply many amps, then you will destroy the diode and it will appear as a short circuit in either direction.

Tom... :)

Does your DMM have a diode testing function? Can you tell us the model of the DMM, or a picture?

I think we scared him off or something :)