ECN Forum Forums General Topics of the Trade General Discussion Area What is the primary purpose of a relay?

For another example, look at telephone exchanges. Before the age of electronic and digital switching, the typical telephone exchange contained literally thousands of relays (not to mention all sorts of other electro-mechanical switching devices).

Some relays could have just a single set of normally-open, normally-closed, or changeover contacts, but others could have a coil operating a dozen sets of contacts.

Other "tricks" were possible, for example by placing suitable magnetic slugs on the core, you could make a relay "slow to operate" or "slow to release." That was often used in complex telephone switching to insure that one relay would always operate before or after another.

And then there were the "unusual" relay configurations, such as latching relays (apply one pulse to the coil to turn it on, another pulse to turn off), dual coil relays (one coil for on, one for off), and stepping relays (multipole switches, with each coil pulse advancing the output position by one "step". They usually had a separate "reset" coil, which would jump the output back to "zero" when pulsed. These were widely used for telephone exchanges, counting pulses from rotary dials to complete calls. Originally called a "Strowger switch" after the inventor.

That's a good question to answer with a question.

What does a relay do? Well, what do you want it to do? Don't forget a whole line of protective relays. I fixed 3, 150Y rate-of-rise relays just last week. They have a power supply, balanced input amp, differentiator, threshold comparator, timing circuit, output relay, and an SCR trip circuit. With all that built in, it is still considered a "relay". Don't forget the time delay relays that might keep your compressors from short cycling or give your fan motors a little time to spin down before reversing. That phase loss/reversal/under/over voltage monitor is a relay. I've used bistable impulse relays to replace Hg switches on an alternator table. I think we used a UHF coaxial relay if we had to switch our transmitter into "emergency multiplexing" mode. And finally, I still have to find a use for those stepping relays I picked up at the electronics surplus. They looked like they would be fun to play with at the time.
Joe

The old IBM accounting machines (402 and 407)could read a card, tabulate up to 20 columns, transfer data from all 80, save data from one card to the next and print out the results at the rate of 120 a minute, all done with relays, brush emitters and ratchet counters. These things were programed with a patch panel that had about 2000 holes you could plug a wire in to select various functions. The wire contact relays in these things could be up to 12 pole double throw. I have a few if someone wants to see one.

The primary purpose of a relay is to relay on a telegraph signal that's getting weak. That would be why it's called a "relay".

Every other use is an afterthought.

[This message has been edited by SolarPowered (edited 05-14-2006).]

... but there was a lot of "afterthinking"

I bet ol' Ben Franklin invented it!

Alan

quote:Every other use is an afterthought

... but there was a lot of "afterthinking"

So does that make any relay not designed for telegraphy, a "time delayed relay"?
Joe

My understanding of any relay is to isolate the Control circuit from the circuit being controlled by magnetic (not electrical) coupling.
Just in the same way that an Opto-isolator does with an LED and a photo-transistor.
Relays can be cascaded to provide any number of contacts (being NO or NC) or the use of higher voltage coils.

Indeed, there was! And is.


I could be wrong, but my recollection is that Samuel F. B. Morse invented the relay, for the purpose I described above.