Relay Basics

by: Andrew Krause

Relays are one of the oldest, simplest, and yet, easiest and most useful devices. Before the advent of the mass produced transistor, computers were made from either relays or vacuum tubes, or both.

A relay, quite simply, is a small machine consisting of an electromagnet (coil), a switch, and a spring. The spring holds the switch in one position, until a current is passed through the coil. The coil generates a magnetic field which moves the switch. It's that simple. You can use a very small amount of current to activate a relay, and the switch can often handle a lot of current.

The relay we are going to look at is the Bosch 5 pin relay. Bosch is a German manufacturing conglomerate (who also happen to own Bosch Telekom and Blaupunkt), but they are not the only manufacturer of this relay. There are several other companies such as Siemens (stop laughing) and Potter & Brumfield. I don't know why they call it the Bosch type relay, but dammit, I don't give a shit either. The Bosch 5 pin relay is the most widely used and versatile relay, and it can handle up to 30 amps, which is more than suitable for most applications.

Looking at the diagram to the right, we see the pinout of the relay. Note that each pin is numbered, 85, 86, 87, 87a, and 30. The 30 pin is set perpendicular to the other pins to let you know where each pin is at (although, most relays are labeled at the bottom).

85 and 86 are the coil pins. Normally, it doesn't matter which way you pass the current, because if you hook it up backwards, the coil will still activate the relay. However, relays sometimes have an odd tendency to turn themselves back on briefly. To counter this, a diode (a one way switch) is placed between 85 and 86. This is referred to as a tamping diode. A diode wall have a very high resistance in one direction, and a very low resistance in the opposite direction. When a tamping diode is used, it is important that you hook the coil up according to polarity. If a tamping diode is used, and you hook it up backwards, you will essentially be shorting a wire out, which sucks, because you can and will burn something up.

30, 87, and 87a are the other three pins. 87 and 87a are the two contacts to which 30 will connect. If the coil is not activated, 30 will always be connected to 87a. Think of that pin as "87, always connected". When current is applied to the coil, 30 is connected to 87. 87 and 87a are never connected to each other. Here, polarity does not ever matter. You can connect 30 up to positive or negative, and that is what you will get out of 87 or 87a. Refer to the picture at left, and perhaps it will make the relay a tad simpler.

As you can see, the coil is in no way connected to the switch part of the relay. This can allow you to completely isolate one circuit from another. You can even use a separate power supply to control the relay.

Now let's talk about applications. One common use of a relay is for multiple amplifiers. Normally, if you run any more than three audio components off your decks remote output, it is recommended that you use a relay to provide higher currents. Also, if you wanted to power something like a fan, or neon lighting, you need a relay to keep up with the higher demands these devices place on your remote. Take a minute and guess if you can see how it is done. All you have to do is connect 85 to your decks remote output wire, and 86 to ground. Then, connect 30 to a fused 12 volts source, and 87 to your outgoing remote line. Remember that the relay can carry up to 30 amps, but if your wire can only handle 10 amps, you should use a 10 amp fuse. 87a is not used, but I recommend taping up that pin, or pulling the wire out of the base, so that it does not short out.

That is one of many applications for this one type of relay, and it demonstrates really well the use of a relay as a switch. One other use of this relay is to select. Remember that when the relay is not energized, 87a and 30 are connected. Say for instance, that you wanted to be able to run your subs in bridged mode, but then switch them the two channel mode when the amp starts to run hot. This will take two relays. The amp we use in this example is a two channel which will combine the right channel negative with the left channel positive for bridged mode, and we are using two single voice coil subwoofers. In either configuration (bridged or two channel), the left channel positive will always connect to the left woofers + terminal, and the right channels - terminal will always connect to the right woofer -. The other terminals we have to play with. Connect the left woofer relay's 30 pin to the - terminal on the woofer, and connect the right woofer relay's 30 pin to the + terminal on the woofer. Then, connect the left woofer relay's 87a pin to the - terminal on the amp, and the right woofer relay's 87a pin to the + terminal on the amp. Now, connect the left woofer relay's 87 pin to the - terminal on the right woofer, and connect the right woofer relay's 87 pin to the left woofers + terminal. Finally, connect the 86 pins of each relay together and ground it, and connect both 85 pins of each relay together, then to a switch, which is connected to a fused 12 volt source. Sound complicated? I hope the diagram helps make it seem simple. With the relays at rest, the amp is running in two channel mode. However, flipping the switch places the woofers in parallel, and safely breaks the connection with the two terminals that are unused. This is not seamless: When you flick the switch, you will hear a pop much like you hear when you plug a woofer in while it's getting juice. Some amps only produce a small pop, but cheap amps tend to make a big deal out of this. On a side note, you can use a thermistor to automatically switch the relays.

Relays really are not a big deal. They are relatively easy to use, and are very forgiving of stupidity. For many applications, a relay is the only way to go.