Tips on Applying Relays


15.3 Coils

Misunderstanding of coil operating power and other coil input characteristics can cause problems.

15.3.1

According to the standards used in the relay industry, AC relays will pick-up at 85% of the nominal voltage, and DC relays will pick up at 80% of the nominal voltage (although automotive relays are typically specified at 60% of the nominal voltage). If other values are needed, the relay user must specify those values. In most industries, it is not normal practice to require a " must not operate" voltage.

15.3.2

Most manufacturers only guarantee that the relay will drop out at greater than zero voltage. If a higher number is needed, this must be specified. Again, most industries do not require a "must not release" voltage.

15.3.3

In certain circuits, the load may be specified as an AC load, typically a lamp load. However, driving the coil may be an electronic circuit that always switches at the same point of the sine wave. Most relays are essentially constant-operate time devices at a given voltage and temperature, the relay contacts may see what is essentially a DC load. This can significantly limit the contact life.

15.3.4

Not generally considered by the user is the giving of sufficient allowance for changes in coil drive at temperature extremes. This can be critical, particularly at higher temperatures, as the coil resistance increases and the coil power becomes reduced. The copper resistance increase of about 4% per 10°C means that a 5 volt relay that operates at 4 V at 23°C would not operate until 4.5 V at 55°C, or 4.8 V at 75°C. This leaves virtually no overdrive or allowance for internally generated coil temperatures, but is not as frequent a problem.

15.3.5

The pick-up voltage of the relay may have to be lower than the power supply voltage due to the voltage drop in the wiring. This is particularly true when using doorbell-type or other small gauge wiring of any appreciable length.

15.3.6

If a relay is energized for a length of time, dropped out, and again energized in a short period of time, the pick-up voltage will increase due to the inherent temperature rise of the coil. Under such circumstances, the 5 volt relay form above may not operate the second time at 75°C even though its pull-in at 75°C was 4.8 volts. It is not unusual for a relay to achieve internal coil temperatures of 30°C to 50°C higher than the ambient.

15.3.7

Relays should not be operated at pick-up voltage. This does not allow for any overdrive to compensate for coil temperature rise. The overdrive also provides faster operate speeds which can significantly affect relay switching life on DC loads applied to the normally-closed contact sets. Relays should be operated at nominal voltage.
Current-operated relays, unless specifically designed for a sliding drive signal, should be operated at 130% of pick-up current.

15.3.8

Some relays may not operate properly or reliably when operated with a ramp voltage. Normally, it is best to use a step voltage so the contacts are properly seated.

15.3.9 Suppressing relay-coil transients

When a relay coil is tuned off, the inductive energy stored in it can create surge voltages up to 1,500 V on a dc power line. Prior to the use of solid-state circuitry, this surge was not critical. Now, with the increasing use of solid-state devices, relay coils must be suppressed to limit the voltage spikes to within 50 V to 80 V.
When the power to a coil is interrupted, the suppressive device absorbs and dissipates the coil's energy or allows the relay coil to dissipate the energy. The measure of successful suppression depends on the degree to which the method affects the drop-out characteristics and relay life. Extreme or improper suppression causes long release time, slow contact transfer, contact bounce on break, and armature rebound. These conditions lead to arcing and contact welding, which reduce contact life.
There are seven common used ways to suppress a relay coil, and each has advantages and disadvantages.

15.3.9.1 Bifilar Coil

A bifilar relay coil ( see definition of "Winding, Bifilar" in Chapter 1 ) has two windings: (1) a power winding, and (2) a shorted winding that absorbs the inductive energy from the power winding. To minimize surge voltages, the shorted coil usually has the same number of turns as the power coil, and about 1.5 times the resistance. This is accomplished by using a smaller wire size. The smaller wire size, combined with the dual windings, considerably increases cost. Also, bifilar windings greatly increase release time (can cause break bounce and armature rebound) unless contact pressure and armature position are adjusted.

15.3.9.2 Resistor

This is the simplest and oldest method of suppression. A properly chosen resistor gives adequate suppression but requires extra power to the coil and across control contacts. Increasing the suppression by lowering the resistance causes longer release time and arcing. A rectifier diode can be added in series with the resistor to eliminate the steady-state power dissipation and permit use of a lower wattage resistor for the suppression. The addition of the diode; however, creates a polarity sensitive relay and care must exercised in applying the relay.

15.3.9.3 Varistor

This device is similar to a resistor in that it continuously draws current when the coil is energized. But since a varistor is voltage sensitive, it does not draw an appreciable amount of current until the inductive voltage appears across it. A proper varistor allows near normal relay operation during drop-out. Varistors are bi-polar devices and thus do not create a polarity sensitive relay. The proper size varistor may cause a packaging problem.

15.3.9.4 Resistor-Capacitor

This combination eliminates power drain when the circuit is on but, like the resistor, increases drop-out time. Also, the capacitor required to do the job usually is too large for the space available. An improperly chosen resistor can damage the controlling device during the capacitive in-rush.

15.3.9.5 Diode

This device is an excellent suppressor, but it affects relay characteristics in the same manner as bifilar windings. It increases release and transfer times and often will cause break bounce and arcing. Diode suppression has been found as the root cause of many "tack welding" problems. Additionally, it is polarity sensitive.

15.3.9.6 Zener-Diode and Zener-Zener

These combinations are compact, give excellent suppression, and do not affect drop-ouot time or relay life. The Zener-diode combination, however, is polarity sensitive. A diode only, shunted upstream from the relay, will bypass the Zener-diode combination at the relay. The most reliable check of the system is to verify the relay action on an oscilloscope.