relays; general info

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.

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

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.

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.