Educational Information about Relays

Educational Information about Relays: how they work, what they are, what they do. (If you need one, order below.)

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!

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. And it is too small to connect to light Bulbs or a large bank of LED's. 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. (I wrote "white of three" on the diagram. In Centrodyne meters, this wire is the white wire in a three wire cable, or on newer harnesses from Centrodyne it is a green wire, not white. In Pulsar brand 2030 meters, there is already a relay installed at the lower end of the cable. It is driven by the white wire in the cable between the meter and the relay. With Pulsar meters, you DO NOT have to add a relay - it's already there.)

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! :)

. . Here's an important Note: some relays come with a "diode" device wired across the coil. This is a one-way street sign to electric current, and allows juice to flow one direction but not the other. If your relay has one of these, and won't work... reverse the wires on pins 85 and 86... the coil leads. Put the wire you have on 86 to pin 85, and the wire now on pin 85 to Pin 86. That way the diode will not fight the current flow, and the relay will come back to life. (It really seems like it is a plot of some kind!) Some diodes are inside the case, others are actually connected to the pins themselves. If yours is outside the case where you can see it, you can just cut the leads and remove the diode. It will be a tiny black or glass shelled part about the diameter of pencil lead or slightly larger. You don't need it for this job.

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 one 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!

    Don't let relays befuddle you. They are, as you can now see, just switches which can carry a large current, and which can be operated by a coil which is connected to a circuit which can only handle a little current. What will these technicians think of next?!

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."

HERE ARE THE PART NUMBERS OF TYPICAL RELAYS BY MANUFACTURER/SUPPLIER. Any of these will work as shown above.

Hella Brand, Model 4RD-333-332-01 "Single Pole Double Throw Relay"
Hella Brand, Model 87499 "Single Pole Double Throw Relay"
Bosch Brand, Model 0-332-209-138-UM "Single Pole Double Throw Relay"
Potter & Brumfield Brand, Model VF4-45F11 "Single Pole Double Throw Relay"
or use Tessco Co. SKU Numbers: 473844, 39028, 57043, 95436 etc. (Tessco Co.'s Phone Number is 800-472-7373 or Go Online "Tessco.com")

ORDER FROM US (below), OR CONTACT YOUR AUTO PARTS STORE:
Tell them you need the same relay that is used in the Power Distribution Panel of a 2005 Dodge Caravan. They can look it up under their part number.

Now, brethern, go ye forth and relay the earth! -Fred Stock