Electromagnetic Relays

General technical information

The fast advance of electronic components, especially integrated circuits, over the past ten years did not lead – as was often prophesied – to a drop in the demand for relays.
Rather there was a trend to optimally unite the advantages of what seemed to be competing components, relays and semiconductors, in the circuit in which they were used.
In many cases relays were replaced by semiconductors, but at the same time completely new applications appeared as electronics progressed that also had an impact on the development of relays.
This resulted primarily in further miniaturization and in relays whose systems are tightly sealed against the effects of the environment.
Because of environmental influences there is also a demand for sealed relays with dry-reed contacts whose design enables greater operating speeds.

Advantages of relays

The relay is notable for a number of characteristics which provide the basis for general purpose use in all fields of electronic engineering.
A complete description of the advantages of relays would burst the bounds of this page, but the main ones will be looked at here.

Division according to functions

The type of relay that a user decides on will depend on the particular application, and especially on the type of drive circuit and the task that the relay has to fullfil.
There are many possibilities with relays, as shown by the following table with its divisions similar to DIN 41215.

Electromagnetic relays

Monostable relays Bistable relays
Monostable neutral relays Monostable polarized relays Relays for AC operation
Bistable polarized relays Remanent polarization relays

Explanations to the terms used above

Electromagnetic relay
An Electromagnetic relay is a component in which an electromagnetically generated force actuates relay contacts directly or by way of connecting links.
It has maximally three switch positions and serves primarily for combining electric information or evaluating it.

Monostable relay
A relay that returns of the off-position after the energizing current has been turned off.

Bistable relay
A relay that remains in the switch position last reached after the energizing current has been turned off.

Neutral relay
A relay in which the transition from the off-position to the on-position is independent of the sense of the energizing current.

Monostable neutral relay
A monostable relay that works independently of the sense of the energizing current.

Bistable neutral relay
A bistable relay in which the transition from the off-position to the on-position is independent of the sense of energizing current.
Note: The switch position assumed can be kept by means of mechanical aids or by magnetic effects (remanent polarization).

Polarized relay
A relay in which the transition from the off-position to the on-position is dependent on the sense of the energizing current.
Note: There are polarized relays in which overexcitation will not cause the relay to leave the assumed switch position and those overexcitation causes another switch position, e.g. the off-position, to be assumed.

Monostable polarized relay
A monostable relay that assumes the on-position when there is energizing current of a certain sense.

Bistable polarized relay
A bistable relay that assumes one switch position when there is an energizing current of a certain sense and the other switch position for an energizing current of opposing sense.

Division according to design features

There are mainly four categories that can be distinguished by design features,
dustproof relays are provided with a cap, usually transparent, as protection against damage and the penetration of dust into the interior of the relay.
Washable relays have a resin-sealed plastic cap to protect the relay system.
Relays with Gas-Protected Contacts are contacts sealed either in a glass tube filled with protective gas or in a metal package.
Vacuum relays are similar to the relays with gas protected contacts, but the contacts are in vacuum.

Shutdown behaviour

Contact load classifications (related to the utilization categories defined in EN 60947-4-1 and EN 60947-5-1):

Load classification Supply type Application
AC 1 AC single-phase
AC three-phase
Resistive or slightly
Inductive AC loads.
AC 3 AC single-phase
AC three-phase
Starting and stopping of Squirrel cage motors.
Reversing direction of rotation only after motor has stopped rotating.
Motor reversal is only permitted if there is a guaranteed break of 50ms between energisation in one direction and energisation in the other.
Provision of 300ms "dead break" time when neither relay contacts are closed during which time the capacitor discharges harmlessly through the motor windings.
AC 4 AC three-phase Starting, stopping and reversing direction of rotation of Squirrel cage motors.
Jogging (inching).
Regenerative braking (plugging).
AC 14 AC single-phase Control of small electromagnetic loads (<72 VA), power contactors, magnetic solenoid valves, and electromagnets.
AC 15 AC single-phase Control of small electromagnetic loads (>72 VA), power contactors, magnetic solenoid valves, and electromagnets.
DC 1 DC/ = Resistive loads or slightly inductive DC loads. (The switching voltage at the same current can be doubled by wiring 2 contacts in series).
DC 13 DC/ = Control of electromagnetic loads, power contactors, magnetic solenoid valves, and electromagnets.


Solid-State Relays (SSR)

Solid state relays (SSR) are relays based on semiconductor technology with electrical isolation between control circuit and load circuit.
The electrical isolation is achieved by an optoelectronic coupling device in the control circuit.
The input is TTL (Transistor-Transistor Logic) compatible.
The switching of the power load in the output circuit is performed by a triac or two anti-parallel thyristors.
By applying a trigger voltage the triac or thyristors in the load circuit are activated, that means a make function.
The relays are suited for switching resistive or inductive loads.
An internal zero point switch ensures zero crossing turn-on.
All versions turn-off at zero current flow.
All solid-state relays are provided with an RC network at the output.
It is recommended to use an additional RC when high output transients (dv/dt) occur and to connect a metal-oxide varistor in parallel to the output for protection against high voltage surges in the power supply.
The most important features of solid-state relays are:

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