2008-08-12

Relays, what they are, how they work, and zowie stuff they can enable!

I'd kept coming across mention of "relays" in electronics publications, but couldn't wrap my mind around them. Like, how can a low-voltage circuit control a high-voltage circuit?
I mean, if they are connected the one will over-power the other, right? Blow that puppy out of the sky over-power, no?

No.

Having just had my satori, my moment of ah-ha-ness, I seek to pass this vision on to others, that they not be mired in darkness as was I. Bear in mind that rampant anthropomorphism follows, the imputation of feelings to non-organic constructs that their interactions may sink more deeply into your psyche, that you may make this knowledge as one with your being.

You have two (or more, yes, more is possible, oh seeker after truth) circuits, wires, running close together, close enough that magnetic fields generated by the passage of current through t'one wire can influence thangs attached to t'other wire, see? Now, t'wires, all o'dem, dey be passing through dis here relay, see? And d'relay, well, hmm, deys several kinds o'relays, norm'ly open and norm'ly closed.

[return to normalish English detected]
[jokes about Vulcan, Irish, and Italian names inspired by the above gratefully accepted]

Normally: refers to the at-rest state of the relay, the state in which its most happy, where it needs neither respond to nor influence it's surroundings.

Open: refers to being non-attached, apart, undedicated, non-conducting, as in an open relationship.

Closed: refers to being attached, together, dedicated, conducting, as in a closed relationship.

An electro-mechanical relay is the simplest kind of relay, where a physical coil lies next to a physical spring, [whether the spring be like a slinky or a flexible strap of metal is immaterial at this level, OK?], and the flow of electrical current through the coil determines the state of the mechanical spring, whether it is relaxed or stressed, at-rest or not-at-rest.

When current is applied to the coil a magnetic field is created which changes the charge of the coil in relationship to the spring, resulting in the spring either being attracted or repulsed by the coil. This causes the spring to move ... um, big whoop you say, right, left something out, I did.

Gate: the spring is part of one (or more) gate(s), and controls if its open or closed. Each gate is a break in a circuit and controls the flow of current through that circuit. If open, no current flows, if closed, current flows. The at-rest state of the gate determines if the relay is normally open or normally closed.

In a normally open relay, the flow of current through the coil causes the spring to change to a closed state, closing the gate, and permitting the flow of current through the wire(s) controlled by said gate. Conversely, in a normally closed relay, the flow of current through the coil causes the spring to change to an open state, opening the gate, and forbidding the flow of current through the wire(s) controlled by said gate.

[Think of it like an electric fence on a ranch, the simplest of them require all gates being closed to allow electricity through the fence, any of them being open breaks the circuit and no current can flow. In theory, anyway; it's still not smart to fiddle with them unless you have the source locked off, as with all electronics.]

So, depending upon the sensitivity of the spring, it doesn't take much current through the coil to produce a strong enough magnetic field to flex it from it's at-rest state to it's not-at-rest state.

Sensitivity: See, I told you anthropomorphism was involved! Sensitivity has to do with how easily something is influenced by external factors, and its just the same with physics as with emotions. And like with people, the more sensitive the relay, the more it has to be shielded from outside influences. A high-voltage circuit controlled by a low-voltage circuit requires a very well shielded relay to insure that the on/off state of the high-voltage circuit is not toggled unintentionally by external sources.

With a properly constructed normally open relay, with the controlling circuit being low-voltage and the controlled circuit high-voltage, closing a switch on the low-voltage circuit will close a gate within the relay allowing current to flow through the high-voltage circuit. This is handy when you want to take a bunch of lights in a room which weren't wired together and control them from one location; [make sure that all circuits are shut off at the source before doing this!] determine your control location, run low-voltage wire from relay-controlled lamps to the control location, install your switch(es) and run a line out to a proper transformer, [double check everything to make sure you did it properly] turn the mains back on and use the new low-voltage switches to control your lights. This is, I believe, how automotive headlights work, and it definitely ties in with stage lighting and sound boards, take a gander at the wire bundles attached to them and they are low-voltage cables.

Umm, I just spent 2½ hours on this, way past time to eat lunch, will do more with this later!

So...

Post this Puppy!

1 comment:

evils-thoughts said...

You have known an electrician for a long time, ok we have not seen each other for years but that’s besides the point, you could have just asked me. I would have explained it as simple as it gets. A relay is a remote control switch. That is all. I have used “high” voltage relays to control “low” voltage as well as the reverse. It does not matter. Any time you can use a switch of any size, you can use a relay to do the same job, but from a remote location.

It’s nice to hear from you.

David/Dietrich