[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
Even 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.
Here are links to the other
Operate and Release Times of Relays
Finding Relay Operate and Release
, Arc, Surge, and Noise
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.
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.
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
3. Decrease the useful life of the system and its reliability by, excessive wear and tear on its
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
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
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 IL
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.
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.
Table 1. Various suppression techniques used in relay circuits along with comments on their general
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.
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. Testing Suppression
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 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 af electro- magnetic switching devices. He has published
several papers on related subjects and holds one potent, with another pending.