Printer Friendly Forum Posts - do all relays need to be diode isolated?

Hey just researching some information and happened to run across information about using a diode to protect the relay coil from a reverse spike.

At least this is what I believe is what the purpose is.

Do all relays need this protection? My application for relays on my alarm are as follows:

Vehicle is a 1992 ford mustang.

1) Relay to reverse polarity of my parking lights. Ford's parking light wire is (+). I don't want my back up battery(dei 520t) to power my parking lights if the main 12v supply is cut. BTW I have not purchased the relay yet. So I will wire a relay as such:

pin 85: (-) ground signal supplied from alarm
pin 86: (+) 12v constant direct from vehicle's main battery
pin 30: (+) 12v constant direct from vehicle's main battery
pin 87: (+) parking light wire

2) Relay to pop trunk. Wired into the (+)12v wire of the 2 wire switch in the glove compartment. I will be tapping into the (+)12v wire. The relay will send a (+)12v supply to this wire to pop the trunk.

pin 85: (+) 12v constant direct from vehicle's main battery
pin 86: (-) ground signal supplied from alarm
pin 30: (+) 12v constant direct from vehicle's main battery
pin 87: (+) trunk release wire

3) Relay that came with alarm viper 350 to kill starter. Wired as a basic starter kill diagrammed here on this page from the12volt

https://www.the12volt.com/relays/page2.asp

4) A DEI 451M to control my 5 wire door locks. This appears to be two relays in on small box.

Thanks for any info! I just joined this site and everyone is very helpful and I'm lucky to have found this site. One day I hope to be knowledgeable enough to help others here. Right now I'm a newbie posted_image

Opps the other relay I will be using is for my Sirens.

One siren is the siren which comes with the alarm system for under the hood.

I have purchased a second piezo siren from radio shack that draws a max of 150amps.

I will wire the relay for this as such

pin 85: (+) 12v from alarm siren output
pin 86: (-) ground
pin 30: (+) 12v tapped into from DEI 520t grey wire 12v supply to alarm's (+) 12v supply
pin 87: (+) 12v to both sirens

Hey all,

Trying to figure out how much current the 520t "diode box" is designed to output/handle. I am already going to upgrade the battery to a 12v 5ah. The 0.7ah is to small in my opinion.

The reason I ask how much current it is designed to supply is because I want to wire a relay to the grey (+)12v alarm power supply to power additional sirens. If I wire the sirens to the main vehicles battery 12v supply and the main car battery is disconnected the sirens will not work when wired through the relay I plan to add.

Here is what I am planning to do with the relay:

pin 85: (+) 12v siren output from alarm
pin 86: (-) ground
pin 30: (+) 12v supply from grey wire coming from 520t "diode box"
pin 87: (+) to siren's

Thanks for your help! posted_image

Not all relays need diodes across the coil - but it never hurts to add them.

All outputs of an alarm that are designed to power relays should have the diode built in to the alarm.

The battery backup should be used to power only a siren (no lights or anything else). This will keep the noise going on longer with the smaller battery. I would highly suggest getting a battery back up siren instead of a battery back up system. With the battery back up the thief can just cut the siren wire(s). With the battery back up siren you can cut all the wires to the siren and it will continue to make noise.

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Kevin Pierson

No relays need protection diodes.
Only the circuit that attach to them do.

And don't confuse a reverse-biased diode across a relay's coil (#85 & #86) with a isolation diode - they are two completely separate and different functions.
One quenches spikes.
The other stops circuits feeding other circuits. (Though that can include the spike....)

PS - your piezo siren draws a max of 150mA, not 150A.

KPierson wrote:

Not all relays need diodes across the coil - but it never hurts to add them.

All outputs of an alarm that are designed to power relays should have the diode built in to the alarm.

The battery backup should be used to power only a siren (no lights or anything else). This will keep the noise going on longer with the smaller battery. I would highly suggest getting a battery back up siren instead of a battery back up system. With the battery back up the thief can just cut the siren wire(s). With the battery back up siren you can cut all the wires to the siren and it will continue to make noise.

Hey, thanks for the response. From my understanding the 520t backup battery is designed to power the entire alarm not the siren alone. So I was going to use a relay for the sirens because my research on the threads from this site indicate that the siren output from the alarm is designed only for a single siren and adding more will damage that output so it's was recommended to use a relay to power the sirens. So instead of the alarm powering both the sirens I was going to just get power from the same point the alarm receives its power.

Also I am hiding all the wiring with a stealth install. They wont see any wires for the siren or the siren itself. I will definitely look into the backup battery siren.

How do I wire the backup battery siren with the other sirens? Use the relay I'm talking about and use the alarm siren output to power the coil for this relay?

oldspark wrote:

No relays need protection diodes.
Only the circuit that attach to them do.

And don't confuse a reverse-biased diode across a relay's coil (#85 & #86) with a isolation diode - they are two completely separate and different functions.
One quenches spikes.
The other stops circuits feeding other circuits. (Though that can include the spike....)

PS - your piezo siren draws a max of 150mA, not 150A.

Opps! My mistake, I can't edit any posts, not sure why posted_image

You are correct it draws a max of 150ma.

So would any of my relays need this isolation diode?

Here is the thread from this site I am referring to.

https://www.the12volt.com/installbay/forum_posts.asp~TID~86348~PN~2~TPN~2

If you scroll down to "dual sport's" diagram, this is what I am referring to.

Neither of those are isolation diodes.
They are both suppression (quenching, protection, etc) diodes that are there to short any negative transient spike from the coil - especially when turned off.

Do you need it? You tell me....
As KP wrote - "it never hurts to add them".

If you are unsure, you can omit them. Then if whatever controls them blows - or if something else blows - you can let us/me know that you did need them.

PS - WRT that link - yes, that imaged text is wrong. I think that's the same text I donged somewhere else. The diode's line/kathode end is towards the +ve side of the coil.

Generally agree with Kevin and Peter, normally, you won't need the diodes when using a switch but ALWAYS fit them across the terminals as a precaution.
I don't agree with Kevin about alarms having internal diode protection, also the circuits in these alarms such as aux and locks are such low current that they WILL be burnt out by a reverse.
ALWAYS diode divide alarm inputs/outputs such as hood and trunk using the same trigger wire, separate door triggers , aux outputs.
Always use 86 as the POS and 85 as the NEG coil terminals, it's the international convention, yes if the relay doesn't have an internal diode it makes no difference, but if you stick to that, less chance of mistakes. At the rates I buy them, 1N4004 diodes cost me less than a cent each, it's worth it for thoroughness and piece of mind.
Remember the Ps, Proper Preparation, Prevents *iss Poor Performance.

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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.

Here is the datasheet to a very common output driver used in alarm systems:

https://focus.ti.com/lit/ds/symlink/ulq2004a.pdf

Page 3 shows the diode between the output and COM pin. I know that DEI used a similar chip in their older alarms - not sure what they are using now but I would guarantee that the diode is built in.

A good illustration of this is the fact that the 451M does not have any diodes in it. However, it plugs directly in to most DEI alarms. In order for this to work the alarm would HAVE to have the diodes built in to the door lock outputs.

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Kevin Pierson

OK Kevin, point taken, I still prefer to use them because the cost and time factor makes ME feel better, incidentally why do they (DEI) ask you to use diodes on the 5902 diagrams.

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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.

Like I said before - they won't hurt!

What diagram are you talking about - I looked through the 5902 install manual and didn't see a single diagram showing a diode added across the coil?

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Kevin Pierson

You might be right, it was the 5901 lock/unlike wires but that used a flip flop output so that was probably the reason. Its 9:30pm here, I'm watching TV so I can't be bothered to track down the reference, there are about 5 revisions to 5901/2 installation diagrams

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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.

Hey old spark,

What could I do to test or check and see if I need a reverse biased, isolating or suppression diode?

And would it be to much to ask you to explain how to wire these up if I do decide to add them for a little extra protection for my wiring.

Thanks again

If you are connecting a relay to a 200 milliamp output of an alarm, it already has a diode. if the output is only capable of 200 milliamps, they know you are connecting a relay to it.

The diodes you mentioned are all the same.

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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 am an idiot wrote:

If you are connecting a relay to a 200 milliamp output of an alarm, it already has a diode. if the output is only capable of 200 milliamps, they know you are connecting a relay to it.

how about the relay I will be using for my sirens? I'm going to be using the (+)siren output from my alarm to power the coil

using multiple sirens with that 520t will give you a couple of mins max if so much at trigger, along with four blinking bulbs, you might barely get anything.

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

The siren output of an alarm is driven by a transistor. It will be fine with or without the diode.

Thanks I am an idiot. I want to run the relay for the sirens just to be sure i don't over draw current from the alarms siren output.

t&t tech - do you say this because of the low aH battery that comes with the 520t is to small to drive the sirens or is it because the 520t diode box cannot handle the current needed?

BTW I am using a relay to reverse the polarity of my headlights so the backup battery will not be activating the parking lights. and my additional siren only requires 150mah.

Wow - away for a day an look what happens...

Just to address the questions to me...

madmanuser wrote:

What could I do to test or check and see if I need a reverse biased, isolating or suppression diode?

Just fit them. Prevention is better than cure.
Otherwise test by seeing if you blow something up without them (the trial&error method described before) or obtain all circuit data pertinent to your set up and determine if protection is needed.

madmanuser wrote:

And would it be to much to ask you to explain how to wire these up if I do decide to add them for a little extra protection for my wiring.

Yes. Just fit them. (Not too much to ask, but it is too much to answer.)
It has already been explained in this thread or at linked diagrams and threads. IMO there is no point repeating it - especially as you still seem to include the isolation function....

Just fit the diodes and be done with it. At under 20c each, why bother?

Hey good points old spark.

I did some more research on this topic and it appears that using only a rectifier diode can reduce the life of the relay unless that diode is run in series with a zener. Here is the pdf from tyco electronics on this topic.

https://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

Have any of you had any issues with relay life using only a rectifier diode on its own without the zener?

Yes and the average Tycho, or Bosch or Omron or Panasonic auto motive relay has an average live expectancy of about 15 MILLION cycles so I wouldn't worry too much about zener diodes.
The whole point is the diode YOU need is a 1N400X.
Use to differentiate inputs, example, units with single trigger for hood and trunk, since in most cases the hood pin is aftermarket, it may fail. If it does it might short out causing the trunk light to illuminate giving a flat battery. Thus diode separation.
Diode across a relay, I know and appreciate what KP and Mr. I said, I just feel safer since I blew out the aux circuits on an older Clifford using a relay without a diode a few years ago.
I'm with Oldspark's last post on this.

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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.

As per Howard.

And from a brief peruse, it seems that article is from "the standpoint of physics".
There is no mention of what reality is (ie, no statistics).

And note the transient with the zenor - 25V - what does that do to a 12V circuit?
Mind you, the article is about the relay and not the circuit, but why not use a 16V zenor or less instead? (Reason - it gets too complicated or the zenors will blow.)
And a diode in series with a zenor?
I'd really expect a better article if it were used as justification for anything. It falls far short or normal research documentation

Let's all stand back here, Mr.I, KP, Oldie and myself have all been doing this a long time and have our own methods which WORK.
Our poster is asking questions from either ignorance or lack of REAL WORLD/PRACTICAL knowledge.
I didn't write this post to have a hassle with KP and Mr.I, two people I respect, I would like to point our poster in the direction of asking specific questions in relation to specific problems.

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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 point taken guys. It wont be an issue. I just didn't want to hide everything and have to dig it up because a relay died due to the use of a diode without zener. But if the life expectancy is 15million cycles then your right it won't be an issue. Once again you guys FTW with all the info I need.

I don't bring up articles to try and prove you guys wrong, I just want your input as to what the article says and if its really something we should consider. I respect everyone's real world experience and will hookup a diode to my relays.

BUT!!!! I have a problem that arose as i was wiring everything up as a test run before final install.

So I am using a DEI 451m. This unit is for the door locks, as you guys probably already know. My car is a ford mustang and uses a 5 wire door lock system.

So I have the yellow ignition wire for the starter cut relay wired up to my ignition wire and it receives (+)12v from my ign wire when in run or start.

What is happening is that when the ignition is turned on then off a slight click is heard in the 451m. Not sure where this feed back is coming from.

The 451m has a red and purple wire that are (+)12v constants, and I have those two wires wired up to the 12v constant directly from the battery. The blue and green lock/unlock wires are connected properly to the alarm.

Everything on the alarm works correctly such as lock unlock when armed/disarmed etc. Is it possible I need a diode for the (+)12v constants going to the 451m? Or is it possible power is being sent from the alarm to the 451m?

Not sure what to do at this point.

posted_image

Alarm is the basic viper 350. I know the alarm does have a function that locks the doors three seconds after the ignition is turn on.

Maybe this sound in the 451m when the ignition is turn off has something to do with a setting in the alarm?

WOW I am a noobie. Went to my car and long behold it was unlocking the doors when ignition is turned off.

Went through the menus using valet button and turned off ignition controlled locks.

Thanks anyways. Onto wiring these diodes before final install..everything seems to be working great so far.

I wish I could edit posts posted_image

Anyway, forgot to add a question.

Thanks for your patience also!

So I reversed the polarity of the parking lamps on my car because I don't want my backup battery powering my parking lamps.

So I have it hooked up like this

pin 85 - ground signal from alarm
pin 86 - (+)12v constant
pin 30 - (+)12v constant
pin 87 - (+) parking lamp wire

My question is where does the 10 amp fuse go?

in between the (+)12v constant and pin 30 or in between pin 87 and the parking lamp wire? BTW the (+)12v constant is already fused within 12inchs of the battery.

Thanks

The wire that feeds constant voltage to 30, place the fuse wherever that wire is connected to the power source.

Cool thanks, hey out of curiosity what is the reason it is preferred in the location you suggest?

Well if you think about it logically, since pin 30 is the pin where current is introduced to the circuit it would be wisest to fuse it here as close to the source as possible as it minimises the risk of possible grounding and not having a fuse to protect the circuit, if you were to fuse at pin 87 and the wiring grounded out from the side of pin thirty? Catch the drift? I thin you're covered since you already fused within 12 inches of the battery, but hey you could go closer, posted_image

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COMMIT YOUR WAY TO JEHOVAH AND HE WILL ACT IN YOUR BEHALF. PSALMS 37:5

European Telecom rules mandate the fuse 4" (10cm) from the source; battery, fuse box, ignition, etc.

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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.

Interesting. But I assume that does not apply to BIG telecoms batteries...
(Australia's telecoms regulating body Austel once mandated fusing for all telecoms-exchange batteries. That requirement was later withdrawn.

Bu distance (from source) specs vary greatly with the authority and the application.

MOBILE comms Peter.
For years, Orange insisted on both POS (fused) and NEG going to the battery.
I used to argue that the spurs on say Mercedes under front passenger floor measured out at 12.6V+ to the nearest ground point but they wouldn't take the chance. Even made us fuse the head unit mute line with a 1amp fuse, a bit like placing a 500amp fuse on a domestic cooker line then wondering why your house just burnt down.

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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 just saw the "European Telecom".
I thought the ignition was strange, but assumed that was the result of the fusing (ha ha), though I wonder what ignition they use in exchanges anyhow aside from diesels.

Mobiles tend to be less centralised and use less power than landlines.
They also use 24V and -ve earth/ground as opposed to +ve earth/ground in traditional 48V telephony systems. That could account for Orange's philosophy depending on what the common connection is between systems. Though normally the grounds would be commoned, that means a 48+24 = 72VDC system which exceeds LV DC requirements (ie - HV DC Licenses etc are required...)

Ah - Mobile as in mobile - not mobiles (cells, GSM etc).
Does that mean all vehicles with mobile comms must have fuses within 10cm of the battery? That's put most cars of the road here. (Some might be physically close to 10cm, but not electrically.)

I'll have to see what fuse my mobile has (if any), though that will be within 10cm.

Or maybe one day I'll read those Rules instead....

A 1A mute fuse - yeah, it's a bit like fusing a voltmeter....

There's a whole British Standard and because the rest of Europe is too lazy to test car products (Thatcham in Berkshire, UK), any British Standard becomes the de facto Euro Standard, and to some extent Oz and NZ.

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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.