In the old days, most people just wired the lights up on a golf cart similar to the diagram below:
Because the 36-volt cart consisted of 6 6-volt batteries strung in series, it was easy to “steal” 12 volts (that’s what most golf cart lights are designed to use) by just connecting up to 2 of the batteries in in the chain of batteries. That is what is indicated by the diagram above. For instance, you could go from the negative post on A battery in the pack to the positive post on the B battery and bingo, you’ve got 12 volts (6 volts per battery). You could, likewise, go from the negative post of the B battery to the positive post on the C battery, and get the same thing (12 volts). It really didn’t matter which “pair” of batteries you pick, as indicated by the drawing.
Of course, you had to remember to turn the light switch OFF after using the lights, or the 2 batteries that were being used for the lights would be drained from the load of the lights. So, someone came up with the idea to make it so that when you shut the key switch off, the lights would automatically turn off, even if you left the light switch on. They accomplished it by the use of a simple and inexpensive device called a relay.
The drawing above represents a typical relay. Notice that it is made up of 2 basic elements: a coil and a set of contacts. The idea is that when the coil is “energized”, the contacts are “pulled” together to provide current flow through a circuit (in this case the cart’s lighting system).
The numbers that I have placed by the terminals of the relay are a standardized set of numbers dictated by an electrical engineering group that defines such things. So, we can see that terminals 85 and 86 are the terminals that need to have a voltage placed across them in order to energize the relay and terminals 30 and 87 are the terminals that will “make” or “break” in order to turn the lights on or off.
Both the coil and the contacts of the relay have “ratings” that need to be observed when placing them in a circuit. The most common light relay for a golf cart will say something like 12V 30A on them. By this, they mean that the coil voltage of the relay is 12 volts. That is what it takes to properly energize the relay. The 30A part means that once the relay is energized, the contacts can withstand a current flow through them of up to 30 amps. The 30A has nothing to do with the current that it takes to energize the coil. It only takes a few milliamps to do that. And, the lights will take much less than 30 amps to operate. The old fashioned incandescent type lights would usually “draw” closer to 10 to 15 amps, and the newer LED type will only draw a couple of amps. So, the 12V 30A relay is more than tough enough to do the job. So, here is a typical circuit that includes a relay to shut the lights off when the key switch is turned off:
The circuitry of the lights themselves is basically the same as it was. It’s just the manner in which the energy to them that is supplied to them that is modified. Instead of the positive end of the 12-volt supply (no matter which 2 batteries are selected to provide it) going directly to the light switch, it only gets connected to the switch when the relay is energized and that only happens when the key switch is in the “on” position.
As you can see from the drawing, the B+ post of the battery has a unique role. It provides 36 volts to the logic switches because their other side goes to B-. However, it supplies 12 volts to the relay because its other side goes to the negative post of the next battery (not B-).
Now, the above illustration only works when the key switch provides B+ to the logic switches mentioned in the first diagram. However, there are models in which the B+ is always applied to the logic switches and the key switch provides B- instead. In that situation, the relay is connected as shown below:
In this case, the key switch is used to provide B- to the solenoid instead of B+.
Of course, there are dozens of different configurations for golf cart lighting systems but the purpose of this article is simply to help someone who has never worked with relays before, understand what they are for. They can use the energy in one circuit (in this case the logic switching circuit) to control the energy in another circuit (in this case the light system) to make or break its path (provide switching). Relays are often used for other “appliances” that are added to golf carts also (like radios for instance).
Another important thing about a relay is that because the current required to energize its coil is very small (compared to the current that the appliance itself actually uses) the switching voltage for the coil can use much smaller and easier to run wiring. Let’s say, for instance, that you wanted to add a horn to your golf cart. Horns usually draw quite a bit of current, perhaps as much as 15 amps. Now granted, the horn is usually only used in short “bursts” so the wiring doesn’t have much time to overheat, but the wiring still needs to be in the range of 12 or 14 gauge or so. By using a relay, the heavy wire that supplies the energy to the horn can go directly from the source of current (the batteries) to the horn through the contacts of the relay, but the switching can be controlled by a relay whose coil circuitry needs only to use perhaps 18- gauge wiring or so. It would look like this:
In this case, we aren’t concerned with the key switch at all. We are just making it so that we can run a nice manageable size wire to wherever we want the switch to be located but still have an adequately sized size to supply the relatively large amount of current that the horn requires.
Relays are very useful devices. They are often used in HVAC (air conditioning and heating) work to control huge amounts of current (like for compressors or heating elements) with very small voltages used for switching (like a thermostat would use). Maybe this will clear things up a little about why relays are used and how they are wired. Ron.
For information about books written by Ron Staley about both electric and gas driven golf carts and their repair, visit the following links.
Electric Golf Cart Repair, both as an eBook and in Hardcopy:
Book: Ronald L Staley: 9780578560557: Amazon.com: Books
Gas Golf Cart Repair, both as an eBook and in Hardcopy:Gas Golf Cart Repair Book: Ron Staley: 9798987911303: Amazon.com: Books