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Topic: Automotive relays diodes shorting? (Read 2383 times) previous topic - next topic


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. 

Runaway Pancake

What's the point of the resistors in series with the coil?
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!


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 
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?

Runaway Pancake

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".

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.]
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

James C4S

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?
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com


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.
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html


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?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


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?

Everything runs on smoke, let the smoke out, it stops running....


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.

Runaway Pancake

Jul 21, 2013, 05:18 pm Last Edit: Jul 21, 2013, 06:07 pm by Runaway Pancake Reason: 1
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.

...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 know how to test a diode then ask.  A picture of your multimeter could be helpful in providing an answer.
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

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