Mechanical Relays.

Design File DF05, May, 2006

When you need to switch a heavy load with a light current, or need to physically isolate a switched circuit, the relay is one of the best components available. Cheap and rugged, the mechanical relay comes in a variety of sizes and rating. But to effectively use a relay, you need to understand how it operates and its limitations.

Notes:

Mechanical Relays

As electronic components go, the relay is one of the oldest. Actually an electro-mechanical device, in the past it has served many roles from a power and signaling switch, to even a logic and memory element. Today its role is primarily limited to signal and power switching applications. Its ability to use a light input current at low voltage to switch high voltage and high current loads, high electrical isolation of the load from the drive circuit and the characteristically low impedance of the output switch make the relay an ideal microprocessor interface device for switching high power loads and signals with low loss.

Relay construction

The electromechanical consists of two parts, a electromagnetic coil which when powered by a drive current magnetically activates either a simple reed switch for light loads, or a more elaborate spring loaded mechanical switch for heavier loads. (Figure 1).

The coil is usually relatively high impedance, allowing the use of a small current and low voltage to activate the switch. The switch itself is usually much higher current and voltage rated. Thus, a low voltage, light current drive can switch very high current and voltage loads, with complete electrical isolation between the drive and the switched circuits.

Relays switches can be normally open or normally closed when inactive, or can have both output available.

The coil makes the relay a current driven device. Drive can be either AC or DC. AC drive types are usually for line voltage applications, and will not be considered in this discussion. The DC drive type is the most flexible, and the type found in most electronic control circuits.

When the coil current is removed from a standard relay, the driven switch state is released by the spring action of the switch or reed. In may applications, this can be a good fail-safe feature. If the switch circuit is designed to be in safe-mode when ever there is no signal to the relay, loss of power to the drive circuit will cause the switched circuit to automatically go into safe-mode.

There are also "latching" type relays which use only a short pulse to change the switch state. The switched is "latched" into on or off state, even when the coil current is removed. To change the state, depending upon the design, the relay is pulsed again, or pulsed with an opposite polarity pulse. This design is good for applications where the relay control circuit needs to be low-powered and the switch needs to remain on for a long period. After pulsed on or off to the active state, the relay does not consume any more power until next state change.

If you need a voltage driven device, you may consider the new solid state relay types. These are not actually true relays. They are a semi-conductor switch, transistor or triac for example, usually controlled by an high impedance photo-isolated signal. They are not as flexible in the type of signals they can drive. We will not cover solid state relays in this discussion, but be aware that they exist.

Relay Ratings

The basic relays rating parameters are, the minimum voltage and current required to switch the relay on, the electrical isolation (as a resistance at a rated voltage) between the drive coil circuit and the mechanical switch, the contact resistance of the switch points, and the voltage and current contact rating of the switch. A life-cycle rating may also be specified, but it is dependent on the load being switched - heavier loads will degrade the switch.

Detailed specification will also specify minimum and maximum switching times. Some relays designed for specific tasks, such as switching high frequency signals, have other applicable specifications. There are a lot of different types of relays - read the manufacturers specification carefully.

Design Considerations, Coil Drive.

First and foremost, when implementing a mechanical relay driver interface, remember that there is an inductor in the drive circuit!

As with all inductor circuits, a charge time is needed for the coil current to build up, so switching is not instantaneous, it may take milliseconds to reach the trigger point.

But even more important is the converse: when switching the drive circuit off, the inductor current cannot be instantaneously shut-off.

When the relay is switched on, the switching device, usually a Darlington or FET transistor in current sink configuration creates a low impedance path to ground or the low potential voltage of the circuit, to switch current on through the inductor. When the drive device switches "off", the drive device goes into high impedance.

The coil on the other hand now becomes a current source with no place to go. Since the current sustained in the inductor by the induced magnetic field cannot be instantaneously switched off, you have the situation of a constant current trying to flow through a suddenly high resistance - something has got to give, in this case it is the magnetic field in the coil. With the resultant collapse of the magnetic field there will be very significant voltage spike at the switching point- most likely high enough to cause the switching device to catastrophically fail.

The solution to the problem is to first select a drive device suitable for switching inductive loads. For example, the MB4001 Quad Relay Controller uses a ULN2003 Darlington array designed to d rive loads with moderate inductive"kick-back."

Then second, clamp any voltage pulses and provide a low impedance path for the current in the inductor to "gradually" decay, preventing a voltage spike.

This type of circuit is called current snubber or clamp if just a diode is used. The ULN2003 circuit contains clamp diodes on each output (figure 3.) The simplest clamp is just a diode of sufficient current rating, connected in reverse bias back in to the power supply to both clamp the high voltage pulse and create a low impedance path for the coil current to recirculate back into the power supply (figure 4).

In most cases this is a sufficient, but because of the low impedance of just a forward biased diode, the decay time of the recirculating inductor current can be longer than desired. This can result in slower switching response times than required.

When making contact, all contact points will exhibit a small degree of micro-welding called contact "sticking". The "sticking" break force is usually well within the forces created by the normal contact return velocity. If the decay time of the coil current is significantly extended by the clamp circuit, thus lowering the return velocity, it it is possible for the relay to remain in the "sticking" state. The solution is to insure sufficient current decay rates to overcome potential sticking problems. A small resistance can be placed in the return path to help dissipate the inductor current (figure 5).

One other solution that works well is to place in the path a Zener diode of approximately two times the voltage of the power supply (figure 6).

Finally, to reduce the potential injected noise from the coil voltage as it is switched on and off, a small capacitor can be placed in a parallel with the coil.

One design consideration often missed is when designing densely packed banks of relays, electromagnetic compliance must be considered. Normally the electromagnetic field required to activate a relay is too high for adjacent relays to false trigger a relay on, but is possible for adjacent "on" relays if their magnetic fields are in sync to significantly delay the turn-off - on times of an adjacent relay. A simple solution is to power adjacent relays in such away that repulsing magnetic fields are created, or use available shielded relays for sensitive applications.

The contacts.

Since the output of the mechanical relay is a magnetically driven mechanical switch, all of the characteristics of mechanical switches apply. See Design file DF01 for a detailed discussion of mechanical switch noise characteristics.

Like any mechanical switch, the main consideration is to select a relay capable of handling the voltage and current required to be switched. The biggest problem is arcing between the contact points of the switch as the switch opens and closes. This is why the rating of the switch is different for AC versus DC voltages.

When driving inductive loads, it is important to include an arc suppression circuit (as we did above) to limit potentially damaging current spikes. In the case of AC loads, a diode is not an option, but a suitable capacitor connected across the contacts can be used.(figure 8.)

In the case of a large capacitor - which itself can hold a large enough charge to cause arcing, a small series resistor can be added to slow the capacitor discharge time, as well as discharge any residual voltage in high voltage applications. It can be both unnerving as well as dangerous to get a jolt from a circuit that is "off", but holding a charge on the filter capacitor!

Other Considerations

Finally, non-linear load affects must be taken into account. For example, switching inductive loads as discussed above present problems when switching off. Other types of loads can present special problems such as potentially high in-rush currents when switched on - just the opposite of an inductor.

An example of this is a tungsten light bulb - one of the commonly relay driven devices. When a tungsten light bulb is switched, the filament has very low resistance when cold. As the filament heats up, the resistance increases. The in rush current can be as much as 10 times higher than the operating current. To safely switch this type of load you must either over-rate the relay, or use a current limiting series resistor.

Conclusion

The ability of the relay to use a light input current at low voltage to switch high voltage and high current loads, high electrical isolation of the load from the drive circuit and the characteristically low impedance of the output switch make the relay an ideal microprocessor interface device for switching high power loads and signals with low loss.

But for reliable performance, when designing carefully consider the electrical loading effects of the drive coil as well as the switched load.