April 1967 Electronics World
[Table 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.
As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
though this article was written more than 40 years ago, the fundamentals
of protecting relays against interference from either internally or
externally generated noise haven't changed. Sometimes a datasheet will
recommend protection and noise suppression techniques, and when that
is that 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, use this article
and the very comprehensive table of application examples.
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
Arc, Surge, and Noise Suppression
By R.M. Rovnyak* / Staff Engineer, Product Design Section, Automatic
Electric Co.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.
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.
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.
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.
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; and radiated - 104
Hz to 1011
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
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.
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
R, 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.
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.
Table 1. Various suppression techniques used in relay circuits along
with comments on their general characteristics.
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 circuits.
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
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.
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
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 af electro- magnetic
switching devices. He has published several papers on related subjects
and holds one potent, with another pending.