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Initially, relays are confusing to those new to electronics. Looking at them from the outside, they are little sealed boxes with multiple terminals. Since you generally can't see what's going on inside the relay, it can be difficult to understand how they operate. For those of you who are just starting out, just remember that, generally, they allow a very small current to control a high current circuit. It's sort of like when you start a car engine. The mechanical effort it takes to turn the ignition key is nothing compared to the effort it would take to start it with a pull rope. Essentially, the relay allows one device to effortlessly control a much larger device.

The Contacts:
As you already know, if you touch two pieces of wire together and if the terminals of a power source are connected to the wires, current would flow from one wire to the other. A relay does essentially the same thing. It touches two conductors together to allow current to flow from one to the other. In the following graphic, you can see a relay with three contacts (labelled A, B and C). A and C are stationary contacts. Contact B is also a contact but it differs from the others because it moves.

The Electromagnet:
In most relays, an electromagnet is used as the actuator (there are also relays that operate on air pressure or a vacuum). When the electromagnet has power connected to it, it pulls the movable contact away from contact A (where it stays when no power is applied to the electromagnet) and and down to contact B. Contact A is the Normally Closed contact. Contact B is the Normally Open contact. As you can see, all the way to the right, there is a spring. The spring returns the movable contact to the Normally Closed contact when power is removed from the relay coil (the electromagnet). In the following demo, you can see all of the components I've mentioned so far. When you press the button, you will see the magnet engage (the yellow lines are the magnetic field). This is what happens when you apply power to the relay coil. As you can see, that the movable contact is attracted by the magnet and is pulled down. For the most part, this is all that there is to the operation of relays. You apply power to the coil. The coil pulls the armature down and contact is made between 2 of the contacts (and, in this case, contact is broken between 2 contacts). as a side note, the 'armature' is the part that moves. The contact is on, or is part of the armature.

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The terms 'normally open' and 'normally closed' refer to a pair of contacts in a relay or switch. For the simplest of relays (like the one in the photo at the top of the page, see close-up below), there is only one pair of contacts, the movable contact and a stationary contact. With no voltage applied to the coil of the relay, the contacts are open. If you used the relay to make/break the connection in a circuit, the connection would be broken with no voltage applied to the coil. If you had a relay with two pairs of contacts (like the one in the demo above), there would be one pair of contacts that could pass current (even with no voltage applied to the coil) and one pair of contacts that could not pass current (until sufficient voltage was applied to the coil). As an example. Let's say that you needed to control two lights, one red, the other green and the green light was lit most of the time. With a relay with both normally open contacts and normally closed contacts, you would connect the green light to the normally closed contact and the red light to the normally open contact. With no power applied to the relay coil, the green light would be lit. The red would be off. When the lights needed to switch (green off, red on), you would apply voltage to the relay coil and the relay would engage. This would break the normally closed contacts (breaking the circuit for the green light) and make the normally open contacts (closing the circuit for the red light).

In the previous example, the green light was connected through the normally closed contacts. Since it was going to be lit most of the time, it would generally be more efficient not requiring the coil be energized. This doesn't mean that it's always best to do it this way. If there was in a situation were a loss of power or a failure of the relay would prevent the red (warning) light from lighting, it may be better to power the red light through the normally closed contacts.

Two Reasons to Use a Relay:
A relay will allow you to use a small current to control a larger current. Generally, a relay coil has a relatively high resistance and will require only a small electrical current to engage the contacts. Typically, the contacts are rated to carry much more current than it takes to engage the relay. This means that a relay can be used if you need to make/break the circuit path where there is a relatively high current flow and the control circuit can only supply a small amount of current. In your vehicle, the ignition switch cannot pass a significant amount of current without being damaged. You probably also know that the engine starter motor needs significant current to be able to start the engine. Since it would almost instantly destroy the ignition switch if you were to try to power the starter motor with the ignition switch itself, manufacturers use a relay (also known as a solenoid) as a buffer between the ignition switch and the starter motor. In old Ford vehicles, the solenoid was mounted on the fender. On other vehicles, the solenoid was mounted onto the starter. If you ever have to work on the starter, you will see that there is at least one small wire and at least one very large wire. The small wire drives the coil of the solenoid. The larger wire supplies power to the actual starter motor. In this application, the relay/solenoid is used to allow a small current to control a larger current. In car audio, the most common use for the relay is a buffer for the remote output of the head unit. Since the head unit's remote output is limited, the relay allows you to power many more devices than you could with the remote output otherwise.

The second reason to use a relay is to isolate two circuits. If you needed to control a very high voltage circuit with a 12 volt controller, you could use a relay. Since a relay coil is 'generally' isolated from the contacts, you typically have complete isolation between the 'input' and 'output' section of the relay. Of course, the input of the relay is the relay coil and the output would be the contacts.

The table below shows just a fraction of the available relay configurations. On the relay above, there was only one movable contact. As you can see below, there are multiple sets of movable contacts on some relays.

relay single pole single throw - spst This is a Single Pole Single Throw relay. Current will only flow through the contacts when the relay coil is energized.
relay single pole double throw - spdt This is a Single Pole Double Throw relay. Current will flow between the movable contact and one fixed contact when the coil is DEenergized and between the movable contact and the alternate fixed contact when the relay coil is energized. The most commonly used relay in car audio, the Bosch relay, is a SPDT relay.
relay double pole single throw - dpst This is a Double Pole Single Throw relay. When the relay coil is energized, two separate and electrically isolated sets of contacts are pulled down to make contact with their stationary counterparts. There is no complete circuit path when the relay is DEenergized.
relay double pole double throw - dpdt This relay is a Double Pole Double Throw relay. It operates like the SPDT relay but has twice as many contacts. There are two completely isolated sets of contacts.
relay 4 pole double throw - 4pdt Yep! You guessed it. This is a 4 Pole Double Throw relay. It operates like the SPDT relay but it has 4 sets of isolated contacts.

Examples of Common Relays
The following are examples of different styles of relays. When you click on an icon, the image will open in a new tab or window. When it opens, it's likely going to be smaller than actual size. For most browsers, you can click on the image to view it full size. Close that window/tab when are ready to view another example.
This is a simple single pole single throw relay. You can see that there is no normally closed contact. The only time that this relay will pass current through its contacts is when the coil is energized. This is a single pole double throw relay. This is the standard Bosch type relay. This one is made by Tyco. This is a three pole double throw relay. The terminals on this one require that it be used in a socket. This is a four pole, double-throw relay.

Relay Specifications:
There are two specifications that you must consider when selecting a relay for use in an automobile, the coil voltage and the current carrying capacity of contacts. The coil voltage for relays used in automobiles is ~12 volts. This means that if you apply 12 volts to the coil, it will pull in and stay there until the applied voltage is removed from the coil. The current rating on relay contacts tells how much current can be passed through the contacts without damage to the contacts. Some relays have different current ratings for the NC contacts (which are held together by spring tension) and the NO contacts (which are held together by the electromagnet). If you need to pass significant current through the NC contacts, you may want to check the manufacturers specifications for the relay.

The Famous Bosch Relay

Bottom View:
The most commonly used relay in car audio and security is the Bosch type relay (AKA: 5-pin relay, AKA: 5-prong relay, AKA: 5-terminal relay...). Although Bosch is no longer producing the relay (Tyco purchased the relay division from Bosch), it's still referred to as a Bosch relay. On this page, it will also be referred to as a Bosch type relay. The photo below is the bottom of the relay. Take note of the markings (85, 86, 87, 87a, & 30) near the terminals. 85 and 86 are the relay coil terminals. 30, 87 and 87a are the various contacts. This is a double-throw relay and has five terminals (two for the coil and three for the contacts). A single-throw relay would have only four terminals (two for the coil and two for the contacts).

Internal Construction of Bosch relay:
The following diagram shows what those external terminals are connected to on the inside of the relay. When there is no difference of potential (voltage) across terminals 85 and 86 (the coil), the relay's movable contact (connected to terminal 30) is held, by spring tension, against the electrical contact which is connected to terminal 87a (the normally closed contact). In other words, when no voltage is applied the the relay coil, terminal 87a is connected to terminal 30. When 12 volts is applied to the relay coil (terminals 85 and 86), the movable contact (connected to terminal 30) is pulled down/in by the electromagnet (coil) so that it physically contacts the electrical contact which is connected to terminal 87. Again, in other words, if battery voltage is applied to the relay coil (terminals 85 and 86) terminal 30 will be connected to terminal 87. The red dashed line shows the path in which electrical current flows from/through terminal 30 to the contact of terminal 87a when the relay coil is NOT energized.

Side View:
The side view of this relay shows the schematic diagram for the relay. This can be found on most relays. This is a simple relay with only one circuit. THIS is the schematic diagram for a four pole double throw relay.

Top View:
On most relays, there are specification that tell you the current rating of the contacts and the coil voltage. Often there is other information. If there is little or no technical information on the relay, there is often a part number. With that part number, you can find the datasheet for the relay. THIS is the datasheet for the Tyco relay. The datasheet for the relay will tell you virtually everything that you'll need to know about the relay.

Remember that the relay coil has to have a difference of potential between terminals 85 and 86 in order for the coil to pull the armature in/down. This means that you may apply battery voltage to either terminal 85 OR 86 and then ground the OTHER terminal. The positive battery voltage OR the ground connection may be broken to make relay switch terminals (87 to 87a).

This flash demo should help you understand how current flows through the relay as the coil is energized and deenergized.

Click HERE to make this applet fill this window.

Click HERE to make this applet fill this window.

Testing Relays:
The following section shows testing for a Bosch type relay but the procedure is the same for most relays. Generally the only differences are the number of poles or whether it's single throw or double throw.

Set your meter to ohms. If your meter is auto-ranging, it will have only one ohm mode and the meter will select the appropriate range automatically. If the meter is not auto-ranging, it will have several ohm ranges (like the one below). If it's not auto-ranging, set it to the lowest resistance range.

Touch your meter probes together. What does it read? This is what it should read when the probes are placed on the terminals of closed contacts on the relay. When no voltage is applied to terminals 85 and 86, this is what you should read between terminals 30 and 87a. On other relays it will be between the common terminal and the normally closed contact's terminal.

What does it read when the probes are not in contact with anything but air? That's what the meter shoud read when the probes are placed on the terminals of open contacts. When no voltage is applied to terminals 85 and 86, this is what you should read between terminals 30 and 87.

Click HERE to make this applet fill this window.

If the meter isn't auto-ranging, set it to the range that includes 75 ohms. Touch the probes to terminals 85 and 86. Do you read approximately 75 ohms? If so, the coil is OK. You will do this with the relay coil disconnected from the power source.

Quenching Diodes:
Anytime that a relay coil is driven by a circuit that is not specifically designed to drive a relay, you should use a quenching/suppression diode connected in parallel with the relay coil. The diagram below will show the connection of the diode. Initially, you may think the diode serves no purpose because the voltage applied to the relay cannot pass through the diode. This is true when the relay is energized. The diode comes into play when the power source is removed from the relay coil. When power is applied to the relay coil, a magnetic field is created and energy is stored in the coil. When power is removed, the magnetic field collapses causing a reverse voltage to be generated (it's called inductive kickback or back EMF). The back EMF can easily reach 200 volts. The diode will absorb the reverse voltage spike. This voltage, if not absorbed by the diode, will cause premature failure of switch contacts and may cause the failure of power switching transistors. You can use virtually any type of rectifier or switching diode (i.e. 1N4001, 1N4002, 1N400x... or Radio Shack part #s 276-1101, 276-1102, 276-1103, 276-1104).

relay with diode

Voltage Graphs:
The following diagram shows 2 different voltage graphs. The top graph shows how the parallel diode quenches the reverse voltage. The bottom graph shows the unsuppressed voltage. This voltage can damage low voltage transistors and switches. You can right click on the diagram to zoom in on the graphs.

Earlier I said that you energize the relay by applying positive voltage to either 85 OR 86 and grounding the remaining terminal. The only thing that changes when using the quenching diode is the fact that the positive terminal and the striped end of the diode must be together. If the positive control lead is connected to the diode's anode (unstriped end of diode). There will effectively be a short circuit to ground possibly causing damage to the control circuit (if the control circuit is not properly fused). A 1 amp fuse will carry more than enough current to energize the relay's coil.

Relays with Internal Suppression Circuits:
There are some relays with internal suppression circuits which make the external diode unnecessary. The suppression circuit is generally a resistor or a diode parallel to the relay coil. The relays with a diode suppressor will have polarity sensitive coil connections. This means that the proper relay coil terminal (the positive terminal) must have the positive voltage applied to it. If the relay is connected improperly, the relay may be damaged or in some cases it simply won't operate.

The following two images show why you need to be careful when using relays with suppression diodes. In the diagram, you can see that the anode side of the diode is connected to terminal 85. This means that terminals 85 has to be used for the ground terminal for that particular relay (this is from the Tyco datasheet). The second image shows a wiring harness for a relay that came with a car alarm. The diode is connected with reverse polarity (compared to the Bosch internal diode). Generally (maybe always with the Bosch type relays) terminal 85 is considered ground when there are internal diodes. If you were to use this relay socket with a Bosch type relay that had an internal diode, there would be no way to make the combination work unless you cut the diode from the socket.

The next image shows a relay with an internal diode supressor. The second image shows a relay with a resistor supressor. Not all relays with internal supression will have the supressor on the schematic diagram but most will.

Pull in Voltage:
The pull in voltage is the minimum voltage required for the relay coil to pull the contacts (30 and 87 on the Bosch relay) together. The pull in voltage is about 8 volts for a typical Bosch relay.

Drop out Voltage:
The drop out voltage is the voltage at which the energized coil will release the movable contact. The drop out voltage is somewhere between 1 and 5 volts for a Bosch relay.

Coil Resistance:
In a DC relay coil, the coil resistance determines the current flow through the coil. The current draw by the coil of a Bosch relay is ~0.160 amps (~75 ohm coil). In an AC relay coil, the resistance does not solely determine the current flow through the coil because the coil has inductance. The inductive reactance along with the DC resistance work together to limit the current flow through the coil.

Remote Input Current:
The remote input current for amplifiers varies with the amplifier and the model. Some draw minimal current. Others draw a little more. The upper limit of a properly functioning amplifier is approximately 50ma (0.05 amps). If you're using/controlling more than 2 amplifiers, it is (in my opinion) much better to use a relay to control the amplifiers. Actually I really prefer having a relay in the remote circuit (no matter how many amplifiers I'm using) because it protects the head unit's remote output circuit in case of a short circuit. The following chart shows the remote input current for various amplifiers I had laying around the shop.

Manufacturer Model Current Draw (mA)
MTX 2300 14
Jensen LXA300 43
Pioneer GMX602 1.5
Autotek 7150 16
Punch 200x2 14-45
Autotek 200x1 17
Coustic Amp162 22
Orion 275SX 28
Crossfire CFA1000D 5
Lanzar Vibe 250 17
Test conditions: 14.3vdc; Fluke model 27 DMM; The meter was inserted in the remote supply line.

Punch amplifiers may draw slightly more current when the power supply fuse blows. This generally causes no problem because the increase in current is still below the current normally drawn by other amplifiers.

There is at least one very popular brand of amplifier that draws as much as 500ma of current when the amplifier fails. This is enough to damage the remote output switching transistor in the head unit if the fuse is missing or is of the wrong value. A relay in the remote circuit will completely eliminate the possibility of damaging the head unit in this situation.

It's been mentioned quite a few times that the Bosch relay's coil has a fairly low resistance (~75 ohms). It has also been suggested that you could use a different relay with significantly more coil resistance so that you draw less current from the remote output of the head unit. The relay below is from Radio Shack. Its stock number is 275-248. It has a coil resistance of ~400 ohms which means that it will draw ~1/5 the amount of current of the Bosch relay. In the following image, the red wire is the fused power source (10A max - even less for small wire like I've used). The blue wire is the remote from the head unit. The black wire goes to ground. The green wire goes to the remote input of the amplifiers and to fans if you have them.

  • Online Source for Relays:
  • Parts Express has a few relays that are suitable for use in car audio applications.
  • THIS is a direct link to a Bosch type relay on the Parts-Express site.

  • Proper Fuse and Wire Selection:
  • The following calculator helps you to select the wire size and fuses when using a relay.
  • The default current draw values are a good starting point. If you know the exact values for your equipment, enter them in the appropriate fields.
  • The smallest recommended wire size is 16g. You could use smaller wire but smaller wire is not as easy to crimp reliably which will lead to unreliable connections.
  • If you are using the relay for fans alone, enter 0 (zero) in all other current draw fields.
  • The distribution block shown is a relatively low current type (like the type used in marine applications).
  • The wire from the dblock to the amp is the remote lead. The battery and ground connections for the amplifier are not shown to reduce clutter.

Click HERE to make this applet fill this window.


Relay control:
A relay can be wired so that it will operate when the ground connection is made/broken (instead of when the 12 volt connection is made/broken). The diagram below shows the connection. Remember that it doesn't matter which connection (power or ground) is made or broken as long as the circuit driving the relay coil is made/broken. Click the switch position selector to toggle the switch.

If the switch has to be a significant distance from the relay (and you put the switch in the 12 volt source wire), the 12 volt source wire will have to run a long way also. If this wire happens to get shorted, it will keep blowing fuses and the short circuit may be hard to find. If you switch the ground connection, the worst case scenario is that the relay will turn on when the wire becomes shorted to ground. This will also make it much easier to find the shorted part of the wire and you won't blow any fuses.

Relay Terminal Connection:
When switching power with a Bosch type relay, if the situation allows, apply power to terminal 87 and use terminal 30 for the output terminal.

If the relay is wired so that terminal 30 is the input and terminal 87 is the output, the circuit will work exactly as the previous example but when the relay is switched off, terminal 87a will become energized. Terminal 87a could be insulated to prevent any problems but wiring it as shown in all of the diagrams will prevent any additional problems.

You should remember:
  1. Relays electrically isolate the control circuit from the circuit being controlled.
  2. Relays allow a small amount of current to control a large amount of current flowing in a separate circuit.
  3. A diode should be connected across the relay coil to prevent a large voltage spike when the voltage source is removed from the coil.

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