How to design a transistor switch


Source: Ben Krasnow

Precautions Switching Inductive Loads

Many times we need to switch inductive load like solenoid valve through a transistor switch and for some tasks require powering a heavy load via relay/contactor. The solenoid valve, relay/contactor and any coil related device primarily acts as an inductor because it’s a coil and the inductors do not like sudden changes in current. If the current flow through a coil is suddenly interrupted for instance when the switch is opened, the coil will react with a sudden very large voltage across its leads causing a large surge of current through it due to collapsing magnetic field within the coil as the current is interrupted abruptly. Surges in current resulting from inductive spike can results in thousands of voltage which will possibly harm neighboring devices within the circuit such as switches, transistors and relay contacts etc.

A way out of this is to use what is known as transient suppressors to interrupt the current through the coil. Typical transient suppressor is a diode across the relay coil in the relay driver shown below.

Fig 1Fig 1 Relay Driver

The circuit above is a typical transistor switch control driven by an input voltage whereby when sufficiently large voltage and input current are applied to transistor’s base lead, the transistor’s collector-t-emitter channel opens, 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 an RC network is used as a transient suppressor especially for ac – actuated relay, but it’s important to use a resistor and capacitor rated for a potentially large transient current suited to the coil current.

Fig 2Fig 2 AC Driven Coil

In the AC Driven Coil with RC transient suppressor shown above, the capacitor absorbs excessive charge, and 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.