A Guide to Switch Considerations - Signals Involving Reactive Loads
Switching circuits that include reactive elements need special care to avoid problems due to transient effects. The methods used to limit transient effects depend on whether the load is capacitive or inductive. When a capacitive load is connected through a switch to a voltage source, the in-rush current may exceed the current rating of the switch and weld the contacts shut. As Figure 28a indicates, when the switch (S) is closed, the peak current (i) is limited mainly by the sum of the wiring resistance and the relay contact resistance. The peak current may exceed the current rating of the relay and cause welding of the contacts. Cold switching is the most effective way to prevent this current surge.
If cold switching is impractical, it may be possible to add series resistors to limit the current to a safe value, as shown in Figure 28b. The value of the series resistor (R) should be greater than the ratio of the applied voltage to the maximum current rating of the switch card. However, if the resistor is too large, it may affect measurement accuracy. The current surges of these two circuits are illustrated in Figure 28c.
Figure 28. In-rush current of capacitive load
Capacitor leakage measurements and insulation resistance measurements of multi-conductor cables are two applications where switching capacitive loads may be a particular problem. Even with resistive loads, the capacitance of a shielded connecting cable may cause relay welding. In this case, the series resistor should be placed as close to the relay as possible to limit the current when charging the cable capacitance.
When an inductive load is connected to a voltage source, the current will increase relatively slowly. However, when the switch is opened, a large inductive reaction voltage will appear across the switch contacts and may damage the contacts. The contact bounce that occurs on closure can also produce an inductive reaction voltage, because the current is interrupted repeatedly. A voltage-clamping device across the inductive load is usually required. Figure 29 illustrates four possible circuits for voltage clamping. For best results, the voltage-clamping device should be located near the load.
Applications that involve switching inductive loads include testing motors, solenoids, and transformers.