Tips on Applying Relays

15.2 Contacts

Many relay application problems occur because of the difference between the relay user's real-life relay load requirements and the relay manufacturer's rated contact load.

15.2.1 Contact Ratings


The meaning of relay contact ratings is not uniformly defined and understood. Unless otherwise indicated, the current rating of a relay contact is a statement of the magnitude of resistive current that can be switched at rated voltage, frequency, and given cycle rate, for the number of operations specified.

15.2.2 Ratings for AC


Relay contact alternating current (AC) ratings apply only for the frequency specified. If the rating specified is for 400 Hz, the 60 Hz switching is usually appreciably less.

15.2.3 Load types


One of the most common problems in applying relays is the assumption that a relay contact can switch its rated current no matter what type of load it sees. Nothing could be more disastrous. High in-rush currents, high induced back EMF's, and the like can erode, or even weld, contact to the point where life is cut drastically. To better understand the effect of various loads, a brief discussion of each load type is given.

15.2.3.1 Incandescent Lamps

The cold resistance of a tungsten filament lamp is extremely low; this results in in-rush currents that may exceed 15 times the value of the steady-state current. Such high in-rush currents can cause contacts to erode rapidly, or even weld. Series, current-limiting resistors or a small, continuous current flow through the lamp can significantly reduce the inrush by keeping the lamp filament warm.

15.2.3.2 Capacitive Load


The charging current to a capacitive circuit can be extremely high. The capacitor initially acts as a short circuit and the current is limited only by the circuit resistance. Sometimes the user may not be aware that his load is capacitive. He should note that long transmission lines, filters for EMI elimination, power supplies, etc., are highly capacitive. A series, current-limiting resistor can mitigate this problem.

15.2.3.3 Motor Loads

Motors draw high in-rush current because at standstill the input impedance is very low. As current is drawn, a torque is developed due to the interaction of the current and the magnetic field. When the motor starts to rotate, it develops an internal EMF which tends to reduce the current. Depending on the mechanical load, the starting time may be very short or very long causing the in-rush current to persist for the same duration. Upon turn-off, the contacts will receive a high inductive voltage kick that tends to produce an arc that can cause contact erosion.

15.2.3.4 Inductors, Solenoids, Contactor Coils, Chokes


Inductors, solenoids, contactor coils and chokes are high inductive loads which do not necessarily produce high in-rush. They may even limit the rise of in-rush current. When such inductive loads are turned off, however, the magnetic field collapses. The stored energy in this magnetic field must be dissipated across the opening contacts and that causes arcing and contact erosion. Contact protection, in the form of R-C networks, diodes, varistors, etc., will tend to reduce the erosive effects.

15.2.3.5 DC Loads

DC loads are more difficult to turn off than AC loads because the voltage never passes through zero. As the contacts open, an arc is struck and may be sustained by the applied voltage unless the arc length becomes too long to sustain itself. The arc energy can seriously wear away the contact. One technique for elongating the arc and forcing it to break is to use "blow out" magnets and/or coils.

15.2.3.6 Dry Circuits

Dry circuits are circuits wherein the contact does not make or break the load. The contact resistance (influenced by contact material, contact pressure, cleanliness of contact surface, environment, and magnitude of voltage and current) may be very high. This occurs because contact wipe and contact bounce do not occur curing arcing and softening of the contact surfaces. This high contact resistance may impede the flow of current to an intolerable extent. The relay manufacturer should be consulted for proper contact materials.

15.2.3.7 Low-Level Switching

Low level switching is generally considered to be in the range of microamperes or a few milliamperes with the open circuit voltage below the melting voltage of the contact material. Absorbed organic materials on the contact surfaces are polymerized by friction-developed heat. These polymers cause high and unstable contact resistance that results in erratic conduction. The relay manufacturer should be consulted for proper contact materials.

15.2.4

When using reed relays in sensitive, instrumentation-type applications, time must be allowed for all contact noise to settle. This is usually only a few milliseconds.

15.2.5

A relay contact rating does not necessarily apply for all loads from zero up to the magnitude specified. The fact that a contact can reliably switch 10 amperes does not necessarily mean it can reliably switch 10 milliamperes.

15.2.6

A relay contact rating specified for an ungrounded metal case relay operation does not necessarily apply when the case is grounded. Grounding the relay case for contact voltage approximately 30 V and higher usually reduces the contact rating appreciably. (Note: For reasons for personnel safety, metal relay case should be grounded-see various military specifications and standards.)

15.2.7

A relay contact current rating specified at a given, single-phase, line voltage does not necessarily apply for the same, nominal, line voltage of a three-phase system, since the phase-to-phase voltage is greater than phase-to-neutral (or ground).

15.2.8

A relay rated to open and close three-phase loads cannot necessarily switch a load between two non-synchronized three-phase power sources. Again, voltages may be out of phase and, therefore, may be higher-e.g., as much as two times.

15.2.9

Testing relays that have low-level current contacts by using a device (such as a tungsten lamp or doorbell) that draws enough current to degrade the low level switching capability must be avoided. Incoming inspectors must be cautioned about this. Use of a two-wire digital ohmmeter to measure contact resistance on relays is not acceptable practice since the test current is normally in the range of only a few microamperes and the probe connecting resistance is also included in the measurement.

15.2.10

Do not parallel relay contacts to switch a load greater than a single contact can handle. Lack of absolute simultaneity of contact closing and opening results in one contact taking all the load. This will lead to premature failures. Relay contacts may be paralleled for improved reliability against failure to make within single contact rating, however.

15.2.11

Except in a few special applications, a double-throw contact should never be connected across a power source with the rated load connected to the pole as in the circuit shown below. Many relays are not capable of switching a load when used in this manner.


15.2.12 Arc Suppression and Relay Contact Protection

The methods described as follows eliminate or minimize the harmful effects of arcing on contacts that interrupt inductive loads. In both military and commercial applications, useful contact life can be extended considerably by the reduction or elimination of contact arcing. As relays, or hybrid relays, work in conjunction with, or alongside sensitive logic circuitry, it becomes very important that the devices neither radiate nor conduct transient voltages at radio frequencies of sufficient magnitude to either damage or cause false triggering of sensitive logic circuitry.

Contact damage is most commonly caused by the transients which occur at contact closing and opening. The appropriate protection is determined by whether the circuit is dc or ac, and whether the load is capacitive or inductive. Contact protection can increase life expectancy by as much as 2 or 3 orders, or in extreme cases may permit successful switching which would otherwise be impossible. The schemes described below should be useful in combating these problems.

15.2.12.1 Capacitive or "In-rush" Type Loads

In-rush transients at contact closure can be destructive to contacts due to the fact that there is always some melting at the actual point of contact. This causes a weld which tends to break asymmetrically in dc circuits, resulting in some metal transfer and resulting in roughened contact in ac circuits. In extreme cases, the weld is sufficiently strong to prevent the contacts from reopening. In-rush transients can be caused by the following loads:
1.) Tungsten lamps
2.) Transformers (when switched in the primary)
3.) AC solenoids
4.) Some kinds of motors
5.) Capacitor input filters
6.) Capacitors without series resistance used for arc suppression.
Corrections for this condition can include:
1.) Choice of a suitable contact material which combines high thermal and electrical conductivity with little tendency to produce welds.
a. Fine silver and fine-grained silver have the longest lives for many applications.
b. Silver-cadmium oxide or silver-tin-oxide can be used in extreme cases of sticking.
c. More exotic material combinations such as palladium-copper versus silver alloys are often used for cyclic lamp loads.
d. Tungsten contacts have been used in some lamp applications (although it is high resistance and tends to form tenacious oxide layers requiring high contact pressures).
2.) The relay should be designed to have:
a. Heavy contact force both closing and opening.
b. Minimum contact bounce.
c. Weld-breaking contact motion such as rocking or shearing motions.
d. Some armature travel before the actual contact separation.
3.) It may be possible to add small values of resistance to the circuit to limit the current transients.

15.2.12.2 DC Inductive Loads

A simple form of transient suppression for DC inductive circuits is a diode which shunts the inductance and which blocks the applied voltage.


The diode permits the load current to recirculate back through the load when the circuit is opened-decaying at a rate which is expressed by the L/R ratio of the load. The voltage transient at the instant of contact opening is limited to the forward voltage drop of the diode. This form of suppression has the following characteristics:
1.) It is small and conveniently mounted on load or load socket.
2.) It is simple to select.
a. The peak inverse voltage rating of the diode must exceed the peak source voltage plus any anticipated transient surges.
b. The pulse current capability of any silicon rectifier diode is sufficient for loads up to 5 amperes. Larger diodes can be used if the load current is higher than 5 amperes. The peak of the transient pulse current will be approximately equal to the steady-state current of the load.
3.) It is economical.
4.) It is not suitable if the polarity of the input can be accidentally reversed.
5.) The decay time for the load current is relatively long.
6.) It is vulnerable to high-voltage transients in the normal polarity.

15.2.12.3 Resistor-Capacitor

A more universal form of arc suppression is a resistor and capacitor in series connected across the contacts to be protected. The capacitor provides a temporary alternate path around the contacts until a contact gap is well established. The resistor is necessary to limit the capacitor discharge peak current when the contacts are reclosed and, therefore, should be in the order of 1 and 1/2 ohm per volt of source voltage to limit the current transient at contact closure to 1 to 2 amperes. Alternatively, the resistor and capacitor may be connected across the load if the entire circuit inductance is concentrated in the load. The capacitor must be sufficiently large to accept the inductive energy of the load, except some dissipated in the resistor, without charging to an undesirable high voltage. Minimum capacitance can be determined either empirically or by calculation.


1.) Find, by trial, the capacitance which is subdues visible arcing. Allow considerable safety factor for the effects of contact roughening and carbonizing, altitude, humidity and variable source voltage and components.
2.) Determine the capacitance by trial which gives the best arc suppression as determined by an oscilloscope connected across the contact gap. This is much more sensitive than visual observation, but can cause confusion due to other circuit transient conditions. Evidence of arcing is a sustained (0.1 to several milliseconds) erratic contact voltage normally between 12 and 30 volts at the instant of contact separation. The oscilloscope trace is frequently a "fuzziness" due to high-frequency oscillatory discharge which may be in the megahertz-frequency range. (See Fig. 15.3).


3.) Approximate the correct value by solving the equation for the stored energy transfer from the inductance to the capacitor:


in which, C = desired capacity (uF),L = load inductance (henrys), I= load current (milliamperes), V_s = source voltage, and V_p = the transient voltage which must not be exceeded in order to prevent a breakdown of either the capacitor or the air gap as the contacts separate.
For currents of less than an ampere or two, contact gaps will usually tolerate up to 300 volt transients (in circuits whose voltage is low compared to 300, such as 30) providing the instantaneous voltage at the beginning of the contact separation is sufficiently low to prevent "drawing" an arc. Since the minimum arcing voltage for silver is 12 volts and the minimum arcing current is about 0.4 ampere, it is desirable to have the product of the suppressor resistance and the load current not greater than 12 (when the load current is greater than 0.4 ampere). While it is not always practical to achieve this, the actual duration of the arcing can made very short (such as a few microseconds) unless the arc is sustained indefinitely as a result of insufficient contact gap. In Figure 15.4, the voltage and current relationships are shown for various lengths of arcs (contact gaps). If a straight line, usually called a "load line", is drawn from the open circuit voltage to the closed circuit current, the largest gap length curve which it cuts shows the minimum gap needed to interrupt the load current safely. An inductive load without suppression causes the load line to be momentarily distorted upward during the current interrupting process while correctly chosen arc suppression networks temporarily deflect the load line downward. To eliminate arcing completely, the load line must be distorted near or below the point 12 volts and 0.4 amperes. (See Electrical Contacts Handbook-Ragner Holm, Part III.)


If it is desired to get a lower instantaneous opening voltage than can be accomplished with a simple resistor-capacitor circuit, it is effective to shunt the resistor with a diode which lets the capacitor be charged through it in the forward direction during the contact opening, but which requires the capacitor discharge on contact closure to be through the resistor. The resistance value in this case is determined by the time available during a contact closure for effective capacitor discharge. (1,000 ohms is usually suitable.)
The circuit in Fig.15.5 ensures that the instantaneous voltage at the moment of contact separation will be essentially zero, and permits the contact gap to become established before the contact voltage becomes sufficiently high to cause an arc to strike (usually greater than 320 volts for normal atmospheric pressure and contact surface conditions). The capacitor must be sufficiently large so the voltage to which it charges as the stored energy of the load is transferred to it does not exceed the voltage breakdown of either the air in the gap, the dielectric in the capacitor, or the diode.


When the contact arcing has been completely eliminated, and closure transients are not greater than an ampere or two, the contact life should be the same as the mechanical life.

15.2.12.4 Switching Speed

The effect of a diode as used in Fig. 15.1 is to cause a slow decay of the load current. Decay rate is determined by the L/R ratio of the load. If no arc suppression is used (see Fig. 15.3), the inductive energy of the load will be transferred to heat and damage to the contact as caused by the arc, whose voltage drop is usually in the range from 12 to 50 volts, depending on the current and gap length. The resistor-capacitor form of arc suppression, with or without the diode (see Fig. 15.2) can be chosen so the inverse voltage rises to a peak between 200 and 300 volts, thereby resulting in a much quicker transfer of the stored energy of the inductive load and, hence, very rapid de-energization of the load after the contacts start to separate. Fast switching can also be achieved by the use of a zener diode as shown in Fig. 15.1B when high voltage transients cannot be tolerated.

15.2.12.5 AC Inductive Loads.

Arc suppression for ac inductive circuits can normally be handled by some combination of the following:
1.) The voltages being switched may be high enough so that hard metal contacts may be used which are more resistant to arcing transfer, but which may have high contact resistance in low voltage circuits.
a. Tungsten is excellent if the contact forces and "wipe" are heavy enough to deal with the oxides which form, and providing the load current is not so heavy that the contacts run too hot. Tungsten is vulnerable to high humidity or moisture. Typical values:
Source Voltage: 18Vac min.
Load Current: 2A max.
Contact Force: 25 grams, min.
Contact Resistance: 0.25-25 Ohm variable.
b. Palladium may be satisfactory for conditions of low sources voltage, or contact force, or for corrosive environmental conditions which make tungsten undesirable.
c. Sintered silver-cadmium oxide has an arc quenching tendency, and has nearly the electrical and thermal conductivity of silver, which makes it suitable for loads that are too heavy for tungsten or palladium.
2.) A resistor-capacitor combination may be connected in shunt with the load so that the combination is essentially resistive. The resistor should be approximately 0.5 to 1 Ohm/volt, or for loads under 0.5A can equal the load resistance, and the capacitance should be chosen to have a time constant or satisfy the equation RC=L/R where L is in henrys, C is in farads, and R in ohms.
3.) Various non-linear resistance can be used to clip the voltage transients which would otherwise appear at the contacts:
a. Back-to-back zener diodes across the contacts or the load.
b. Neon bulb across the contacts.
c. Varistor across the contacts or the load. "Thyrite" or equivalent which shunts the load and carries about 10% as much current as the load will prevent transients greater than about 2 times the source voltage.
4.) For conditions where extremely long life is required, the following treatment may be justified. Circuit A in Fig. 15.6 protects against all circuit inductances which may be in the system but permits about 3 mA residual current in the load when the switch is open (for 115 V circuit and R = 100 kilohms). If this is undesirable, Circuit B shown in Figure 15.6 may be preferred, although it does not protect against inductance in the power source.
For 115 VAC service, the diodes must have a peak inverse voltage rating of 400 V, the capacitor must have a DC working voltage of 200 VDC, and 100 kilohms resistor will dissipate nearly 1 watt. The reset time during a switch closure may be as long as a second.