plain ol Bill

I'm curious (and electrically challenged also) about what a relay is and what they do in a circuit. Could someone that understands electrical stuff explain it in plain enough english I could understand it.

Willys55

It allows a circuit that runs a higher ampherage to be activated by a smaller amphere switch/circuit. (smaller wires and lower voltages[sometimes]to activate or control higher amps,larger wires and voltages.....Like headlights, starters, blower motor etc...etc

Flying Jester

You know how when you flip a switch, the light goes on? A relay lets electricity filp the switch. it says "If electricity is flowing between these two pins, I'll let electricity flow through these pins".

It's used because a normal flip-switch is often not heavy duty enough to handle the flow of the circuit, or because it's a more subliminal trigger (like heat via a thermostat).

And damn, ****** beat me to it!

dyegunswin

yeah basically low current controls high current. current passes through a coil in the relay and electromagnetism pulls a little metal "arm" or lever over, which touches a contact, which in turn closes the circuit allowing current to flow through the circuit.

Running Free

http://www.the12volt.com/relays/relays.asp

5-90

A relay is a "switch controlled by a remote switch," in simplest terms.

What a relay does is:
- Allows you to use a small switch in a convenient location to control a large load.
- Allows you to use some varieties of "hidden switch" (viz: a magnetic reed switch) as a security device to cut out a critical circuit - like the starter motor solenoid. Also allows the use of an "exotic" switch - say, a pressure mat, a mercury tilt switch, a reed switch (as before,) water flow over a pair of contacts (I'd set a couple of those up for "flood alarms" for people,) or any of a number of other actuation methods. Either the switch will only tolerate a low current; or the use of a current-limiting device on the switch side is necessary for safety, but a high-current load must be switched for function.
- Allows a low-current electronic circuit to control a high-current accessory (typically something that draws a rather higher load than a reasonably-priced transistor can handle - a transistor can be thought of as a "solid-state relay" with no moving parts.)
- Allows you to run a more direct lead from the power source to a high-current accessory (reducing conductor losses.)
- Allows you to eliminate the switch (user interface) as a "bottleneck" for current transfer.

Typical automotive relays (DIN relays and variations) can be had with ratings from 20-40A, while mini-DIN relays (about half the size) tend to be good for around 10-30A. Critical current ratings for a relay:
- "Trigger": This is the current that the switch has to be able to handle. Typically 150-300mA (.15-.30A)
- "Make": This is the current that it can switch ON reliably (this is often the nominal rating.)
- "Break": This is the current that can be switched OFF without causing immediate contact failure. This is typically a bit higher than the "Make" rating, for safety's sake.

There are three typical switching arrangements for a relay in the automotive world (applies to both DIN and Mini-DIN relays):
- SPST (Single Pole, Single Throw): This is a simple ON/OFF relay, and will have four terminals (two for the trigger coil, one for the power source, and one for the load.)
- Changeover: This relay will always have one circuit ON and one circuit OFF, and the two circuits will change state when the coil is energised. These are the most common five-terminal relays, and may also be used be readily used in place of SPST relays.
- Twin-Circuit: This is also a five-terminal relay, but the two load terminals will either be both ON or both OFF. I like these for running pairs of lamps - one lamp gets wired to each output terminal, so the lamps are on a parallel circuit starting at the relay. Just seems to work better, to me.

Terminal Identification - Since the DIN relay (and its variants) are probably the most common version out there, here's how the terminals are identified and wired.
- 30: Power feed from battery/distribution
- 85: Trigger coil ground
- 86: Trigger coil feed from switch
- 87: Relay Output Normally Open (OFF)
- 87a: Relay Output Normall Closed (ON)

The SPST variant will be lacking terminal 87a. The Changeover relay will have all four, and the Twin-Circuit job will have two pins labelled 87 and no pin labelled 87a.

A DIN relay will be a small cube, about 1" on a side, not including contacts. The Mini-DIN relay will be about 1/2" wide, 1" tall, and 1" deep. Contacts on both are standard 1/4" spade-style quick-connects (about 1/4" wide and 3/8" long.) Terminals on "Power Relays" (up to 70-80A) will be somewhat larger.

It's also possible to go much higher in current ratings - at this point, they become known as "solenoids" and are in round metal cannisters (and no, I don't suggest using Ford starter motor solenoids as constant-duty anything. The windings tend to overheat and fail.)

Instead of the typical "leaf" contacts used in a relay, a solenoid uses a plunger passing through a much larger coil and the moving contact is on the plunger. Solenoids are nearly always SPST - either ON or OFF. Trigger current can be 500-1,000mA (0.5-1.0A,) and constant duty current throughput can run up to 200A without too much trouble.

A solenoid will have two large (typically about 3/8") posts - one on either side - for the high current leads in and out. There will be either one or two smaller (#8 or so) posts - if there is only one, it gets wired to the switch or trigger lead and the mounting bracket provides the ground. If there are two, then one will go to the trigger and the other to ground - the coil is not polarised, so it doesn't matter which is wired to what. (Likewise, the high-current posts aren't polarised, either. You can wire them up either way.)

Solenoids carry a few ratings as well:
- Constant-duty rating: This is the maximum current that can pass through the contacts, all day every day, without melting down.
- Surge current: This is the maximum short-term current that can pass through the contacts without welding them together or melting them. Typically given as a current figure over time. The typical 200A solenoid, for instance, carries a 500-700A @ 15 seconds surge rating (a third part of this rating may be the "resting time" - during which the constant duty rating should not be exceeded to allow the assembly to recover. This is typically ten to thirty minutes.)
- Break current: Similar to relays, the "break current" is the current that may be "broken" (circuit opens) without arcing, burning, or pitting the contacts beyond utility. Again, typically somewhat higher than the constant-duty rating (a 200A relay will look like: 200A @ 100%dc; 650A @ 15 sec., 15 min.; 250A break. Reads as, "200A at 100% duty cycle, will handle 650A for 15 seconds with a 15-minute break, and will reliably break 250A.")

That's probably more than you wanted, but I'm hoping this will help others as well. Did I miss anything you were after?