April 1967 edition of Electronics World had a series of articles on
designing systems with electromechanical relays. Even in today's high
solid state relay world, there are still lots of applications for electromechnical
relays. Only a handful of people actually design them, but the application
tutorials provided therein are as valuable to today's engineers and
technicians as they were 45 years ago.
April 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
Here are links to the
other relay articles:
Operate and Release Times of Relays,
Finding Relay Operate and Release Times,
Arc, Surge, and Noise Suppression
See all the available
Electronics World articles.
Operate and Release Times of RelaysSince graduating from the engineering school at the University of Kentucky
in 1937, the author has held supervisory positions in just about every
production and engineering department at Guardian Electric. From 1960
to 1963 he served as Chief Design Engineer and has been Assistant Chief
Engineer since 1963. He is a Registered Professional Engineer and holds
many patents on relay, switch, and stepper designs.
By Warren Wright / Asst. Chief Engineer, Guardian Electric Mfg.
Definitions of these important characteristics and
methods that are used to modify these parameters.
All too often relays are placed in control and logic circuitry without
enough consideration being given as to whether or not the relay operational
time characteristics will assure proper functioning of the circuit under
various operating conditions. A working knowledge of which factors affect
relay operating time can give the circuit engineer or technician confidence
in his design.
Since graduating from the engineering school at the University of
Kentucky in 1937, the author has held supervisory positions in just
about every production and engineering department at Guardian Electric.
From 1960 to 1963 he served as Chief Design Engineer and has been
Assistant Chief Engineer since 1963. He is a Registered Professional
Engineer and holds many patents on relay, switch, and stepper designs.
In general, relays are electromechanically operated
switches, thus there are two items which must be evaluated when considering
the time elements of relay function. These are the electrical characteristics
and the mechanical characteristics.
But first, let's define
the terms and then consider their relationship to total relay function.
Definition of Terms
The operate time of
a relay is the time interval from the instant of coil-power application
until completion of the last contact function.
The release time
is the time interval from the instant of coil-power cut-off until the
completion of the last contact function. (See Fig 1.) Note that the
operate and release times do not include contact-bounce times.
When coil power is applied, coil energizing current increases until
the magnetic flux is sufficient to move the relay armature and its contact-actuating
members. Upon removal of the coil power, magnetic flux does not collapse
instantly, but decreases for some period of time - depending on the
circuit, the coil, and the magnetic structure. When the magnetic flux
drops below the "hold-in" value for the particular relay, the armature
and its contact-actuating members return to the normal or de-energized
With these fundamental characteristics in mind, we
can now consider the various relay designs and the effect of circuit
characteristics on operate and release times.
For d.c. relays, the operate time of a specific relay design may
be reduced by three methods. First, we can overdrive the relay. This
is done by increasing the control voltage, decreasing the coil resistance,
increasing the control voltage and adding a series resistance, discharging
a capacitor at an over-voltage charge into the coil, pre-energizing
at some value below pickup voltage (the lowest voltage at which the
relay always operates), using dual-wound coils - one coil for overdrive,
the other to hold the armature in the operated position, using a series
resistor shunted by a capacitor, using a positive temperature coefficient
resistor in series with the coil, and using a series resistor shunted
by an N.C. switch - the switch being operated by the relay being controlled.
Second, we can reduce the pickup voltage of the relay by mechanical
means, such as by reducing return spring pressure, reducing the armature
gap, or reducing contact pressures and gaps.
Third, we can decrease
the mechanical inertia by reducing the mass of the moving elements such
as contacts, armature, and contact actuators.
For d.c. relays,
the inherent release time of a specific design may be increased by
using a parallel capacitor and series resistor, a parallel shunt resistor
or switch, parallel diode, or by reducing the residual magnetic air
Relay manufacturers produce many varieties of relays with
operate and release times ranging from minimal values of less than one
millisecond to some of the more exotic solid-state relays with maximum
times of 30 minutes or more.
When specific time characteristics
are needed, the relay manufacturer can usually provide relays to match
the circuit requirements either from "standard" relays or as "specials"
designed for a specific function.
The National Association
of Relay Manufacturers (NARM) has not set standards with respect to
fast or slow response. In general, relays which have operate and release
times under 3 milliseconds are considered fast-operate and/or fast-release.
Relays with function times of 50 milliseconds or more are usually considered
slow-operate or slow-release. Relays with function times between 3 and
50 milliseconds are medium-operate and release and this is the range
into which most general-purpose relays fall. Relays which are purposely
designed for slow function time are classified as time-delay relays.
These are covered in another article in this special section.
Relay manufacturers produce a bewildering range of relays for use
in over a hundred "usage" classifications, with thousands of varieties
and modifications in each classification. It is obviously impractical
to analyze all of these types, therefore we will only cover some of
the most popular general-purpose relays.
The graphs of Figs.
2 and 3 show the effects of ambient temperature changes on the attract
and release times of some typical general-purpose, d.c.-powered relays.
From this we can see that the attract or operate time increases with
temperature and the release time decreases with temperature rise (although
circuit components may modify this).
When we consider that relays
heat up either under extended energization or repeated cycling, we
can expect a relative shift in operating time characteristics, depending
upon the frequency of operation.
Fig. 1. Typical d.c. operation of a relay with a resistive
Relays from different manufacturers, but of the same type, generally
have similar operating-time characteristics. It will be noted that some
relay types change more with temperature than others - this is inherent
in the design of the relay but. in general, the time change is within
±10% over an ambient temperature range of 0°F to 160°F.
factors affect timing, such as changes in operating voltage or current.
in which the attract time varies inversely with coil power and the release
time varies directly with coil power. Where operation time is critical,
the relay should be evaluated in the circuit under the most adverse
combination of variables.
Power-supply inductance will increase
the operate time of d.c, relays depending upon the L/C ratio of the
circuit. Resistance in the power-supply line will tend to decrease the
operate time. Relays which are wired in parallel would aggravate these
effects. Arc suppressors or coil shunts will increase the release time
Basically, d.c. relays are reasonably consistent
in their timing characteristics under the same operating conditions,
but this is not the case with a.c. relays.
Other variations in both attract and release times are due to the
added factor of the instantaneous (turn-on or turn-off) voltage change.
The source voltage varies from zero to peak 120 times per second, thus
any voltage from zero to peak may be applied across the coil at the
instant of either "turn-on" or "turn-off".
Examination of a typical
60-Hz sine wave will show that most of the time the instantaneous voltage
at "turn-on" or "turn-off" will be higher than the pickup voltage of
the relay, so that the time characteristics are usually within a reasonable
range. Frequently, however, the probability of turning on or off at
lesser voltages catches up with us and the operating time suddenly changes.
This voltage variation is further complicated by the magnetic flux distribution
between two or more functional pole faces. Remember that a.c. relays
generally have one core face shaded by a copper ring to cause a phase
displacement in the magnetic flux. This phase shift is necessary to
provide relay hold-in during current reversal. Thus, we have an operational
time range rather than a specific operation time. The manufacturer generally
specifies the average function time for a.c. relays.
Fig. 2. Variations in operate or attract times at
Fig. 3. Variations in the release times at
various ambient temperatures for six
various ambient temperatures for the
general-purpose d.c. relays.
same six general-purpose d.c. relays.
Modification of operational time for a.c. relays may be accomplished
in a manner similar to that for d.c. relays, but the actual time of
operation will vary because of the sine-wave voltage applied. We must
conclude, therefore, that straight a.c. relays should not be used in
circuits where precise repeatable narrow-range operational times are
The mechanical characteristics which affect operational
time apply to both d.c. and a.c. relays and what affects one type will
usually affect the other type in the same manner.
in mass, whether it be in contact actuators, the contacts, or armature
will slow the operational time. Release time will be increased by minor
contact welding or sticking, mechanical wear, and residual magnetism.
We have not attempted to give charts showing precise operating
characteristics of relays because the timing is subject to so many variables
from relay to relay type, depending upon adjustment, type and number
of contacts, voltage variation, source impedance, line resistance, wear
factors, etc. Information on timing characteristics is available from
the manufacturer and should be used whenever operate and release timing