A Relay is an electro mechanical switch used for driving heavy current loads in electronic circuits. Relay interface the electronic circuits with high voltage AC and there is complete galvanic separation between the two. When current passes through the relay coil, magnetic field develops which attracts a lever and changes the switch contacts. The contacts can be used to connect AC or DC loads which may then turn On / Off the load depending on the contact connections.
A relay allows one circuit to switch on a second circuit which is completely separate from the first one. For example, a battery operated circuit can operate AC loads through a relay. There is no electrical connection inside the relay between the two circuits so that total galvanic isolation is possible.
The coil inside the relay is the most important part which passes only DC current from the circuit. When DC current passes through the coil, magnetic field develops which attracts the moving contact so that the contacts may break or connect. The relay coil passes relatively large current of 30 to 100 milli amperes. The relay coil voltage and resistance are important parameters to select the appropriate relay for the circuit.
Different types of relays like 5 V, 6V, 12V 24V etc are available to operate in different DC voltages. Generally a supply voltage slightly higher than the rated voltage is necessary to operate the relay. But some relays require lesser voltage than the rated voltage for proper functioning. But it is better to give 2 or 3 volts excess than the rating of the relay for proper switching since there will be slight voltage drop when the circuit works. For example a 12 volt relay requires 14 volt DC for clean switching.
Relay Coil Resistance
Coil resistance of the relay is also important. It is the DC resistance of the relay. Relays are available with 100 to 500 Ohms coil resistance. The circuit must provide sufficient current to operate the relays. The current passing through the relay coil can be determined using Ohms law.
Relay coil current = Supply voltage / Relay coil resistance
For example, a 12 volt relay having 400 Ohms coil resistance passes 30 mA current.
12 / 400 = 0.03 Ampere or 30 Milli Ampere.
So it is important to consider the coil resistance also before using the relay. If the relay coil is rated 400 ohms and the power supply is 12 volt 300 mA, the relay will not work.
Typically a relay has three contacts. A moving contact called Common(Comm) and two fixed contacts. These are Normally Closed (NC) andNormally Open (NO). AC / DC is connected to the Comm. Contact. NO contact is used to connect the circuit / Gadget when the relay triggers. In this case, the circuit/Gadget remains off when the relay is Off. NC contact is used to break the supply when the relay energize. In this case the circuit / gadget remain operating until the relay energize. Contact rating is also important to drive the load. Typical relay contacts are rated for 4 Amps current. If the load is a high current one like Motor, Heaters, Inverters etc, relays having high rated contacts ( 16 Amps or higher) is necessary. These types of relays are called as Industrial type relays. If the contact rating is lesser than the load current, contacts may heat up and fire may break due to sparking.
Based on the number of contacts relays are classified into
SPDT – Single Pole Double Throw – These have a single Comm, NC and NO contacts
DPDT – Double Pole Double Throw. – These have 2 Comm, 2 NC and 2 NO contacts. DPDT relays can be used to drive two loads separately so that on will switch on the load and other will switch off the load depending on the connections in the NO/NC contacts.
These are small sized relays that can be directly soldered on the PCB to make the circuit board compact. Like other type s of relays, PCB relays are available with the rating of 5 V, 6 V, 12 V, 24 V etc. Coil ratings may be 100 to 400 Ohms. Pin connection of PCB relay is shown below.
The diagram is that of a standard type relay. Pins may vary, so Multimeter testing is necessary to confirm the pins before connecting the load.
1. Test pins A and B. If it shows Ohms resistance, the Pins are Relay coil pins
2. Test other pins for continuity to confirm NC, NO and Common.
These are used to drive very high current loads. Coil rating is suitable to carry high current. Contacts are also suitable to carry high current without heating.
These are Relays rated to use in industrial applications to drive heavy loads. Its coil rating will be as high as 16 Amperes or more. Contacts are also suitable to carry high current without heating.
Reed relay consists of two iron strips surrounded by coils. When the current passes through the coil; magnetic field develops around the coil. This magnetic field magnetizes the iron strips and they attract each other to complete the circuit. Reed relay can be used to drive a large current load with relatively small DC current.
Relay Driver Circuit
Transistors or most of the ICs cannot drive the relay directly since the relay requires its rated voltage and coil current. So relay driver circuits using transistors are used to drive relay. The circuit shown below is the relay driver using the common NPN transistor BC 548.
The relay coil is connected between the collector of transistor and positive rail so that when the transistor conducts, current passes through the relay coil and the relay energize. The Comm and NC contacts are always connected so that current (AC or DC) passes to the load. When the relay energize, the Comm and NC contacts break and switch off the load. When the load is connected between the Comm and NO contacts, the load operates only when the relay energize. Always connect the Phase line (AC) or Positive line (DC) through the relay contacts for safety reasons. Give adequate Sleeving to the contacts, if AC load is connected.
Diode D1 is necessary to remove the back e.m.f when the transistor switches off. Without which the transistor will be destroyed since very high e.m.f develops in inductive loads such as relays.
Capacitor C1 is necessary for the smooth switching of the transistor. It gives a short lag before the transistor switches on and after the base current ceases. This prevents the “Relay clicking” or “Relay chattering” when the base current of the transistor fluctuates. Relay clicking causes contact sparks and heating. C1 also keeps the base current of the transistor steady so that the relay switches on smoothly.