Zero crossing AC relay control circuit

An AC relay control circuit comprising a pair of back-to-back, series connected zener diodes control current flow through a control resistor. The control resistor is connected in parallel with parallel, reverse-connected light emitting diodes forming the light emitting portions of optical isolators. The photo transistor portions of the optical isolators are connected in parallel and control the generation of a trigger signal adapted to control the closure of relay contacts. Regardless of the instantaneous state of the AC voltage waveform (e.g., positive or negative), during the higher portions of the AC voltage waveform, one or the other of the LED's is forward biased and emits light, causing its related photo transistor to be turned on and the trigger signal to be at a low level. As the AC waveform approaches the next zero crossing point, it drops below the breakdown level of the zener diodes, causing current flow through the control transistor (and the forward bias voltage) to terminate, causing the previously lit LED to go out. The trigger signal then rises to a high level adapted to cause relay contact closure. The time between trigger signal rise and the zero crossing point is exactly related to the relay closure time.

BACKGROUND OF THE INVENTION 
This invention is directed to relays and, more particularly, to control 
circuits for controlling the closure of relay contacts when AC is applied 
across the contacts. 
In many electronic control systems AC power is applied to a load through 
relays. In many instances relatively low voltage control systems are 
utilized to sense and manipulate data and, in accordance with that data, 
generate control signals. The control signals, in turn, control the 
closure of relay contacts so that AC power is applied to the desired loads 
at the desired point in time. The loads may be in the form of relatively 
large banks of lamps (lampbanks), adapted to display information in 
numeric or alphanumeric form, visible at relatively large distances such 
as by personnel located in an N/C machine tool area. Alternatively, the 
load may comprise electric motors or other electric devices requiring 
relatively high voltage power. The AC power may be single phase or 
multi-phase (e.g., three phase) power depending upon the nature and 
requirements of the particular load. 
One of the major disadvantages of relays controlling AC is the arcing that 
occurs when the relay contacts are closed at some point other than the 
zero crossing point in the AC waveform. More specifically, if relay 
contacts are closed when a relatively high voltage is applied across the 
contacts, arcing occurs. Arcing causes deterioration of relay contacts 
and, thus, reduces relay life. In order to overcome this disadvantage, in 
recent years, relatively high voltage semi-conductor devices (e.g., 
silicon controlled rectifiers, triacs, etc.) have replaced relays in many 
electrical systems. However, these devices have voltage and power limits 
that prevent their use in many other systems. Moreover, in many instances, 
particularly when high power is involved, these devices are more expensive 
than desirable. Thus, while relays have certain undesirable features, they 
also have certain desirable features, particularly in the area of cost and 
power transfer capability. In some situations they are the only device 
available for use. 
It will be appreciated from the foregoing brief discussion that it would be 
desirable to provide a circuit for controlling relay contact switching 
such that the instantaneous AC voltage existing across the contacts is at 
a minimum level (preferably zero) when the contacts close. Obviously, 
switching at a minimum voltage level will result in lower arcing and, 
thus, longer contact life. 
Therefore, it is an object of this invention to provide an AC relay control 
circuit. 
It is a further object of this invention to provide an AC relay control 
circuit adapted to sense the impending occurrence of the zero crossing 
point of an AC voltage, and control the closure of relay contacts at the 
zero crossing point. 
It is another object of this invention to provide an inexpensive, yet 
reliable, zero crossing AC relay control circuit. 
It is a still further object of this invention to provide an AC relay 
control circuit adapted to prolong the life of the contacts of relays. 
SUMMARY OF THE INVENTION 
In accordance with principles of this invention, an AC relay control 
circuit that senses the impending occurrence of the zero crossing point of 
an AC voltage and generates a trigger signal adapted to cause relay 
contact closure at the zero crossing point is provided. A pair of 
back-to-back serially connected zener diodes control the current flow 
through a control resistor. The control resistor is connected in parallel 
with parallel, reverse-connected light emitting diodes (LED's) which form 
the light emitting portions of optical isolators. The photo transistor 
portions of the optical isolators are connected in parallel and are 
adapted to produce a trigger signal for controlling the closure of the 
relay contacts. Regardless of the state of the AC wave form (i.e., 
positive or negative), during the higher voltage portions of the AC wave 
form one or the other of the LED's emits light. This light emission causes 
the related photo transistor to be turned on and the trigger signal to be 
at a low level. As the AC waveform approaches its next zero crossing 
point, and drops below the zener diode breakdown point, current through 
the control resistor drops to zero. This current flow termination removes 
the forward bias from the lit LED causing it to be extinguished, whereby 
its related photo transistor stops conducting. At this point both photo 
transistors are non-conducting and, the trigger signal rises to a high 
level. The high level trigger signal allows (if gated with another control 
signal), or directly controls, the closure of the relay contacts. The time 
between trigger signal rise and the zero crossing point is exactly related 
to the closure time of the relay. Thus, the relay is adapted to close 
exactly at the zero crossing point whereby the voltage across the relay 
contacts is exactly at, or very near, zero when the contacts close. Since 
the voltage across the contacts is low when contact closure occurs, relay 
contact arcing and the resultant deterioration of the contacts is 
essentially eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic diagram of a preferred embodiment of the invention 
and comprises: two zener diodes designated ZD1 and ZD2; three resistors 
designated R1, R2 and R3; two light emitting diodes designated LED1 and 
LED2; and, two photo transistors designated PT1 and PT2. 
LED1 in combination with PT1 forms a first optical isolator and LED2 in 
combination with PT2 forms a second optical isolator. While various types 
of optical isolators can be utilized by the invention, one suitable device 
(comprising two optical isolators) is the ILD 74 sold by Litronix 
Corporation, Cupertino, Calif. 
The same AC voltage that is to be applied across the contacts of a relay 
adapted to be controlled by the invention is applied across a pair of 
terminals designated AC and neutral. The AC terminal is connected through 
R1 to the cathode of ZD1. The anode of ZD1 in connected to the anode of 
ZD2. The cathode of ZD2 is connected through R2 to the neutral terminal. 
It will be appreciated from the foregoing circuit description that current 
will flow through R1 and R2 as long as the AC voltage applied to the 
AC/neutral terminals is greater than the breakdown voltage of either ZD1 
or ZD2. Assuming the AC voltage is standard 115 volt AC line voltage, and 
the zener diodes have an 80 volt breakdown level (for example they are 
IN3003 zener diodes) current will flow through R1 and R2 as long as the AC 
voltage is above 80 volts. When the AC voltage drops below 80 volts, the 
zener diodes will terminate current flow through R1 and R2, one preventing 
current flow in one direction and the other preventing current flow in the 
other direction. Termination of such current flow will terminate the 
voltage drop across R2. 
LED1 and LED2 are connected in parallel with R2, but in reverse directions. 
During one entire half of the AC cycle, regardless of the actions of ZD1 
and ZD2, one or the other of LED1 and LED2 will be biased off. The other 
LED, as long as there is a satisfactory amount of current through R2 will 
be forward biased and emit light. Thus, light emission will occur only 
when the AC wave is above the voltage breakdown point of ZD1 or ZD2, 
whichever is operative. In this manner, LED1 and LED2 in essence sense the 
polarity of the AC voltage and only emit light when the voltage in either 
direction is above a predetermined level, set by ZD1 and ZD2. 
The emitters of PT1 and PT2 are connected together and to a ground 
terminal. The collectors of PT1 and PT2 are connected together and to a 
trigger terminal. The collectors of PT1 and PT2 are also connected through 
R3 to a bias voltage source designated +V. R3 is a pull-up resistor that 
prevents floating of the system. 
The ground terminal is adapted for connection to one side of the coil of 
the relay to be controlled. As used herein the term "relay" refers to both 
AC and DC relays since either may couple AC power through its contacts. 
When the invention is used in combination with a DC control system, the 
trigger terminal, for example, may be connected to the clock input of a JK 
flipflop having a control signal applied to a control input of the 
flipflop. A related output of the flipflop is, in turn connected to the 
other side of the relay coil. When a control signal produced by a control 
system, adapted to indicate that a relay contact closure is desired, 
occurs, the next occurring trigger signal causes the relay contacts to 
close. In this manner, the control signal indicates that a closure is 
desired and the trigger signal, as will be better understood from the 
following discussion, times the closure to occur at the zero crossing 
point of the AC cycle. 
When the invention is used in combination with an AC control system, the 
trigger terminal, for example, may be connected to the control terminal of 
a TRIAC. The TRIAC, in turn, is connected to the coil of an AC relay to 
apply power thereto. 
As noted above, ZD1 and ZD2 may be IN3003 zener diodes and the optical 
isolators may be ILD 74's. If such devices are used, and assuming the AC 
input is 115 volts and that it is desired to provide a trigger signal 3 
milliseconds before the AC crossing point, R1 (which is a current limiting 
resistor) may be chosen to have a value 2,000 ohms and R2 (which is a 
control resistor a value of 50 ohms. In such a circuit R3 may be chosen to 
equal 5,000 ohms. 
It is pointed out here that three milliseconds has been chosen as an 
exemplary value because many relays have a three millisecond delay time 
between the time a trigger signal is applied to the coil of the relay and 
the closure of the relay contacts. If the time delay of the relay to be 
used with the invention is other than 3 milliseconds, obviously, zener 
diodes having a voltage breakdown at corresponding greater or lesser 
voltage levels, as necessary, must be chosen. The choice is based on the 
invention's requirement that the time delay between the onset of the 
trigger signal and the zero crossing point must be equal to the time delay 
between the application of a trigger signal to the related relay and 
closure of the relay's contacts. 
In summary, ZD1 and ZD2 control current flow through control resistor R2. 
When current flows through R2, because the AC voltage is above the 
breakdown level of ZD1 and ZD2, one of the LED's will be biased off and 
the other LED will be biased on. The lit LED will, in essence, cause the 
trigger terminal to be grounded. However, as the AC wave approaches the 
zero crossing point, the current through R2 will terminate. The current 
termination point is determined by the voltage breakdown level of the 
zener diodes. When current flow terminates, as best illustrated in FIG. 2, 
the emission of light from the previously lit LED will terminate because 
it will no longer be biased on. This action will cause its related photo 
transistor to be turned off, whereby the trigger voltage will climb to a 
predetermined level. 
The trigger voltage, as noted above, may be used to directly control the 
closure of contacts of a related relay or be combined with a control 
signal through a gate to control the closure of the relay contacts. 
After the zero crossing point, when the voltage breakdown level in the 
opposite direction is reached, the other LED will start to emit light, 
whereby its related photo transistor will ground the trigger terminal. At 
this point, the trigger signal terminates. Thereafter, as the voltage in 
the reverse direction starts to again approach zero, the now lit LED will 
go off and the trigger signal will again rise. Hence, a trigger voltage 
exists across each zero crossing point. 
It will be appreciated from the foregoing description that the invention 
provides an uncomplicated AC relay control circuit. The control circuit 
senses the impending occurrence of the zero crossing point of an AC 
voltage in either direction. When the impending occurrence is sensed, a 
trigger signal is produced that is adapted to cause relay contacts to 
close at the zero crossing point. Such closure, substantially reduces 
arcing across the contacts and, the resultant destruction of the relay 
contacts.