Educational Information about Relays: how they work, what they are, what they do.
Basic concept one:
A SWITCH IS A THING MADE TO CONNECT AND DISCONNECT POWER TO SOME DEVICE. SO, WHAT IS A RELAY?
A relay is a remotely operated switch. Huh? OK, let's look at it this way... lets invent a thing that can turn other things on and off without having to touch it directly. Still not clear? OK Let's look at this way, in terms of your taxicab.
On the roof of your cab you put a toplight. It has several light bulbs in it which pull a pretty good current when they are lighted. Now, if you want them to come on when the headlights go on, you connect the wire to the headlight switch, right? WRONG! :( The power required to light that toplight will cause an overload of your headlight switch, and eventually it will burn out. You'll smell it burning a little at a time, then, kapoot! Bad Plan!
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See, (Basic Concept No. Two:) the current through a wire is limited to the current capacity of the wire, just like the water pipe can only carry what will fit inside it - beyond that it will burst. The wire will heat up and can burst into flame in worst case. So how do all those toplights that I see switching on and off with the meter work? Well, they have a relay in the circuit. A relay is a switch that can carry lots of juice, controlled by a little juice which can be switched on and off. |
Basic concept three: When you put electricity through a coil of wire, you create a magnet. It is a magnet while the juice is flowing, and is NOT a magnet when the current stops. Now look at the relay above. There is a coil of copper wire, wrapped in yellow tape, which has a steel bar in the center. When the coil is turned on (by connecting A+ battery to one screw terminal and chassis ground (A-) to the other) it turns into a magnet. The steel bar concentrates the magnetic field. The flat plate at the top of the relay is pulled down by a force stronger than the spring, and suddenly the output contacts are pressed together. That, my friend, is a "switch". When you disconnect one of the coil connectors, the current quits flowing, and the spring pulls the contacts apart, turning the "switch" off. Now the beauty of the thing is this; those contacts can carry 30, 50, maybe 60 amps of current. That's plenty for a toplight or headlights or a horn. But the COIL the caused them to go on and off only takes a little current - less than an amp - probably a whole lot less. You can safely connect the COIL (INPUT CIRCUIT) to the headlight switch without burning it out. Then connect the contact circuit (OUTPUT) to a heavy lead from the battery and up to the roof. Light up the whole roof with it - it won't care!
Let's look back at
that toplight again: Where I work, we use a toplight with all kinds of stuff on
it: The local regulators, in their infinite bureaucratic wisdom, have directed
us to make the sign light ("Airport Taxi" part) come on when the ignition
key is switched on, and the vacant light to be lighted when the key is on
and the meter is NOT hired. Then, when the meter is started, the vacant
panel must go off and the amber markers must go on. Now, the meter has
ONE WIRE which changes states (off to on) when the meter gets turned on.
So, what's a mutha to do?
Well, we used a relay - but a relay with a twist! This one has not one pair of "contacts", but three contacts! Notice the headlight relay at the top has 4 connections; two for the coil and two for the contacts. The little black relay here has 5 connections. Terminals numbered #85 and #86 are the coil connections, same as above. And the #30 connector is one of the output contacts. It is held in contact with #87a by the spring. When the coil is turned on, it disconnects the 87a lead and connects #30 (the "common") to #87. So all you do is connect that vacant light to #87a, and connect your amber "hired" lights to #87. Then you connect #30 to a key switched source that comes on with the ignition key. (That's the accessory circuit in your car.) That same accessory circuit is connected to the sign lights. Oh, the coil is connected to ground #86 and the signal wire from the Centrodyne Taximeter to #85. When the meter is turned on, the signal wire goes hot, and the coil is powered on, pulling the #30 contact OFF #87a and ON to #87.
Now, when the operator turns on his ignition key, the sign part comes on, and since the relay also gets power to #30 and the coil is NOT powered up, the vacant light gets juice through the #30 - #87a circuit. When he/she punches the meter on button, the coil of the relay gets power, and the current flowing through the relay coil makes a magnet, pulling the movable #30 contact off of 87a and onto 87. The vacant light goes out, and the amber markers go on, making our regulators happy, and the cab driver not have to pay a $1000.00 fine. Oh, GOOD! We support that! :)
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Just for
general information, Pulsar meters have the relay in a little shrink wrapped
pouch connected at the lower end of the cable. If you closely examine it,
you will find it also has five connections, and works in a similar fashion
to the Centrodyne relay I have explained above.
Also, if you start snooping
around electronic boards in equipment of endless varieties, you may find
relays with dozens of connections. They are all the same as these, except
they have many more contact sets in the output. If you have on set, as you
now know, you need five connections, if you have two sets, you need 8
connections: two for your coil, and three terminals for each contact set,
right? If you have three sets, you'll need 11 connections, and so forth.
Hey, mon, I tink you goddit! |
Another use for a relay is to separate circuits, and share the intelligence. Let's say you have a circuit like a very high voltage industrial circuit in some machine. You want a little indicator light to go on or a computer device to be turned on when that original circuit is turned on or off. You can connect a relay with a proper coil voltage specification to that heavy duty circuit, and switch on or off the other equipment without connecting it directly to the high voltage. So a relay can also act like a "buffer". As the guy in "Godfather" said to the congressman, "Yah, the family has alot of buffers."
Now, brethern, go ye forth and relay the earth! -Fred Stock