Elininating noise in electromechanical relays

Careful selection of the correct type of electromechanical relay can avoid many of the effects of electrical- or heat-generated noise, explains Ian Purcell.

Circuit designers are constantly faced with the challenges and demands to produce ever more complex applications with an ever-decreasing level of power available.

Although this has definite benefits, certainly in terms of heat dissipation and product reliability, it does place major demands on the choice and quality of the discrete components used within a circuit.

Obviously, even the best design cannot always compensate for the lack of inherent stability or interference generation within the application itself.

The choice of an electromechanical relay to be included in a low-level signal circuit can be a crucial factor in determining the levels of electrical noise within an application.

In basic terms, the electromechanical relay is a comparatively simple component and when included in an application, it is somewhat governed by its inherent operating characteristics.

When small DC or AC voltages are switched with electromechanical relays, noise can be generated, causing possible corruption of the signals to be switched due to heating and material effects.

Specific constructional techniques during the circuit design, along with a careful selection of materials by manufacturers, can be employed in order for such noise generation to be significantly lowered, if not completely eliminated.

Electrical noise levels occur because of the unavoidability of transition points between materials along the paths travelled by the signal.

The transition points are always a compromise between the practicalities of the relay design, compared against an ideal electrical characteristic.

This is due to manufacturing requirements that dictate that the metal connections are routed into the interior of the relay via sealed or moulded glass entry points moulded into the metal casing.

To prevent mechanical stress between the glass insulation and the pin passing through it when the temperature changes, the coefficients of thermal expansion of the two materials must be matched.

Unfortunately, this results in the use of contact materials that produce noise voltages if heated.

The effective temperature differences largely depend on temperature distribution within the relay and thereby on the relay construction and the geometry of the connections.

Moreover, the manufacturers' datasheets only very rarely give information on the thermo electromotive force of relays.

With all monostable relays - regardless of whether in reed relays or conventional designs - the heat radiated by the relay coil is the main cause of temperature gradients along the contact path, and thereby for the thermo electromotive forces.

Thus the use of bistable (latching) relays can be particularly successful where noise generation is an issue.

This is because the source of heat is not present with polarised bistable relays.

Bistable relays operate by pulsing either a single coil with a known voltage polarity to "set" the contacts, then reversing the polarity to "reset" the contacts.

Alternately, dual coils can be used as dedicated "set" and "reset" being pulsed in sequence.

Obviously, the use of bistable relays in such application implies the availability of reversible polarity pulse currents to drive the relay coil(s).

However, for those circuits not able to offer anything other than a normal continuous energising voltage, it is possible to use suitable circuitry to create a "pseudo" monostable operation with a bistable relay.

In the simplest case, this is achieved by using what is known as a "C circuit" in which a single bistable relay coil is switched in series with a capacitor.

The current flow through the coil is limited to a few milliseconds, with very little heat generation.

During the remaining operation, only a leakage current flows which causes no significant heating effect in the relay coil.

The relays coils are "set" during this charging time.

Once the energising voltage is released, the stored capacitor charge is released back through the relay coil but in the opposite direction causing the contacts to "reset".

An alternate method of offering low heat generation in the coils can be achieved by utilising dual bistable coils where the input pulses can be directed by a flip-flop circuit, causing the relay to be set and reset alternately.

However, the disadvantage of this module is that one contact is needed for supervision of the switch position and is thus not available for switching tasks.

A further noise variable that can specifically affect small AC voltages can be observed with reed contacts and with relays constructed according to the reed principle - designs with their contact springs in the magnetic field of the relay coil.

When the contact is closed there is a damped oscillation with a frequency of between 2 and 3kHz, corresponding to the inherent resonant frequency of the tongues.

Only a part of the magnetic flux created in the coil flows through the contact tongues, whereas the remainder forms a magnetic field within the coil.

In operation, the tongues continue to oscillate for a short time at their resonant frequency without opening once they have moved and the bounce process is ended.

If the contact springs are not absolutely parallel to the magnetic field lines within the coil, a voltage is induced in the tongues.

The amplitude and the timing of the induced voltage depend very much on the dimensions, the form and design of the tongues, as well as their position in the magnetic field.

However a typical level for standard contacts can be extremely significant on very low-level signal switching circuits.

With low thermal electromotive force designs, where the tongues are coated with a good thermal and electrical conducting material that damps the inherent resonance, the maximum values can be reduced.

Furthermore, by adjusting the contact tongues in the magnetic field area, the noise voltage can be practically eliminated.

As the examples of the various scenarios with diverse relays and contact materials discussed in this article demonstrate, the effects of generated electrical or heat generated noise levels could prove a major problem for circuit designers.

However, with careful selection of the correct type of relay, and therefore optimising the compromise element in the transition points for the signal, many of the possible problems can be successfully eliminated at source.

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