Electromechanical relays are often used as actuators, as switches to route power to electrical devices, or for signal routing within a device or between different instruments. In data acquisition applications, they can be found connecting a variety of transducers to a single measuring device.

Most electromechanical relays are driven electromagnetically; by passing a current through a coil a magnetic flux is generated which, in turn, causes an armature to move. This movement prompts isolated electrical contacts to open or close, thereby completing or breaking a connection. As with any mechanical device, there are various "wearout" mechanisms to consider. Therefore, not only is it important to first choose the correct relay for the application, but also to protect the relay against early failure.

Differentiating Relay Characteristics

There are three common types of relays used in switching and signal routing: the reed relay, the mercury-wetted relay, and the armature relay -- all of which have distinctive advantages and disadvantages. In general reed relays can switch much faster than armature relays, have a very low contact resistance and offer the added benefit of being hermetically sealed. On the down side, they can’t handle the higher voltage- and current-carrying capacity of armature relays. Mercury-wetted relays have long lives, don’t suffer from contact bounce, and have very low contact resistance. However, they are position-sensitive, and must be mounted in the correct orientation to operate properly. While some armature relays are also sealed, others are not. Armature relays usually exhibit very low resistance, although they generally have slower switch times, and are somewhat more susceptible to arcing than the other types mentioned.

Then there are solid-state relays, which are often used when switching high-power circuits, such as ac line voltages, is required. Solid-state relays have the advantage of having no moving parts and being arc-free. However, they generally have a higher "on resistance" than is acceptable in low-level signal switching.

Table 1 compares -- in general terms -- some of the different relay characteristics.

Relay Reliability: Of the above-mentioned relay types, all but solid-state relays rely on the closing of metal-based mechanical contacts. When these electrical contacts -- which are covered with a thin surface film -- are moving to close, a large electrical field is generated between the two contacts, which can initiate an arc. Similarly, an arc can form when these contacts open. This is particularly true if the load being switched is inductive. Arcing and the corresponding welding of contacts are just two of the phenomena that affect relay contact reliability. Other factors that affect contact life include the types of loads being switched, high-power or high-voltage switching, the heat capacity and thermal resistance of the contacts themselves, and the surrounding ambient temperature.

The maximum voltage, current, and power specifications of the relay contacts must be within the expected signal levels being switched. Switch contacts can often carry more energy than they can break at a switching point. In all cases, the lower the energy switched, the longer the life of the contacts. Because switching systems can accumulate a large number of switch closures, prolonging relay life can be very important.

Relay Derating: Relay ratings should be derated based on the type of load being switched. Loads can be classified into five general groups.

Resistive Loads: This is the type of load assumed for the rating of a relay. The load is a simple resistive element, and it is assumed that the current flow through the contacts will be fairly constant, although some increase may occur due to arcing during "make" or "break." Ideally, a relay with a purely resistive load can be operated at its stated voltage and current ratings and attain its full lifetime. Industry practice, however, is to derate to 75% of the relay’s stated capacity.

Inductive Loads: Inductive loads are difficult to switch, due largely to the inductor wanting current to continue to flow even as a contact is being broken. This stored energy in the inductor will induce arcing, and an arc-suppression scheme of some sort is often employed. Industry practice is to derate the relay contacts to 40% of the resistive load rating when switching inductive loads.

Capacitive Loads: When a capacitor is charging, it resembles a short circuit. Consequently, the in-rush current from a capacitive load can be very high. Series resistors are often used to limit this in-rush current; without such a limiting resistor, contact welding may occur. It is suggested that a relay be derated to 75% of the resistive rating when switching a capacitive load.

Motor Loads: When an electric motor initially starts, it has very low impedance, and requires a large in-rush current to begin building a magnetic field and begin rotating. Once it is running, there is a back electromagnetic force (emf) generated, which can cause a large inductive spike when the switch is opened. The end result is a large in-rush current at "turn-on" and arcing at "turn-off." Industry practice is to derate to 20% of the resistive rating when switching a motor load.

Incandescent Loads: An incandescent lamp is considered a resistive load. However, the resistance of a hot tungsten filament is 10 to 15 times that of its cold resistance. The in-rush current into a cold filament is very high, and can easily damage relay contacts. Relay values should be derated to 10% of the resistive load rating. When possible, a current-limiting resistor should also be placed in series with the filament to limit this in-rush current.

Table 2 summarizes relay switch derating factors based on the type of load switched:

Suppression Circuits

As previously mentioned, it is often desirable to limit the surge current into the relay contacts. Whenever a relay contact opens or closes, electrical breakdown or arcing can occur between the contacts. This can cause high-frequency noise radiation, voltage and current surges, and physical damage to the relay contacts. A simple resistor, inductor, or thermistor in series with the load can be used when driving capacitive loads to reduce the in-rush current, while techniques to clamp the voltage can be used for inductive loads.

Clamps, a diode, zener, varistor, or resistor/capacitor (RC) network can also be placed in parallel with the load as a snubber or suppression circuit. RC networks and varistors bear a closer inspection.

RC Protection Networks

When designing RC protection networks, the protection resistor (Rp) is selected as a compromise between two resistance values. The maximum acceptable relay contact current (I_max) determines the minimum value of Rp. Assume that the maximum allowable relay current (I_max) is 1 A dc or ac rms. Thus, the minimum value for Rp is V/Io, where V is the peak value of the supply voltage.

The maximum value for Rp is usually made equal to the load resistance (RL). Therefore, the limits on Rp can be stated as:

Note that the actual value of the current (Io) in a circuit is determined by the equation:

Where V is the peak value of the source voltage, and RL is the resistance of the load. The value for Io will be used to determine the value of the protection capacitor (Cp).

To determine the value of the protection network capacitor (Cp), there are several factors to consider. First, the total circuit capacitance (C_tot) must be such that the peak voltage across the open relay contacts does not exceed the maximum voltage rating of the relay. For a rating of 300 Vrms, the equation for determining the minimum allowable circuit capacitance is:

where L is the inductance of the load, and Io is the current value calculated earlier.

The total circuit capacitance (C_tot) is actually made up of the wiring capacitance plus the value of the protection network capacitor Cp. Therefore, the minimum value for Cp should be the value obtained for the total circuit capacitance (C_tot). Note that the actual value used for Cp should be substantially greater than the value calculated for C_tot.

Using Varistors

Use a varistor when adding an absolute voltage limit across the relay contacts. Varistors are available for a wide range of voltage and clamp energy ratings. Once the circuit reaches the voltage rating of the varistor, the varistor's resistance declines rapidly. A varistor can supplement an RC network, and is especially useful when the required capacitance (Cp) is too large.

A relay’s full potential life can be realized -- if the correct relay type is chosen -- if voltage, current and power ratings are kept within the relay’s ratings (derating as appropriate for a given load type), and if snubber circuits are added as required.

References:

HP 34970 Data Acquisition and Control Unit Users Manual
Electronic Engineer’s Reference Book, Edited by FF Mazda
Electronic Engineer’s Handbook, Fourth Edition, Donald Christiansen

Biographical sketch:

Mark D. Bailey is the voltmeter product manager at Hewlett-Packard Company’s Electronic Measurements Division. Bailey joined HP in 1983, where he worked first in service support and then in on-line field sales support. Since then, he has been involved in customer research, product definition, and several product launches involving the

HP 34401A digital multimeter, the HP 33120A function generator, and most recently the HP LogicDart advanced logic probe.

Prior to joining Hewlett-Packard, Bailey worked for nine years as an R&D design engineer at Tektronix, Inc., where he was project leader on several different instruments. Bailey holds a B.S. in Electrical Engineering, which he earned at Brigham Young University. He also owns MDB Sound, a business specializing in sound reinforcement.

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