Circuit : Andy Collinson
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This article describes conventional naming practises for switches and relays, in electronic circuits and schematics.
Switches
Switches come in a variety of shapes and sizes and are pretty straight forward to wire, but problems may arise with multi-pole multi-contact switches. In schematics switches are always drawn in the OFF position.
Switches can be non-locking, i.e. a single button that when pressed makes contact only as long as the switch is pressed, then breaks the circuit when the button is released. These switches are called "push-to-make", see left image. You can also buy switches that work the opposite way, i.e. push-to-break, an example is shown on the right.
Many switches are latching, their state changes permanently depending on the position of the button or lever. Common varieties are toggle switches, slide switches etc. The common on/off switch has a single contact arm and one contact or position, this is known as single-pole single-single throw or SPST. A switch with a single contact arm and two positions ( 3 contacts ) is called a Single Pole Double Throw SPDT switch, see left image. Often a circuit is required with two sets of contacts. These switches are called Double Pole Double Throw switches (see right image).
Rotary Switches
In some circuits, e.g. a power supply with multiple outlets, a frequency generator with different output waveforms, a switch with more than two positions is required. A Rotary Switch (see left image) is a prime example of a multi-way switch. Rotary switches can have a single pole (wiper arm) or multiple poles and multiple outlets. The one shown left has 2 poles and 6 outlets, abbreviated to 2P6W.
An expanded view of the contacts of a 2P6W switch is shown below, right.
Rotary switches are always drawn in the OFF position, therefore the wiper arm will always be connected to the first contact. The diagram at left shows a 2 pole 6 way (2P6W) switch. A switch is designated by the letter "S", so "S1" is the first switch. If a switch has more than one pole, then a dashed line will be drawn between the poles. The first set of contacts or arc is designated S1a. The next set S1b, etc. The contacts are numbered 1 through 6, contact 1 being the "OFF" position. The right image shows a close up of a 2P6W contacts, 1a represents first contact on arc a, 4b the fourth contact on arc b, etc. The actual schematic may just show the position of the first contact or first and last contact for clarity, see left. With all multi pole switches, the arcs are tracked simultaneously.
Identifying Rotary Switch Contacts
The left image shows a 4 pole 3 way (4P3W) switch, with four arcs, only 1 arc is in use. To identify the contacts, first turn the
switch anti-clockwise. Using a multimeter on ohms function take one of the wipers (inner contacts), shown as a green wire, and tap
around the switch with the other probe. When a reading is obtained, this is the first contact (red wire) in the picture. Now move the switch one
click to the right. The second contact will be adjacent to the previous contact (blue wire) in the image. Using different colour wires makes it
easier to identify when wiring rotary switches. It is a similar procedure for all rotary switches, the right image is a 3P4W switch.
Multiple Gang Switches
Less common, but still used are multiple pole, multiple way switches. The picture on the left shows a 3 Pole 6 Way switch, the first pole (also called a Gang) is seen nearest the top shaft. If a circuit calls for a 3 Pole 6 Way switch, then one way to substitue this is by using a 1P6W and a 2P6W switch. Using separate switches would require that you turn each switch individually or couple the switch shafts mechanically.
Relays
Like switches relays are available in all shapes and sizes. Some relays are miniature and can fit a standard IC socket. The left image is a small DIP miniature relay. The right image shows a DPDT relay that can switch loads of up to 2A at 24 Vdc. Relays that can switch line voltages of 240 VAC and high currents are generally physically larger than the miniature variety.
Relays and Contacts
As with switches, relays are always drawn in the OFF or un-operated state. Contacts may be drawn in different ways as shown below; but the middle contact (labelled 2 below) is the wiper arm and will move when the relay is energized. This contact is called the changeover contact, abbreviated CO. The top contact (labelled 1) is the normally closed, NC contact (relay not energized) and contact 3 known as normally open, NO. When power is applied to the relay coil, the NO contact is then mechanically moved and electrically connected to the CO contact.
Relay Contact Labelling
As shown above, bottom right hand corner the relay has two changeover contacts. The designation for a relay coil may be, RL or just R. If there is more than one relay in a circuit then the usual method is to designate relays as RLA, RLB, or RA, RB etc. The number of contacts that belong to a particular relay coil is shown with a forward slash "/" then a number. In this case RLA/2 has two contacts and these are appropriately labelled RLA1 and RLA2.
Motor Reversing Circuit
I will finish this page with a motor reversing circuit.
This circuit uses 4 relays and 3 push buttons for Stop, Forward and Reverse control of a DC motor. RLD has just one changeover contact whereas RLA, RLB and RLC have 2 CO contacts each. Basically RLD will stop the motor, RLA latches the motor in a forward direction and RLB latches the motor in the opposite direction, whilst RLC and its contacts provide the reversing for the motor. This circuit is quite messy and it is common practice to see the relay coil and its associated contacts separated some distance in the actual schematic.
The following circuit is identical (electrically) and you should now be able to see that the circuit uses the conventional naming rules of switches and contacts in this article.