Circuit schematic *
|Make & break relay||The most simple form of relay. The circuit between terminals 30 and 87 is made on energisation of the relay and broken on de-energisation, known as NO (or vice-versa for a NC relay).|
|Changeover relay||Two circuits (terminals 87 and 87a ) have a common terminal (30). When the relay is at rest 87a is connected to 30, and when the relay is energised 87 becomes connected to 30 (but never both at the same time).|
|Relay with double output|
Terminal 87 is linked to pin number 87b, giving double outputs from the single NO contact.
Relay with dual contacts
The armature contacts both terminal 87 and (in this case) 87b at the same time when the coil is energised, creating a dual NO output
A blade or ceramic fuse is connected between terminal 30 and the NO contact, providing built-in protection for the high current circuit. The fuse is normally mounted in a holder moulded as part of the relay body so it can be replaced if it blows.
When voltage is removed from terminals 85/86 and the coil is de-energised, the magnetic field that has been created around the coil collapses rapidly. This collapse causes a voltage across the coil in the opposite direction to the voltage that created it (+12V), and since the collapse is so rapid the voltages generated can be in the order of several hundred volts (although very low current).
These high voltages can damage sensitive electronic devices upstream of the +12V coil supply side, such as control modules in alarm systems, and since it’s common to take low current alarm output signals to energise relay coils, equipment damage is a real risk.
Using a relay with a diode across the coil can prevent this damage by absorbing the high voltage spikes and dissipating them within the coil/diode circuit (this is known as a blocking or quenching diode). The diode will always be installed in the relay with the stripe on the diode body facing towards terminal 86 (reverse biased) and it is important that +12V is connectedthis terminal, not 85 (as per the DIN standard) or the diode could be damaged.
Relay with resistor across the coil
A high value resistor performs a similar function to that of the diode in the previous configuration by absorbing the high voltage spikes created by the collapsed magnetic field on de-energisation of the coil. The disadvantage of a resistor is that it allows a small current to flow in normal operation of the relay (unlike a diode) and is not quite as effective as a diode in suppressing voltage spikes, but it is less susceptible to accidental damage because resistors are not sensitive to polarity (i.e. it doesn’t matter whether +12V is connected to terminal 85 or 86).
* All schematics shown with the relay at rest (de-energised)