S108 Datasheet, PDF (3/8 Pages) Solid State Optronic – Zero-Volt Cross Switching

S108 Performance & Characteristics Plots, T_A = 25°C (unless otherwise specified)
S108
Zero Cross
700V / 0.8A
AC Solid State Relay
I =5mA
F
ZERO-VOLT SWITCHING
This solid state relay has been designed with a driver circuit that controls the operation of two back-to-back silicon controlled rectifiers
(SCRs), each responsible for one half of the AC cycle. If an AC signal is examined, the turn on, turn off and zero-volt switching can be
seen. Figure 3 shows a typical 60 HZ, 120VAC signal with a corresponding relay input signal:
1
Relay Input Signal is
Turned On
5 _{Relay Input Signal is}
Turned Off
Input Signal
5V Threshold
(Zero Cross Voltage)
2
SCR #1 Turns On when
5V threshold is reached
6 SCR #1 and Relay
Turn Off at Zero Volts
Load Signal
3
SCR #1 Turns Off,
SCR #2 Turns On
at Zero Volts
4
SCR #2 Turns Off,
SCR #1 Turns On
at Zero Volts
Figure 3: Zero-Volt Switching Cycle
Figure 3 shows the sequence of zero-volt switching operation. At Stage 1, an input signal is applied to the relay. The relay will not turn
on until the threshold voltage of 5V is reached. Once this point is reached (Stage 2), SCR #1 (designated as the SCR which controls
positive AC voltage) turns on. However, SCR #1 only conducts for an instant, as the cycle quickly crosses zero. At this point (Stage 3),
SCR #1 will turn off and SCR #2 (negative AC voltage) turns on. Likewise, at the next zero cross (Stage 4), SCR #2 will turn off and
SCR #1 conducts again. Even though the input signal is terminated at Stage 5, the relay will continue to conduct (typical SCR
behavior) until Stage 6, when SCR #1 crosses zero and ceases to conduct. Please note that turn on can likewise begin on the negative
phase of the AC cycle with a -5V threshold, though only the positive phase is shown here.
© 2013 Solid State Optronics • San José, CA
www.ssousa.com • +1.408.293.4600
Page 3 of 8
S108 /H/S/TR
Rev 2.10 (11/25/2013)
001766