Relays, one of the earliest electronic devices, invented by Joseph Henry in
1835, seem simple enough: energize an electro-magnet to pull an armature
that closes a switch. De-energize the electro-magnet so that a spring
returns the armature to its original position to open the switch. The
switch can pass more current than is needed to energize the electro-magnet,
so relays act as digital amplifiers.
Basic relay terminology
There is a naming scheme use to indicate the number of switches and
terminals a relays contains. Poles refer to the number of switches
controlled by a single electro-magnet. Throws refer to the number of
contacts each switch has: single-throw (for a basic on-off switch) or
double-throw (for an on-on switch with both "normally open" and "normally
closed" contacts in addition to the common one). These combinations are all
- SPST - Single Pole, Single-Throw
- SPDT - Single Pole, Double-Throw
- DPST - Double Pole, Single-Throw
- DPDT - Double Pole, Double-Throw
- QPST - Quad Pole, Single-Throw
- QPDT - Quad Pole, Double-Throw
Figure 1 - Schematic symbol for SPDT Relay
- Coil X and Coil Y: Power input into the
- C: Common
- N.O.: Normally Open
- N.C.: Normally Closed
Figure 2 Physical Diagram
Although seemingly simple device, their details are quite complex and relays
have many "interesting" properties which are either useful to exploit or are
pitfalls to be avoided.
The pickup voltage is the minimum voltage across the coil needed to close
the switch. For example, for one particular "12V" relay, the pickup voltage
The dropout voltage is the maximum voltage below which a closed switch will
open. For example, for the same "12V" relay, the dropout voltage is 3.3
Why are pickup and dropout voltages so different (or different at all) you
might ask? The equation for the force generated on the armature is:
F = 2*pi*(N^2) / (A * (R0 + x/A)^2)
- N is ampere-turns
- A is area of pole piece
- x is distance between pole-piece and armature
- R0 is reluctance of the iron pieces
The key point is that the force is stronger when the distance x is smaller
(when the relay is closed), so lower "ampere-turns" (current) is needed to
generate the same force.
The resistance of the copper wire used in the coil increases with
temperature, so the current through the relay will decrease if the voltage
across the coil is held constant. The magnetic field strength is directly
proportional to current, so the pickup and dropout voltages do vary with
The most troublesome part of a relay (or any switch) is the switch contacts.
There is no ideal material for contacts:
The switch contacts bounce when the relay is opened or closed, so you do not
get a clean electrical edge.
If particles of dirt get between the contacts, the switch will not be
closed. Some relays are sealed and have bifurcated contacts (have two
contacts in parallel) to help prevent this.
The contacts will erode over time due to mechanical wear and electrical
An arc may be strong enough to cause the contacts to weld together so that
the relay "sticks" closed.
Soot from arcing can coat the contacts so badly that an electrical
connection is not made when the relay is closed.
It turns out that having too low a current through the contacts also
leads to reliability problems. A minimum "wetting current" is useful to
clean oxide coatings off of the contacts. Oxide can build up on contacts to
such a degree that a connection is not made even when the contacts are
When a relay is energized, there is a minimum amount of force needed
beyond the minimum needed to just close the contacts. This increases the
surface area of contacting region to provide a better connection.