Diodes used in switching circuits require snubber circuits. It can protect a diode from overvoltage spikes that may occur during reverse recovery. As illustrated in Fig. 2.7, a conventional snubber circuit for a power diode consists of a capacitor and a resistor linked in parallel with the diode. The capacitor charges during forward conduction of the diode, and it discharges during reverse recovery. The resistor dissipates the energy stored in the capacitor.
What is so special about using capacitors instead of inductors for snubbing? Why don't all diodes need snubbers?
Inductors are useful components to have in an electrical system because they can store energy temporarily while reducing voltage across themselves. This is particularly important for diodes which cannot handle high voltages between their anodes and cathodes. Without a snubber, if voltage were to rise sharply on the anode side of the diode while the cathode side was still conducting current, the diode would break down instantly. A snubber uses this property of inductors to reduce the voltage on the anode when the diode goes into reverse bias mode. A capacitor has the opposite behavior - it will charge up when current flows into it and it will discharge when current leaves it. Because diodes need snubbers, it makes sense to use components that can store energy temporarily to reduce the effects of sudden voltage changes on the diode.
The resistance R in the RC snubber circuit restricts the discharge current of the capacitor at the time of SCR firing. Capacitor C charges to full voltage V before the SCR is activated. When SCR is activated at the maximum capacitor voltage, it discharges through the local route created by capacitor C, resistance R, and SCR. Resistor R should be large enough so that it does not limit the current into the capacitor when it is charging.
RC snubbers are used in high-voltage applications where the use of magnetic circuits is undesirable. They also provide some protection for optical devices such as photodiodes. An example application would be where a laser diode is being powered by a battery with a voltage that varies with age or state of charge. The laser diode needs to operate at a constant voltage to prevent damage to its internal circuitry. An additional resistor in series with the cathode of the diode provides this protection against varying battery voltage. If even just one of the batteries goes bad, then the laser diode will shut off automatically.
Snubbers are often included in power supplies to protect other components from over-current conditions caused by impedance mismatches in the output stage. For example, if a load includes both a resistive and a capacitive component, then an inductor or flyback diode can supply current to the load while preventing any dangerous reverse voltage from appearing on the supply source.
Circuit breakers and contactors, for example, frequently require extra switching capabilities in addition to their basic connections. Remote status indication, trip function indication, electrical interlocks, and start circuit retainers are examples of these. Auxiliary contacts provide this additional functionality.
Auxiliary contacts are normally closed (NC) contacts that open when they are activated by a signal from the main circuit breaker or contactor. They then stay open until they receive another signal from the main device. This second signal can be either a new command to close the contacts or an indication that the original command has been cancelled. Open auxiliary contacts do not conduct electricity so they cannot cause damage if one is accidentally left open.
Auxiliary contacts are used where remote operation or multiple control of the main device is required. For example, if a circuit breaker serves several floors of a building, it can be controlled from each floor with an individual auxiliary contact on the panel board. When the contact on one floor is opened, all others remain closed. The first floor's contact to be opened will automatically send a signal down the line, causing all other floors to be opened. This is useful in reducing the risk of fire because the presence of any open circuits will trigger a total shutdown of the current.
Auxiliary contacts can also be used as electrical interlocks.