Relay Suppression Diode Failure

Only 70uJ, unless your measured numbers are wrong. Perhaps relay installed backwards so that diode is forward-biased when relay is powered?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
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Usually when a component 'cooks', it is due to current and not voltage. When the relay switches off and the coil collapses, it produces high voltage but does not have high current potential due to the lack of capacitance.

I would suggest double checking to see if the relay coil is wired in the right polarity.

Thomas

Just to keep thermal things in perspective:

The average power dumped into the resistance of the coil is close to

1/2 * (V^2/R)= 99 mW. The peak energy dumped into the diode is very approximately 11 mW (15 mA times about .7 volts drop), but the average power dumped into the diode over the 4 second cycle is only about 1.6 uW (by simulation), because most of the stored inductive energy is dissipated in the coil resistance during discharge. So the temperature rise in the diode, is far more likely to be caused by its close proximity to a coil dissipating about 6000 times as much power as the diode is. All this assumes that the diode was not defective, to start with.

Even if you activated the coil every 0.004 second, for 0.002 second on time, the ratio only drops to about 46 to 1.

I think you might measure the copper coil resistance during the test to use it as a temperature monitor.

The energy stored in the relay inductance is i^2L/2.

13v and 850 ohms gives 15.3mA.

15.3mA in 600mH gives 70uJ of stored energy.

If the diode had to absorb all the stored energy (which it doesn't) the power dissipated would be 70uJ every 4 seconds or 15.5uW.

The estimated junction temperature rise from dissipating 15.5uW is zero.

More likely you had transient voltage spikes on the 13v supply (possibly caused by whatever you were switching with the relay) which caused the diode to break down and then cook because it and the transistor were shorting the 13v supply. That or you had the coil wired the wrong way round and were trying to short out the supply with the diode from the beginning.

(snip)

(snip) Okay, please tell me what what duty cycle and frequency produces the most power dissipation in this diode. And what power is the diode dissipating under those conditions?

I stated this once before but some one kind of thought I was a fruit cake. The fly back energy released from the coil produces current in the diode, if you keep pulsing the relay, the diode does not have any time to cool down., it will at some point avalanche due to heat etc.

Most likely if you look at the spec's you may find more specific data on the relay about duty cycles., if that data isn't there, then the OEM didn't do their home work.

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Hmm, we use relays at work (DC) that employ AC type TVS diodes to clamp the coil, above the rated voltage on the relay. they don't seem to have a polarity issue.

Now, I have seen a few times in machines we have a circuit that now and then goes hay wire and causes one of these relays to chatter until the machine powers down or the diode shorts and then blows the control fuse which then nicely powers the machine down for you! :)

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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Almost certainly the case, and the "proper" polarity was determined by an old meter movement ohmmeter where the red lead (+) is negative and the black lead (-) is positive:-)

Or perhaps the PCB layout dude/dudette was more familiar with modern component layouts that generally show the part from the *top*. Everything else is generally symmetrical on TO-5 relays. ;-)

Best regards, Spehro Pefhany

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yeah but 15 pulses per minute isn't going to be much of a strain on that.

Bye. Jasen

Depends on the ambient temperature and how many microKelvins you are away from destroying the junction... it could just be the nanostraw that breaks the camel's back.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Thanks for all the inputs. I also calculated the energy dissipated in the diode and determined it was very low. The relay had to be in correctly, because it worked for 3+ minutes before failing. After the failure the relay, you coild still operate the coil, but debris from the exploding diode had made its way between the contacts.

I start looking at other causes.

THAT is NOT a correct conclusion. It just took that long to cook the diode. The relay (diode) IS in backwards.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

How do you know that the diode didn't fail, immediately, but the contacts didn't get contaminated enough to fail until 3 minutes later?

The OP needs to learn how to use an Ohm-meter ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Use the relay which has already failed to perform further tests.

Clip out the damaged suppression diode, and solder in a new 1N4148 or equivalent, in series with a 1 ohm resistor. Clean the debris from between the contacts.

Operate the relay in the 2 second on, 2 second off regime. See if the diode overheats; feel it with your finger but don't burn yourself. It shouldn't heat up at all.

For a more sophisticated test, get a 2 channel (or 4 channel) oscilloscope which can do trace math. Connect one channel across the diode to monitor diode voltage. Connect the other channel across the 1 ohm resistor to monitor diode current. Set the scope to display the product of the two measurements. Monitor the V x I trace for exceptionally high spikes. If they occur, find out why.

I guess its possible, This is a high rel application. (Military relay) The diode is contained inside the relay can. It passed in house tests at the manufacturer and was then screened by an independent QA house before being installed. It also passed functional testing, although I don't know the duration of those tests.

Take an un-mounted relay in your hand. Ohm the coil... both polarities... to determine diode direction. Operating direction should be cathode to plus.

My bet is that you have it backwards.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

My bet is you should have been more explicit in describing "cathode".