anti spike diodes on relay coils

I've noticed that anti-spike diodes across relay coils are often encouraged to be utilized by some posters here. I understand that they are certainly a good idea and will cause no harm, but are they really necessary?

I'm a DEI guy. I have yet to see a stipulation in an install guide to use anti spike diodes, or relays fitted with them. DEI mentions "do not exceed 200ma" in every outputs' description, but never asks for anti-spike protection.

Again, I know it will cause no harm to use anti-spike diodes. But it's just another thing to incorporate into the install, and on a complex system utilizing many outputs will add significant time.

DEI doesn't ask for them. If you don't generally use them either, have you had any problems that you attributed to relay coil spikes? Are they really necessary? Or just a "good idea"?

Thanks for reading, and offering an opinion.

I've always stressed them, about 8 years ago I blew an Intellistart lV and the aux outputs on a Clifford Alarm so it's been my religion ever since!
A standard automotive or any 12V+ coil relay "spikes" at 200 volts.
In theory sending that back to the chipped or transistorised output on a DEI or any other brand can fry that circuit. Early last year a newbie did just that after I suggested the diodes and he didn't use them.
Another eminent forum member Chris Luongo NEVER uses them.
The bottom line is if you haven't used them yet and had no problems, carry on but I'll still advise people to use them.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I'm with Howie. Often not needed, but like a 'useless' fire extinguisher, when needed it's too late - or that's when you find how useless it wasn't.
Some say the have been useless because they have NOT blown anything, but they had the diode so how do they know it wasn't the diode that has saved them? (That's like the old post-y2k argument LOL.)

DEI probably don't specify diodes because they are of good design and hence probably have diodes on all outputs.
It's for the other designs (or if the onboard diodes have blown) that you add your own.

Davep, you joined in a thread posted today where someone actually appears to have done that with a Viper 3203.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

It's difficult to judge an individual's qualifications from just a post or two. Who knows what that guy did? I suggested "programming error" in that thread because that's been the source of most of MY unexplained nuances with my system. It's taken me time to learn the DEI nomenclature, terms, definitions, and sort through the clunky Japanese to English translations.

I posted my question in this thread regarding the use of anti-spike diodes because DEI doesn't say to use them, where they say "do not exceed 200ma" for every wire this applies to. I think that if DEI was concerned about relay coil spikes, they would demand anti-spike diodes, or the use of relays fitted with them, the way they demand that current limitations not be exceeded.

I was curious to know what others who do this stuff all the time had done, and found in their experience.

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I have no idea what that guy did to blow his 3203 up. Nor has he come back and post the resolution to his problem. I did notice that he is triggering "door poppers" through relays. If his B+ source for the relay contacts AND the relay coil are common, hey may well be hitting the 3203 with spikes through the common B+ connection at the relay from the popper solenoids when they deenergize .

Solenoids that draw several amps are quite different from a relay coil that draws 120Ma. The spike is significant with a solenoid. So you may be right that the use of an anti-spike diode may have saved his 3203. But in this instance the diode would be most effective when placed across the solenoid terminals, not the relay coil.

OK, thanks for your input.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

OK. Thanks for killing my thread.

Killed? What did I do?

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

A couple of thoughts...

In answer to your main question, I have never had a controller die in a way I could directly attribute to a relay coil spike. Not on any
of my installs nor any that I have removed / replaced installed by others. I never see coil quenching diodes in these removals but then
again, I rarely see any solder joints in them either.

Some manufacturers claim their R/S's output are "overload protected". That might mean a current draw overload will not damage
their controller's circuits and it might mean a voltage spike won't hurt it either. I don't know but any extra protection is good. While I
don't add quenching diodes to all low current (-) outputs, ones that drive relays that can receive multiple coil energizing pulses in quick
succession do. Call it cheap insurance or better safe than sorry, either way, I haven't had any "come backs" with failed R/S outputs
that controlled relays.

Actually, your question is quite valid and something that could be asked the next time you contact DEI Tech Support for assistance. It
would be interesting to hear their response.

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Soldering is fun!