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.

• Electrical isolation
  There is electrical isolation between the input and output circuit (coils/contact side) and on the contact side also between the different contact units and the individual contacts.
  A measure of good electrical isolation is the isolation resistance and the dielectric strength.
  Depending on the material and climate, isolation values of approx. 10.⁷ to 10.¹⁵ Ω (Ohm) can be achieved.
  The rms value of the breakdown voltage is usually between 500 and 4000 V.
   
• Simple matching to driving source
  Through the selection of a suitable winding and relay type, it is possible to match relays to any operating voltage (dc or ac) from a few volts to several hundred volts.
  Furthermore, since the operating voltages – depending on the ambient temperature and operating mode (continuous or pulsed) – may fluctuate within wide limits without impairing a relay in its functioning and hum voltages caused by ripple in the power supply likewise have practical no effect, the matching of relays to a given power supply becomes all the easier.
   
• High overload capacity of input (coil side) an output (Contact side)
  The data given in the lists of the manufacturers concerning the loading capacity of the coil generally apply to stationary operation; the coil may be substantially overloaded short-term.
  The rated short-term current is defined in VDE  0435/9.62, § 5, as a measure of the overload capacity. This is the current that the winding can sustain for 1 s without being damaged by excessive heat. This current may be greater by a factor of 10 than the rated current.
  
  One has to differentiate between the overload with closed contacts and the overload when switching.
  When the contacts are closed the problem is mainly a thermal one. The shorter the overcurrent load, the greater the loading capacity.
  Although welding effects can occur at extremely high loads in addition to excessive heating up, the problem of welding contact is one that mainly occurs when contacts make at high peak currents, e.g. when lamps are turned on or a capacitor is discharged.
  The welding of the contacts is a statistical event. But a load limit can be stated below which, to a certain degree of probability, no welding will occur.
  This is dependent on the contact material, the voltage on the open contact, the tendency of the contacts to bounce and the time constant governing the decay of peak current.
   
• Switching ratio of relays
  The two “on” positions of a relay contact are characterized by clear criteria.
  In the open state the reverse resistance is 10.⁷ to 10.¹⁴ Ω and in the closed state 10 to 100mΩ; the contact resistance of the relay is thus of the order of the line resistance of the etched tracks (approx. 0.5 Ω/m).
  When it switches the relay goes from one state to the other, even if the input quantity is constantly changing; there are no interim values.
   
• Economics
  in the design of circuit economics play a decisive role in addition to the technical and production-related demands.
  It has often been found that relays present the most economical solution for the implementation of many switching functions.
  In the overall costs of a circuit it is not the price of a relay alone that is a determining factor; a power supply may cost more than the circuit for which it is needed.

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 fulfill.
There are many possibilities with relays, as shown by the following table with its divisions similar to DIN  41215.

Electromagnetic 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):

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:

• High switching rate and long life.
• Zero crossing turn-on, zero current flow turn-off.
• Bounce-free switching, as no moving parts.
• No mechanical switching noise.
• Low control power, TTL compatible.
• Unaffected by shock and vibration.
• Use of plastic materials for sheathing against environmental influences.

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