April 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Even though this article was
written more than 50 years ago, the fundamentals of protecting relays against interference
from either internally generated or externally generated noise haven't changed. Sometimes
a datasheet will recommend protection and noise suppression techniques, and when
that is the case, the manufacturer's advice should be followed (unless you have
a really good reason to deviate, possibly voiding a warranty). When you find yourself
on your own with the design, whether a new creation or modifying an existing
circuit, use this article and the very comprehensive table of
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
Arc, Surge, and Noise Suppression
By R.M. Rovnyak* / Staff Engineer, Product Design Section, Automatic Electric
When relays are used. in switching circuits, conducted and radiated r.f.
interference as well as contact erosion may occur. Here are some of the techniques
that are employed in order to minimize these harmful effects.
This article will discuss the suppression of interference associated with a relay.
The entire system contributes to the noise problem and all factors such as grounding,
shielding, bonding, wiring and component layouts. and choice of inter-system connection
points must be adequately engineered to minimize noise.
Switching loads may:
1. Develop voltage and/or current transients of sufficient magnitude to damage
or destroy components within the system, rendering it inoperable.
2. Produce unwanted disturbances which can cause circuit malfunction either within
or external to the system.
3. Decrease the useful life of the system and its reliability by, excessive wear
and tear on its components.
Table 1. Various suppression techniques used in relay circuits
along with comments on their general characteristics.
The high-magnitude transients are associated with breaking inductive loads such
as the coils of relays. The problem is most severe when the inductor is rapidly
switched to the "off" state. Under these conditions the voltage can be very large
and is of opposite polarity to the supply voltage. This presents a hazard to polarity-sensitive
devices or it may initiate a high-energy discharge across a set of contacts or the
insulation between windings on the coil or elsewhere.
Load switching may also generate coincident parasitic disturbances or RFI (radio-frequency
interference). This broad classification spans a frequency spectrum of about ten
decades and can be classified into three types: induced, conducted, and radiated.
The bandwidth associated with these are approximately: induced - 10 Hz to 106 Hz; conducted - 103 Hz to 108 Hz; and radiated - 104 Hz to 1011 Hz.
Disturbances by conduction are derived from such things as dynamic regulation
from the supply or down the line within the system, loop imbalances, recirculating
currents from inductive loads, and poor connections. They are not particularly associated
with anyone type of load but are more dependent on the magnitude of the current
being switched. Such disturbances are minimized by applying good techniques in inter-circuit
connections, component layout, wiring practices, and the proper choice of hardware.
Inductive coupling between circuit loops or between an inductor and a loop can
result in more than sufficient energy to cause circuit malfunction. Careful analysis
of normal circuit current paths will pinpoint the need to either inhibit the source
or minimize the pickup. Physical isolation between source and susceptible pickup
points and the use of twisted-pair leads to reduce the area included within the
loops are the principal cures although magnetic shielding is frequently required.
Arcing which occurs upon both make and break of a load by a set of contacts is
a source of radiated electromagnetic interference. All load types (R, C, L) with
open-circuit voltages above about 12 volts can initiate an arc at the contacts.
This occurs in relay circuits as opposed to semiconductor switching. Suppression
of some sort is required, the type depending on the load. The objective is to prevent
or minimize the energy in the arc.
The final area of concern is the erosion of the contacts. We first choose a contact
material which is optimum for the load and limit the arc energy by the application
of suppression elements.
If we put a series RC network across the contacts and make R small, the impedance
under the transient condition may keep the switch voltage (which includes applied
and self-induced voltages) small immediately after the contacts break. This arrangement
is preferred for most inductive loads, the contact voltage at the instant of opening
being limited to ILR, where IL is the load current just prior
to the break. In the case of the load being substantially removed from the switch,
however, it may be best to suppress at the load unless the sole objective is to
prevent contact erosion.
Two other factors associated with inductive load switching are interwinding capacitance
and magnetic flux leakage. The capacitance is effectively in shunt with the coil
and will draw high, short-duration charge currents. It is occasionally necessary
to include a small inductor or resistor ahead of the load to limit the surge. The
leakage flux will induce a voltage in a coupled loop and, if intolerable, magnetic
shielding must be used.
Capacitor in-rush currents must be limited to reasonable values or high-energy
arcs will be sustained on contact closure. Intense RFI will be generated unless
the contacts weld first. Capacitive loads per se are routinely taken care of in
the design; however, lead and winding capacitance, often overlooked, can play havoc
with a system susceptible to such occurrences.
With any type of suppression scheme it is necessary to first establish the main
circuit suppression requirements - such as to protect a solid-state device, minimize
contact erosion, etc. - before deciding which technique represents the best compromise.
The selection of a suppression technique depends on the objectives to be obtained
and the price one is willing to pay. The objectives can be categorized in three
main areas: protection of components from destruction or abuse due to the transient;
reduction of the erosion rate of the contact to increase the useful life of the
contact; and reduction of the disturbances produced by switching a load. The relative
ease with which each may be accomplished is in that order and, as a general rule,
one that satisfies the more difficult requirement will also take care of the less
difficult ones, e.g., a suppression scheme that inhibits RFI will also provide long
life and protect associated components.
The price paid for achieving the required degree of suppression may be anyone
or more of the following: cost including component and installation; more power
consumption; increased space or weight requirements; or longer release times, in
the case of relays. The circuit designer should approach the selection of a suppression
technique by: first, deciding the objective; second, determining the effectiveness
of the various techniques to accomplish the objective; and finally, resolve the
best technique, based on a trade-off in parameters, including the reliability level
of the suppression elements themselves.
Table 1 shows various suppression techniques used in relay circuits, with comments
on each regarding general characteristics. It should be pointed out that, except
for meeting elementary objectives, the final determination of values and suppression
schemes is empirical.
Evaluation of suppression is a two-phase operation, observation of the transient
magnitudes and degree and nature of the arc and then testing under operating conditions
for the performance characteristic desired. A fast oscilloscope and a probe that
does not load the circuit are required to observe the transient and arc. A small
resistor, suitably located, may be needed in order to observe the current in some
The nature of the load characteristics will determine the approach and interpretation
of the results to arrive at the most suitable suppression scheme; i.e., the prime
factor in an inductive load is the surge voltage, in a capacitive load it is the
surge current, and in an arc it is its power level and energy content. If the load
is a relay, the effect on timing may be important.
A common method of measuring this effect is to trigger the scope at the instant
the coil is de-energized either from the decay slope of the energizing pulse or
the induced transient and measuring the time until contacts change state by displaying
the voltage drop across a resistive load switched by the contacts. It is good practice
to use a dual-trace preamp and to display the trigger because false triggering can
lead to confusing results.
Switching electromechanical devices with semiconductors presents no particular
hazard if the transient voltage, current, and time dependency (dV/dt rate in the
case of an SCR) are maintained within rated limits. Three basic things should be
kept in mind: 1. The peak voltage across the switch is the static off-voltage plus
the transient peak when the suppression is across the load. 2. The peak turn-off
current will be the load current at time of switching. 3. The decaying current must
recirculate decaying to zero, and will seek the part of lowest impedance which should
be, by design, the suppression elements.
Testing the effectiveness of RFI suppression involves a fairly complex approach.
Relative estimates can be made for conductive disturbances by monitoring with an
oscilloscope, for radiated interference by observations of arc characteristics,
and the coupling interference must be built into the circuit as previously described.
Beyond this, standard apparatus and measurement schemes must be employed, and the
reader is referred to the following Military Specifications for further guidance
in this area MIL-I-26600, MIL-I-6051C, MIL-S-10379, MIL-I- 11748B, and MIL-I-6181D.
*The author is a 1961 graduate of Indiana Institute of Technology with a B.S.E.E.
degree. His efforts have been in design and development of electromagnetic switching
devices. He has published several papers on related subjects and holds one potent,
with another pending.
Posted April 30, 2019
(original February 13, 2012)