Types of Circuits - Control Circuits

A control circuit is a special type of circuit used to control the operation of a completely separate power circuit. Consider a 1,000 horsepower, large industrial motor driving a water pump. The motor is connected to a high voltage electrical supply of 2,400 volts.

When this motor is energized, it must draw enough current to get the water moving and it is common for a motor to draw about six times its normal operating current for a short period of time. When we were talking about controlling light bulbs, it was safe to operate a simple switch on the wall. But now this large amount of current flowing when the motor starts can be troublesome. The first concern is the operator's ability to safely close the switch. The second concern is that when the operator opens the switch to turn the motor off, the electricity will continue to try to complete the path. This will tend to arc between the contacts of the switch as it is opened. This arcing is not only dangerous but also damages the switch by severely burning the contact points. A control circuit is used to ensure that the motor is started and stopped in a safe manner for both the operator and the equipment.

A common control circuit example is the thermostat to the air conditioner in a house. The thermostat is part of a low-voltage control circuit that controls a relay that actually energizes and de-energizes the power circuit to the air conditioning compressor.