Thread: Did Ya know the difference? 4-pin relay / 5-pin relay

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For years I have known about relays. Use them for fan, headlights, horn----in fact anything that has a high current draw, where you don't want the high current carrying wires inside the passenger compartment, or else don't want the high current carrying wires to travel too far, which causes a voltage drop. You mount the relay on the firewall in the engine compartment, route the heavy wire from the battery to the relay, then from the relay to the high current device. Then you have a relatively small wire, carrying a low current that runs from inside your car where the switch is located out through the firewall to the relay. When you turn on the switch inside the car, this "small" current flows through an electromagnet inside the relay, then to ground, which pulls the contacts inside the relay closed, and lets high current from the battery flow through the device which you want to power up.
Now I realize that this is pretty basic stuff, and that this requires 4 pins on the relay for wire connections. Last week I bought a relay, and when I got it home, it had 5 pins----a real head scratcher. After a bit of internet searching, I found out something interesting. A 4-pin relay either is "on", letting current flow, or else its "off" and not letting current flow.
A 5 pin relay, lets current flow in either of 2 different paths. Thats what the extra "pin" is for. When the relay is in the "off" position, it lets current flow out through one of the pins-----when the relay is switched to the "on" position, it switches the current to flow through the other pin, acting like a 2 position selector switch. If one of the pins is not hooked up, then the relay acts the same as a 4 pin relay, giving either "on" or "off".
This is kind of neat, and I really never knew this before.

Yah, this is pretty common in commercial electrical appliances, and in refrigeration/air conditioning - my business. There should be markings on the relay showing which is NO (normally open) and NC (normally closed). It's easy to prove with an ohmeter. Where people get messed up is with "open" & "closed." Think of a light switch - when the points are touching (closed) current can flow and the light will be on. Flip it "off" and the points will "open," current can't flow, and the light goes out. The coil voltage and switched voltage don't necessarily have to be the same, tho' I'm not sure how that would apply to a car. Maybe using 12VDC in a 6 VDC car ?? I use them a lot for having a 24 volt coil connecting/disconnecting a 110 or 220 (or 480) VAC circuit.

On a 5 pin automotive relay, the pins are numbered, 85, 86, 30, 87, and 87A

As near as I can tell, 85 is connected to the switched low current wire ) from inside the car), 86 is connected to ground (or through a switching device which goes to ground to complete the circuit, like a 2 terminal thermostatic fan switch in the engine water jacket), 30 is connected to the heavy (#10 ga.) wire from the battery, 87 is the terminal that carries the electricity from the relay through to the "device" being switched (fan, horn, etcetera) when the relay is energized. and 87A is the alternate terminal that carries the electrical current to a different device when the relay has no current fed into terminal #85. If you want to run the relay as a simple switch, you don't hook anything to terminal #87A. ---this is based on tests conducted in my garage.-----Brian

Yes, you can make up cool things using relays.

As a bit of useless trivia the first digital computers used relays as just desribed (C/NO/NC). Two were connected together to form what came to be known as a flip-flop, later replaced by tubes (valves), then transistors, then the modern IC.

Cheers, Mark

see if this will help brain
http://www.the12volt.com/relays/relays.asp

Standard SPDT relay, Brian.

O.K. Guys---The reason that I'm dickin around with relays is this---I met a fellow in town here with the sweetest 30 model A closed cab pickup that you ever seen. He bought it as a roller with a busted Ford 302, and bought a 350 G.M. crate motor and 350 Turbo package to install. He is a nice guy, but he isn't real up on automotive wiring, and the old Ford motor ran a mechanical fan. He wants to run an electric fan with his crate motor, so asked me about how to wire the fan, where to get the thermostatic control switch, and how to wire it all as a package.----I just went through all this with mine a short time ago, so I am helping him out. The post about relays was just a "general interest" post.

Denny---Nah, it waren't a test. Old poops like me and you know most of this stuff, or some version of it. There's a whole world of young-uns growin up out there to take our places, and sometimes they're not sure. Hell, sometimes I'm not sure.
This was just a general interest post, to get them thinking. It gives me something to do on a long weekend besides driving my wife crazy. I've cruised the roadster pickup so much I'm conservin gas, playin on the computer.

FYI, those type of relays are fairly common in the aircraft industry.