Mechanical Relays

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).

Must Operate Voltage

The must operate voltage is the minimum voltage that is guaranteed to activate the relay. You should make sure to drive the relay with a voltage that is higher than this when you want to activate the relay. For example, a relay rated at 12 VDC will typically have a must operate voltage of around 9 VDC.

Must Release Voltage

The must release voltage is the maximum voltage that is guaranteed to deactivate the relay. You should make sure to drive the relay with a voltage that is lower than this (or disconnect the coil) when you want to deactivate the relay. For example, a relay rated at 12 VDC will typically have a must release voltage of around 1.2 VDC.

Normal Position of Contacts

The normal position of contacts describes whether contacts are normally open or normally closed when the relay is not energised.

• Normally open (NO): Contacts which are open-circuit when the coil is not energized, and become a short-circuit when the coil is energized. This is probably the most commonly used contact style, although NC is popular also.
• Normally closed (NC): Contacts which are short-circuit to common when the coil is not energized, and become open-circuit when the coil is energized.

Contact Configuration

There are a number of different configurations relay contacts (the output pins) of a relay can have. The National Association of Relay Manufacturers (NARM) (now called the Relay and Switch Industry Association, RSIA) define 23 electrical contact “forms”. These apply to relays, but are used to describe switches. Below are the most common ones that apply to relays:¹

• Form A: Contacts are NO and become closed when the relay is energised. Also known as SP ST NO.
• Form B: Contacts are NC and become open when the relay is energised. Also known as SP ST NC.
• Form C: There are three output pins, with a common to one NO and one NC contact. When the relay is energised, the NO contact will be connected to the common and the NC contact will be disconnected from the common. It is performed in a break-before-make sequence (see Form D for the opposite). These are also called NO-C-NC or SP DT (B-M) relays.¹. Can be used as a Form A or Form B relay by ignoring one of the outputs.
• Form D: Similar to Form C, but with a make-before-break sequence rather than a break-before-make. This is an uncommon form. Can be abbreviated as SP DT (M-B).
• Form X: Is equivalent to two Form A relays in series, which are mechanically linked and actuated at the same time. Called a double make relay, and can be abbreviated as SP ST NO DM. These are commonly found in contactors and toggle switches designed to handle high power inductive loads.^{1 2}
• Form Y: Is equivalent to two Form B relays in series, which are mechanically linked and actuated at the same time. Called a double break relay, and can be abbreviated as SP ST NC DB.²

Many of the forms above are abbreviated into the form <num. poles> <num. contacts> <contact type> <make-break order>. For example, SP ST NO or SP DT (B-M). This comes from the Engineers’Relay Handbook which states:

Sequence of abbreviations. When abbreviations are used to designate a contact assembly, the following order is used: (1) Poles (2) Throws (3) Normal position (4) Double make or break (if applicable). Example: SPST NO DM refers to single pole, single throw, normally open, double make contacts. — Relay and Switch Industry Association (RSIA): Engineers’ Relay Handbook.²

This is a placeholder for the reference: tbl-relay-acronyms summarises the acronyms used.

Acronym	Description
NO	Normally open
NC	Normally closed
DP	Double pole
TP	Triple pole
DT	Double throw
B	Break
M	Make
DB	Double break
DM	Double make

This is a placeholder for the reference: fig-relay-contact-forms shows a diagram of the different relay contact forms.

A diagram showing the different relay contact forms.

Relays are typically described with the number of these “forms”. For example “1 Form C” or “2 Form A”.

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

Make-Break Order

The make-break order describes the exact sequence of switching when the relay is activated.

• Break-before-make: The relay will first disconnect the normally-closed (NC) contact before connecting the normally-open (NO) contact.
• Make-before-break: The relay will first connect the normally-open (NO) contact before disconnecting the normally-closed (NC) contact. Thus all contacts will be briefly connected to each other when the relay is energised.

This is a placeholder for the reference: fig-make-break-order shows a diagram of the two different make-break orders a relay can have.

A diagram showing the make-break order of a relay.

Dry Contacts

It is common in the electrical industry to refer to “wet” or “dry” contacts. In the context of relays, “dry contacts” are normally used. This is when the relay does not provide electrical power to the output contacts. The relay simply switches the contacts together, in the same way a mechanical switch would. It is up to the external circuitry to provide the power to the load. As shown in This is a placeholder for the reference: fig-dry-contacts-relay-schematic, one contact could be connected to 240VAC (or any other voltage), and the other contact to the live of an external load (which already has a neutral connected to it). When the relay is activated, the contacts will be connected together.

A schematic showing the basic concept of “dry” contacts on a relay. The user has to provide their own power/voltage source to power the load.

It is not clear to me what a “wet” relay contact would be. Potentially this would be if one contact of a NO pair was permanently connected to 240VAC (and not connected to the outside world), with the other contact being available to connect to an external load. When the relay is activated, the available contact would be “energized” to 240VAC and power the load.

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 Information³.

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 open⁴³.

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 Life⁴.

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).

A simple latching relay circuit. The RESET push button can be replaced with short if you only need the circuit to reset on power off.

Enclosure Rating

The enclosure rating is the amount of environment protection the relay has. A common thing to denote is whether or not the relay is sealed. Two common ratings are:

• Flux protection: Not fully sealed, but enclosed enough to prevent any issue with flux.
• Sealed: Fully enclosed.

If you are planning on submersing a PCB with a relay on it in an ultrasonic bath, make sure you are using a sealed relay. If not, the cleaning solution will more than likely enter the relay and cause damage. It can either cause an immediate fault (i.e. direct short-circuit) or slowly corrode the contacts over months. I have personally experienced this first hand with a product that started failing a few months after it was installed in the field due to it being cleaned ultrasonically. The PCB had a relay on it that was not sealed, and after cutting open a relay on a failed unit, the contacts were seriously corroded.

Common Relay Packages

Most PCB mount relays are through-hole components. My guess is that they have not gone SMD like many other components because they are quite bulky and heavy — using SMD pads would probably pose a risk of the pads being ripped of the PCB. Through-hole leads gives much better mechanical strength.

Most PCB-mount relays have an asymmetric lead configuration so that it cannot be installed incorrectly.

You can get DIN mounted relay “sockets” for mounting relays onto DIN rail, as shown below:

An Omron relay on a DIN mounted relay “socket”.

Supplier Links

• DigiKey
  • Main relay page: https://www.digikey.com/en/products/category/relays/14
  • For “power relays” (over 2A): https://www.digikey.com/en/products/filter/power-relays-over-2-amps/188
• TE: http://www.te.com/catalog/relays/menu/en/16453

Product Links

The G5Q/G5LE is a popular range of though-hole PCB mount relays from Omron. The G5Q series can go up to 250VAC at 10A.

Footnotes

1. Wikipedia (2024, Sep 16). Electrical contact. Retrieved 2024-12-02, from https://en.wikipedia.org/wiki/Electrical_contact. ↩ ↩² ↩³

2. Relay and Switch Industry Association (RSIA). Engineers’ Relay Handbook. Retrieved 2024-12-03, from https://web.archive.org/web/20170705143411/http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx. ↩ ↩² ↩³

3. Panasonic. Relay Technical Information. Retrieved 2021-12-29, from https://www.panasonic-electric-works.com/pew/eu/downloads/technical_information_relay_en.pdf. ↩ ↩²

4. TE Connectivity. Application Note: Coil Suppression Can Reduce Relay Life. Retrieved 2021-12-29, from https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN. ↩ ↩²