A relative amount of power for operating the relay coil is the benefit of relay but there are various usage of relay in control motors, lamps, heaters, Ac circuits which use a lot of electrical power. For more information join the electrical training course to make your career in this field.
An output device (actuator), electro-mechanical relay arrives in a lot of sizes, shapes, and designs. There are lots of applications, uses in electronic circuits. For reducing the electronic power or computer type circuits, electrical relays are used for switching relatively high voltages or currents for both ON or OFF state controlling, relay switch circuit is required.
The types and design of relay switching circuits is huge, but lots of tiny electronic projects utilizes MOSFETS and transistors. As the transistor can offer rapid DC switching (ON-OFF), it is used as main switching device. Let us see few relays over here.
NPN RELAY SWITCH CIRCUIT
A coil driven NPN transistor is a typical relay switch circuit relying on the input voltage level. The transistor is disconnected and acts as an open switch when the base voltage of the transistor is zero (or negative). There are no collector current flowing and the current devices causes the coil relay to be de energized and there will not be any current flow through the relay coil if there is no current flow in the base.
In the base if a positive current is drawn for NPN transistor saturation, the B to E current flow controls the larger relay coil passing through the transistor from the Collector to Emitter.
For the famous bipolar switching transistors, the flowing currant’s amount in the relay coil inside the collector between 50 to 800 times and for driving the transistor to saturation, the needed base current is made to flow. The beta value or the current gain of the general purpose BC109 is a 290 mA data sheet.
NPN DARLINGTON RELAY SWITCH CIRCUIT
For switching small loads like LEDs and miniature relays, the ancient NPN transistor relay circuit is very apt. For switching larger relay coils or currents crossing the threshold range of BC109 transistor can be achieved using Darlington Transistors.
The current gain and sensitivity of a relay switch circuit can increase greatly using a pair of transistors of Darlington. Two similarly connected Bipolar transistors can be shown into one device with standard: Emitter, Base, and Collector connecting leads.
The first transistor’s collector current TR1 becomes the base current of the second transistor TR2 because two NPN transistors are connected.
Darlington switching pair configured from two individual transistor have a small value resistor (100 to 1000 ohms) placed between the Emitter and Base of the main switching transistor, TR2 for ensuring the fully OFF turns. A flywheel diode again is used for protecting TR2 from the emf generated back during the relay coil DE-energizing.
EMITTER FOLLOWER RELAY SWITCH CIRCUIT
The Standard Common Emitter Configuration having a relay coil can be connected to Emitter terminal for forming an Emitter Follower Circuit. There is a direct connection of input signal directly to the base and the output is driven from the Emitter load.
EMITTER DARLINGTON RELAY SWITCH CIRCUIT
The Darlington transistor version of the ancient emitter follower circuit has TR1, a small positive base current causing a greater collector current to flow via TR2 because of multiplication of the two Beta values.