Shavano Music Online

    Using Mechanical Relays - Part 1

    07/02 - Jens Moller - http://www.colomar.com/Shavano/relay.html
    Relays are handy devices - they allow you to swap electrical connections around by applying electrical energy to them. The things that are switched can be many things, such as low level audio signals and lighting circuits.

    Before we go too much further, lets go over the parts, the operation, common markings and limitations of relays.

    First off, there is a set of switches in a relay - the types of switches can vary quite a bit, depending on what the needs for the switch are. Some are for very high power and you will find that the current carrying capability is often marked. For low level audio signals, its not all that important, however if you want to make a controller that turns of banks of lights, you need to make sure that the switch contacts can handle the current.

    Physically, the size is often a good indicator of the power that the switch contacts can handle - if the contacts are large, they can handle more power. Some contacts are very tiny, and these cannot handle much power at all.

    Relays, just like other common switches, use a common naming convention to determine how they can be configured. 2 common types are shown above. These 2 types are:

    • DPDT - Double Pole, Double Throw
    • SPDT - Single Pole, Double Throw

    Switch Configuration

    The number associated with the Pole indicates how many circuits that can be switched. The number associated with the Throw indicates how many different connections that you can make with each circuit.

    For example, a DPDT switch can take 2 seperate (and unrelated) circuits, and switch between 2 different connections for each. A SPDT is almost the same, except it only has 1 circuit that can select between 2 different connections.

    The current can flow either way thru the switches - its not important. Because of that, the markings will not say IN or OUT, but rather tell you the conditions the switch in its OFF mode.

    The Connection names are:

    • C - Common - this is the common connection point that each switch position shares.

    • NC - Normally Closed - When the relay is NOT energized, this position makes contact with the Common connection. When energized, this position will disconnect from the Common connection. This is the position that the relay will be in if the power is lost in any system that is controlling the relay.

    • NO - Normally Open - When the relay is energized this position will have contact with the Common connection. If the power is lost to the relay (not energized), then this contact will disconnect from the Common connection.

    The NC and the NO positions can never contact each other - but the Common will always be in contact with either the NC or the NO position.

    You will notice that there is a dashed line connecting the switch itself (connected to the Common) on the DPDT switch. This tells you that the 2 individual switches change in unison - when one is connected to the NC position, the other one is also connected to the NC position. When the relay is energized, then both switches will be in the NO position. Pretty simple stuff once you realize that the NC and NO always refer to when the relay is not energized.

    On the DPDT switch, I marked the individual switches with numbers - 1 for the first switch set and 2 for the second switch set. Its not always marked this way, however, the relay itself normally includes this information on the case of the relay or in documentation that comes with it.

    The Coil

    The next section of the relay is the coil - on the diagram, its shown with 2 connections marked A and B. There are 2 types of coils - one is for DC (Direct Current) power and the other is for AC (Alternating Current) power - don't mix the 2 and make sure that you use the right one for your needs.

    The coil will often have a resistance value associated with it, and it will always have the voltage rating that the coil will operate at. The voltage rating is important, in that you need to match that voltage up with your control circuit that will energize the relay.

    If you are designing your own control system, you can use what ever voltages that you have available. Knowing the resistance of the coil can help you calculate how much current will be required to operate the relay. The operating Voltage, divided by the coil Resistance will tell you how much Current is required - for example, a 12 volt coil with 100 ohms resistance works out like this: 

            Volts / Resistance (in Ohms) = Current (in Amps)
    for our example: 
    	12 Volts/100 Ohms = 0.12 amps
    If you have 10 of these relays in your controller, and all could be on at the same time, you would need to have a power supply that can provide 1.2 amps (10 X 0.12 amps).

    When you apply power to the coil, you energize it. This causes a magnetic action to occur and pull the switch contacts to the NO position. As long as you match the power to the coil voltage, it will last a long time. If you don't have enough voltage and current, it will not pull reliably. If you have too much voltage, the coil will run hot and may eventually fail.

    I prefer lower voltages to operate the coils, simply beucause its safer to touch 12 Volts than it is to touch 120 Volts or 220 Volts. My choice is often made by the application and the length of the controller wires. The cost is often a factor - if you use AC power availble from the wall outlet, you do not have to buy a transformer to power the relays. In most cases, I have spare low voltage power available, so I prefer to use the lower voltages.

    Other Relay Configurations

    There are many other types of mechanical relays out there. Some examples are:
    • SPST - Single Pole, Single Throw - basically just an on/off switch, however it may have either the NC or the NO contact, depending on what its intended use is.
    • 4PDT - Like a DPDT, except it has 4 seperate switch sets in it instead of just 2
    • Mechanical Sequencers - these have ratcheting switch positions where the connection is moved from one position to the next - it could be a Single Pole with 20 individual Throw positions - these used to be used a lot in signs that had moving light displays. They are not that common anymore since you can do many of the same things more reliably using electronic switching.
    • Latching Relays - Kind of like the Mechanical Sequencers, however these often use magnets to hold the relay into its last known position (so if the power drops off, then the switch contacts remain where they were last set to). These can be operated with momentary contact switches to energize the coils only when you need to change the switch position.

    Questions? Comments? .

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