LEACH ® INTERNATIONAL
7. HOW TO PICK A RELAY
How to keep
an airline pilot
straight and narrow
Relays have been around for many decades, yet have survived evolving technologies and still retain a significant position in today's electrical and electronic systems. There have been changes in materials and changes in design, but the relay is still essentially a simple electro-mechanical switch. With today's ever-increasing need for the switching function, the relay offers advantages over other switching techniques. It can be used singly without auxiliary circuits (aside from a power supply); it exhibits very high isolation between controlling and controlled circuits; it can result in a simple, inexpensive circuit fast enough even for today's high speed world; and it can be compatible with semiconductors.
On the other hand, this compatibility is sometimes not achieved because one relay parameter or another is not considered. A relay is a very simple device - how much engineering time should be spent on it? Let's find out.
THE HIGH AND THE MIGHTY
The cockpit of a commercial airliner witnessed the results of a lack of consideration for relays. The flight was on schedule, at cruising altitude and making good time. On auto pilot and smooth - when all at once the stories of the last layover were interrupted by bells, buzzers and flashing red lights. Not Christmas - autopilot failure. The rest of the flight was on instruments and the seat of the pants. A safe flight, but a busy one.
What happened? Several transistors in the autopilot computer and controller had been burned out, and this was traced to negative, high voltage spikes on the 28-volt DC line. The cause? Transients generated by relay coils in the system. The solution? Transient suppression. The reason for problems? Overlooking the fact that the relay coil is an inductor.
System designers usually take into account the problems associated with the making and breaking of currents by the relay contacts. In this case, arc suppression had been included on the load side of the relay to extend contact life and reduce the RFI generated by arcs. (Contact protection is treated in a later installment.) The designers had, however, neglected the fact that the relay coil, too, is a non-resistive load and is thus capable of generating interference. The energy stored in the coil inductance is seen as a back EMF across the coil when the drive is removed. This voltage is usually greater than 750 volts and can be as large as 3000 volts in a 28-volt circuit. Few components are designed to withstand voltages of this magnitude.
WHAT TO DO
The first step in curing circuit interference is to limit the magnitude of the coil generated spike. Any of the circuits shown will do this.
The diode in Figure 1 is probably the most popular form of voltage suppression used today. A single diode (D1) can be used, but this is frequently burned out by the application of the wrong polarity to the coil. Diode D2 prevents this type of damage. Diode D1 provides a very low resistance re-circulating path for the energy in the coil, and thus offers the highest degree of suppression available. Because of the low resistance, however, the time constant for energy decay is quite high, and the dropout time of the relay with a diode across the coil is increased at least 2 times - and often 10 times - the normal value for the unsuppressed relay. This slows the rate of separation of the relay contacts, and can increase arcing damage to the contacts. This actuation delay can also be very critical when several circuits are operating interdependently.
One method of suppression frequently used by relay manufacturers, the bifilar coil, is shown in Figure 2. This is manufactured by winding two coils in parallel simultaneously, then shorting the secondary coil inside the case. The resistance of this secondary coil determines the effectiveness of the suppression. The maximum back EMF generated is approximately equal to the applied coil voltage times the ratio of the coil resistances (EMF = V x R bifilar/R coil).
This last equation seems to point up the bifilar method of suppression as one that can deliver an extreme amount of back EMF limiting. This is true, but you never get something for nothing. The bifilar coil in a relay has a considerable effect on relay performance. For instance, the smaller the resistance of the bifilar coil, the smaller the back EMF, but also the longer the dropout delay. The transfer time of the relay contacts may increase as much as 5 times.
In addition to this, because a current flows in the secondary coil, a magnetic field is generated by it. As the armature moves, the air gap changes and causes a change in the magnetic flux, which in turn causes an increase in self-induced current in the secondary winding. This increase not only slows down the motion of the armature, but may even reverse its direction. If this is the case, break bounce occurs. This of course can cause arcing, which damages the contacts and shortens contact life.
BUT ZENERS ARE BETTER
A better method of suppression is shown in Figure 3. The zener diode can be placed in series with the shunt diode (D1) shown in Figure 1. Or, two zener diodes can be used back-to-back instead. This latter arrangement has the advantage that it is not polarized. The peak back EMF from the coil is now limited to the breakdown voltage of the zener diode. The breakdown voltage obviously should be chosen to be greater than the applied voltage on the coil. The increase in the drop out time when using this technique is negligible. In essence, the zener diode affects the relay performance only when that performance is out of its normal range; the rest of the time the relay behaves as if the zener were not there. (This is not the case with the bifilar coil, which affects the relay's performance at all times.)
The series RC circuit of Figure 4 is effective for voltage limiting, but is usually used only with coil currents under 100mA because of the voltage drop through the series resistors. This voltage drop can be eliminated by placing the resistance and capacitance both in the shunt path.
Figure 5 represents a common relay interface - the transistor driver. Because of the low currents required by relay coils, economical drive circuits can be readily designed. The transistor eliminates the possibility of arcing that exists when a switch activates the coil directly. Even if a switch is used to turn the transistor on, the transistor serves as a buffer amplifier so that a much lower (and hence less troublesome) current is being interrupted. Operate time can be tailored by varying the value of the capacitor. The zener shown prevents transistor burn-out when the coil is de-energized. The ground side of the relay coil is the preferred location for this type of circuit to make it "fail safe."
IF YOU NEED MORE
No component values have been given for any of the suppression circuits because these are dependent on circuit and relay coil parameters and choices will have to be made on that basis. The aim of suppression is voltage spike reduction, but contact life can be reduced if the techniques are improperly applied. If you have a specific application with which you would like some assistance, don't hesitate to contact us. We have an application group with years of experience at applying these techniques to practical, reliable circuits and would be happy to help you.
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