Precautions Switching Inductive Loads
Many times we need to switch inductive load like solenoid valve through a transistor switch. In fact, some tasks require powering a heavy load via a relay or a contactor. The solenoid valve, relay or contactor and any device with coil primarily acts as an inductor. This is because it’s a coil and the inductors do not like sudden changes in current.
Watch this video for thorough understanding.
Source: Ben Krasnow
However, if you interrupts the current flowing through a coil suddenly, for instance when you open the switch. The coil will react with a sudden very large voltage across its leads. Thus causing a large surge of current through it due to collapsing magnetic field, within the coil as the current is interrupts abruptly. Surges in current resulting from inductive spike can results in thousands of voltage. The inductive spike will possibly harm neighboring devices within the circuit such as switches, transistors and relay contacts etc.
Transistor Switch Circuit
In the transistor switch circuit, a way out of inductive spike is to use what is known as transient suppressors. The suppressor interrupts the current through the coil. Typical transient suppressor is a diode across the relay coil in the relay driver shown below.
Fig 1 Relay Driver
The circuit above is a typical transistor switch control driven by an input voltage. When sufficiently large voltage and input current applies to transistor’s base lead, the transistor’s collector-to-emitter channel opens. Thus allowing current to flow through the relay coil. Take note of the diode across the relay coil to eliminate imminent damage to the transistor by voltage spike through the switching of the coil.
Sometimes they use an RC network as a transient suppressor especially for relay using ac. But it’s important to use a resistor and capacitor with rating for a potentially large transient current suitable to the coil current.
In the AC Driven Coil with RC transient suppressor shown above, the capacitor absorbs excessive charge. Then the resistor helps control the discharge to safe limit. A typical RC network for small loads driven from the power line uses 100 Ω Resistor and 0.05 µF Capacitor.