Mechanical Relays

Authors:

Published On:

Oct 25, 2012

Last Updated:

Sep 11, 2024

Mechanical relays are electronic components which can switch a load on and off from an isolated control signal using an electromagnet and contactors.

Relays are used for things such as:

Turing on mains voltage devices from 5-12V circuits, while providing isolation
Switching large currents
Providing isolation between two circuits (although opto-couplers are normally used for this)

Relays can also come packaged with other functionality such as built-in timers.

Terminology/Parameters

The Coil: The windings which turn the relay on when you apply a voltage
The Contacts: The output pins when get connected or disconnected when the relay turns on
Rated Coil Voltage: The recommended voltage that should be applied to the coil to turn the relay on.
Rated Coil Current: The current the relay coil will draw when the rated coil voltage is applied to it.
Contact Current Rating: The maximum current that the relay can conduct through the contacts

The DEC DH1U-5VDC relay that can switch 250VAC at 15A.

The Tyco T9AS5D12 relay. Coil is driven with 12VDC and switches 240VAC. Notice the two different contact ratings, 10A for the N.C. (normally closed) and 20A for the N.O. (normally open).

Contact Arrangements

Normally open (NO): Contacts which are open-circuit when the coil is not energized, and become short-circuit to common when the coil is energized.
Normally open (NO): Contacts which are short-circuit to common when the coil is not energized, and become open-circuit when the coil is energized.

Single-pole, single-throw (1A)

Double-pole, single-throw (1C)

Schematic symbol for a relay with a normally-open (NO), normally-closed (NC) and common contact.

Inductive Kickback and Flyback Diodes

The coil of a relay is basically an inductor. Due to the rapid $\frac{di}{dt}$ when the relay is switched off, the inductive coil will generate a large negative voltage to try and keep the current flowing (remember: inductors “resist” the change in current). This is called inductive kickback. This voltage spike can cause havoc in neighbouring components (e.g. killing the transistor used to switch the relay on, or arcing across switch terminals) if not suppressed. A flyback diode in anti-parallel across the coil of the relay will clamp the voltage spike to approx. no more than $-0.7V$ .

Always add a “flyback” diode in anti-parallel across the coil of a relay to quench voltage spikes due the rapid di/dt through the inductor when the relay is switched of.

It is absolutely necessary to connect a diode in the circuit as a means of preventing damage from the counter emf --- Panasonic: Relay Technical Information¹.

However, there are some drawbacks to using basic flyback diodes. On relay switch-off, the flyback diode decays the magnetic field of the coil far more slowly than if there was no flyback diode protection present. This causes the relay to take longer for the “clapper type” relay contacts to open. When relay contacts open, small “microwelds” occurs at the contact interface. Usually, the contacts are moving apart with enough force/velocity that these welds are easily broken. However, the slower open time with a flyback diode can cause the relay to “stick” — this is when the contacts become welded together and do not open²¹.

This diode shunt provides maximum protection to the solid state switch, but may have very adverse effects on the switching capability of the relay --- TE Connectivity (Application Note): Coil Suppression Can Reduce Relay Life².

The solution? Add a Zener diode in series with the general purpose diode to increase the voltage drop when the coil is turning off. This causes the magnetic field to collapse faster (closer to an open-circuit), but still provides circuit protection. Choose a Zener voltage so that the Zener voltage plus forward diode drop does not exceed the maximum voltage of the switching element (or other circuitry).

Adding a Zener in series with a general purpose diode as shown will improve the turn-off time and help prevent sticking, yet still provides over-voltage protection.

Latching

With a little external componentry, a mechanical relay can be made to latch-on after triggered, and will only reset once a reset button has been pushed (or power disconnected).