LearnTronics
Electronic Parts
Discrete Components
Switches
Relays
A relay is a switch controlled by another electrical circuit. In a traditional electromechanical relay, current through a coil creates a magnetic field that moves an armature and changes one or more sets of contacts.
Relay symbols
Coil and contact are electrically separate
COM transfers from NC to NO when energized
Common protection for a DC relay driver
Coil and contacts
The relay coil is the control side. The contacts are the switched side. They are usually electrically isolated from each other, which allows a low-voltage control circuit to operate a separate circuit without making a direct conductive connection between the two.
Relay contacts use the same terminology as ordinary switches: SPST, SPDT, DPDT, NO, and NC. A common SPDT relay contact has three terminals labeled COM, NO, and NC. With the coil de-energized, COM is connected to NC. When the coil is energized, the armature transfers COM to NO.
Contact forms
| Relay form | Equivalent contact idea |
|---|---|
| Form A | Normally-open contact; commonly equivalent to SPST-NO. |
| Form B | Normally-closed contact; commonly equivalent to SPST-NC. |
| Form C | Changeover contact with COM, NC, and NO; equivalent to SPDT. |
A relay may contain several identical forms in one package. For example, a relay with two Form C contact sets is electrically similar to a DPDT switch whose actuator happens to be an electromagnetic coil.
Coil voltage
A relay coil is designed for a particular operating voltage or voltage range. A 12-volt relay should not simply be assumed suitable for 5 volts or 24 volts. DC coils also have resistance and therefore draw a predictable current when operated near their rated voltage. Coil current can often be estimated from the specified coil resistance using Ohm's law, but the relay's own datasheet remains the authority for operate and release requirements.
The flyback problem
A DC relay coil is an inductor. When its current is suddenly interrupted, the collapsing magnetic field can generate a voltage high enough to damage the transistor or electronic circuit driving it. A diode connected across a DC coil is commonly used to provide a safe path for this current. The diode is reverse-biased while the coil is energized and conducts briefly when the drive is removed.
Driving a relay from logic
A microcontroller or logic output often cannot safely supply the relay coil current directly. A transistor or MOSFET can be used as the coil switch, with the logic output controlling that device. For an ordinary DC coil, the flyback diode is then placed across the coil, not across the transistor. Other suppression methods may be chosen when fast relay release is important.
Contact ratings
The coil rating and contact rating are separate specifications. The fact that a relay has a 5-volt coil says nothing by itself about the voltage or current its contacts can safely switch. Contact ratings may also be different for resistive, inductive, motor, lamp, AC, and DC loads.
Protecting relay contacts
Motors, solenoids, transformers, lamps, and capacitive loads can be much harder on contacts than a simple resistor. Depending on the load, arc suppression, a snubber, a diode, a varistor, or an appropriate inrush-limiting method may greatly improve contact life. The protection method must suit whether the switched circuit is AC or DC and how quickly the load must release.
Reed relays
A reed relay places one or more reed switches inside a coil. Energizing the coil creates the magnetic field that operates the reeds. Reed relays can be compact and fast and are often useful for low-level signals and instrumentation.
Latching relays
A latching relay retains its contact position without requiring continuous coil power. Some use two coils, while others reverse the polarity or use a mechanical latching mechanism. They are useful where coil power must be minimized or where the selected state should remain after control power is removed.
Solid-state relays
A solid-state relay (SSR) performs a relay-like function without moving contacts. Its input commonly drives an optically isolated semiconductor output stage. SSRs can switch rapidly and silently and do not suffer mechanical contact wear, but they have on-state voltage drop or resistance, leakage current when off, heat-dissipation requirements, and load-type limitations that differ from mechanical relays.
Relays in early switching systems
Long before semiconductor logic became practical, relays were used to repeat telegraph and telephone signals and later to implement control logic. Large banks of relays could perform sequences and logical decisions. This made relay switching one of the important technological ancestors of electronic digital logic.
Return to the Switches overview.