relays; general info

A lot of people get confused about relays.
Think of them as a switch.
A current amplifier.
A voltage converter.
A changeover switch.
Most common shape today is the Bosch/Tycho (who designed the shape) "CUBE" because of its 1" sides.
There's also a smaller "mini cube" half size.
Layout. The switch parts are the terminals 87 and 30 4 or 5 pin (terminal)
The changeover parts are 87 30 and 87a 5 pin (terminal)
The coil when energised does the switching.
30 is also known as the Common (C).
When the coil is energised 87 aka Normally Open (NO) and 30 (C) are joined.
With a 5 pin, at rest 87a Normally Closed (NC) and 30 are (C) joined.
On energising the coil 87a (NC) and 30 (C) are disconnected, and 87 (NO) and 30 (C) are joined thus the changeover part.
The coil terminals are known as 85 and 86.
International convention lists 86 as the POS (+) side, I try to follow that.
It's relevant if you are going to diode the coil.
Reason to diode the coil.
When the relay shuts down, the coil emits an EMP or spark or spike across those terminals of about 200 volts, the diode usually 1N4004 has a "peak inverse" voltage of 400 volts, thus sufficient to quench this spark.
If you are switching the relay from say an alarm ECU ("brain") there's a good chance you will fry it.
I believe it should be mandatory, others disagree but I've seen it happen to me and others when they've avoided my advice so it's up to you.
Some relays are ready made with these diodes internally, the diode icon will be on the casing. If so it will ALWAYS be 86 as POS (+) coil.
Resistors are now superceding the diodes.
4 or 5 pins? The price difference is so small that it's not worth only getting 4 pin units, just don't use 87a!
When salvaging relays from cars be very careful to note the pin configuration the type shown on this forum is actually a type II, type I mixes 30 and 85.

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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.

Nicely put. Nicely summarised.
Well done Howard!


To novices I start with your first 2 points - it's just a switch, but it's like a remote controlled switch that can also amplify current.
(For those into basic electronics, I describe a relay as a digital transistor ((except that the base is electrically independent)).)

The 3rd point (voltage converter) falls out of the "remote" aspect - ie, switch contacts are electrically independent of the input (coil) so they can switch "anything", eg....
But there I also add a warning about contact voltage ratings - ie, don't use 12V rated relays for 230VAC etc. That's stressing arcing etc, ie safety, NOT contact life.

As to always buying SPDT - I agree and advise likewise. (I still don't like calling then 4 & 5 pin due to the number of dual-87 5 pins. But for mini (cube) and micro types, a schematic usually exists on the body. But not so for other relays.   [As to electricians that call a 2-way light switch a 3-way or 3-terminal light switch (because each switch has 3 pins)...])

And of course the spike warning. I use the a car ignition coil as a literal example - even across their primary winding!

Funnily enough I forgot to mention SPST and SPDP or twin 87s and 87b.
That's for another time.
Give the punters a piece at a time.

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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.

the way i have always remembered 87a contacting 30 when the relay was off is "the "a" means "at rest""

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yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.

Nice one.
By the way always look at the connections.
As Oldspark pointed out there are also "look alikes" that aren't.
5 Pins twin 87 double output and the hard to find 87b format, twin outputs that are separated.

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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.

Hmmm, I like that. I reckon that beats my "must be 87a because it's not 87" and I know 87 is the normal contact meaning on/connected when the relay is on/energised.
Besides, if I strike an 87b or 87c, "that's not 87"...??! (But I know "a" is the NC contact from experience).


itsyuk's "rule" reminds me of how I remember what normally means in NC & NO contacts. Is it "normal" in normal operation?
No - its normal state is on the shelf in its box (awaiting our application... LOL).
Ergo, NC & NO refer to an un-energised or "at rest" relay. It doesn't matter if NO (Normally Open) is "normally closed" for 99.99% of its life.

Of course one of my old fave rules - which of blue or brown is active or neutral? Easy - if you touch the brown you poo (doodie) yourself!

years ago the only relay you could buy at walmart was the 87b version.they sold it to replace one in a dual foglite kit. you ran one light from 87 and the other from 87b.

ive made a few bucks over the years after a person bought a walmart 87b unit and couldnt get their device to shut off. then they would call me to figure out the problem. or they bought a relay from radioshack and couldnt get both walmart foglites to come on.  LOL

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yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.

Sssh! Don't give away our trade secrets.
I actually thought of putting the caveat, look CAREFULLY at what comes with 87 but after a couple of convenience threads where I want to kick sense into the OP, what's the point. Especially when they seem to do their stupids when my soccer team lost and I'm wizzed off to start with.


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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.

The description about the diode protection is partially misleading. The diode is connected in the direction of the spike current so it will not be using reverse voltage of the diode. The diode will not offer any resistance to the current in the direction of the spike but will let the current flow freely so the voltage will be minimal and the spike current will dissipate easily.

Hmmm, I don't disagree...
But misleading is dependent upon interpretation which only the reader can decide.

But it is an excellent point!

I found Howard's line (no pun) acceptable though it did take a double read.

My ideal would be turboled's description with Howard's qualification that the PIV of 400V for the 1N4004 (or 1kV IN4007) is enough to withstand the typical spike of around 200V ... etc.

I'd probably omit the resistance stuff and merely say the diode "shorts" (out) the (negative voltage) spike (that occurs across the coil ends).
The parenthised (text) is optional. "shorts the spike" is enough, but some want more of a description.

And then how much - shorts the spike to the ~0.6V diode drop, plus the diode's IR drop which will never exceed 12V anyhow...?

Alas the "simple info" dilemmas. Howard gave a bullet-point type of summary and gave lots of bullets. (Yes Howard, I know we both prefer dum-dums, or better.)
Where is the info line drawn, and how do we proof read for alternate interpretations?

Of course this thread could be an excellent platform to tweak Howard's OP for an eventual info page to augment the12volt's reference library (tabs etc).

That info page could be what I refer to as an executive style Master page/thread.
IE - the first post has a succinct summary. Later posts can expand on each summary - adding explanations, examples, tips, caveats, warnings. Such expansions may be links to other Masters or threads or references.
In general such Masters or threads should be closed to general replies, but there are many options.


Woops- there I go again with my mechanisms for other forums, or quality club & corporate documentation...
Rant ends.

Both very good points but I blew an alarm aux for not doing that so from a practical point of view the diodes are mandatory.
I've already had a "what should I do" post about 6 months ago, the fool didn't bother with diodes and blew his (low current) lock/unlocks AND all his aux outputs.
It would be nice if the alarm manufacturers bothered to diode the aux and lock/unlocks internally.
I haven't applied much theory which I'm not that good at it's more practical info.

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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.

howie ll wrote:

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It would be nice if the alarm manufacturers bothered to diode the aux and lock/unlocks internally.

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And ain't that the issue? If a device is reasonably expected to be interfaced to a realay, shouldn't it come protected for that relay (coil)?
Now that's an interesting test for our Consumer Laws LOL. But if specified to connect to diode or spike protected relays, and since they are a commercial offering... (BTW - do those resistor-snubbed relays always prevent damage to circuits?)

I have always included protection in my designs, and I've just discussed the same over at mp3car for a dude that's interfacing an Arduino to an Audi (instead lieu of his messed up OEM dash). There its the 2 reverse-biased diodes (else one Zener) from each of the rails (+ & gnd) to the input.   
And people here know my dislike for diode (and resistor etc) protected diodes, instead preferring to supply that (snubber) component myself - even in the relay base or connectors if it's not part of the existing circuit.


But Howard was definitely right including the protection diode in his OP. (We certainly didn't mean to suggest otherwise.)
Although for typical mechanically switched applications it may not matter, it's too important for the fewer times they are connected to other circuits. And the latter is becoming more common as people connect to modern switches which are likely to be electronics that drive other electronics (and are unlikely to have any protection for relays; obviously).
Of course you installers connect to such circuits as the norm, but for us ignorant DIYers..

That reminds me, I was going to use the term "quench" or "snub" instead of "short" in my last reply - something that sounded less tech but adequately conveyed what the protection diode does - in ALL dialects (and google translators).
Oh - & I was going to investigate if a series diode (to a relay coil) adequately protects. Though that doesn't quench the spike, I have read from good authority (hmmm, who would that be?) that that does protect. Ah - another simple 5 minute task (if that)... maybe during my shower?

Belt and braces Peter, I've often done a line diode AND a quencher, I buy them by the 100 anyway.

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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.

Certainly IMO no harm - unless the diode drop is an issue (like, hardly...).

And it may add one thing I like for critical circuits, namely redundancy. I've never liked the thought of an across-coil protection diode failing in open circuit mode - even if such IN400x failures are rare (assuming 400V or higher PIV, ie, 1N4004 or 1N4007, not IN4001 etc).


The series diode is also nice if anyone manages to connect +12V to the output. And yes, even that is a protection mode I sometimes allow for - more in case of wet relays or stray screwdrivers and dropped screws etc, or "flying lead" installations.