Hello All About Circuiverse,
I'm working on some line voltage motor control and need to drive an AC relay coils from an isolated DC source. I know that there are other options (SSR's, electromechanical relays, etc.) but I am looking for small and inexpensive as part of a commercial product. I thought I had found the perfect solution in the IXYS CPC1972 (datasheet here). From the datasheet: "The CPC1972 is an AC Solid State Switch using optical coupling with dual power silicon controlled rectifier (SCR) outputs to produce an alternative to optocoupler and Triac circuits."
The supposed advantage of antiparallel SCRs over a triac is that they don't have a limited dv/dt for commutation and thus don't need snubbing - an essential characteristic when driving inductive loads. However, I just received the parts and tried to switch the 120VAC coil of a G7L relay (data sheet here). Once turned on, I can remove the input LED current, and the load stays on! It won't commutate and turn off! It will turn-off a few seconds later sometimes but it's very inconsistent.
In my troubleshooting, I disassembled one of the relays and found a simple rectification circuit ahead of the relay coil (capacitor across line + bridge rectifier). That's not too unusual since the manufacturer can just use a DC coil and drive it with the ac signal. I checked the current waveform to ensure it was actually crossing zero and it was (waveform below).
This breaks my mental model of SCRs and seems to defeat the whole purpose of this device. Please help me to understand and remedy the issue.
I'm working on some line voltage motor control and need to drive an AC relay coils from an isolated DC source. I know that there are other options (SSR's, electromechanical relays, etc.) but I am looking for small and inexpensive as part of a commercial product. I thought I had found the perfect solution in the IXYS CPC1972 (datasheet here). From the datasheet: "The CPC1972 is an AC Solid State Switch using optical coupling with dual power silicon controlled rectifier (SCR) outputs to produce an alternative to optocoupler and Triac circuits."
The supposed advantage of antiparallel SCRs over a triac is that they don't have a limited dv/dt for commutation and thus don't need snubbing - an essential characteristic when driving inductive loads. However, I just received the parts and tried to switch the 120VAC coil of a G7L relay (data sheet here). Once turned on, I can remove the input LED current, and the load stays on! It won't commutate and turn off! It will turn-off a few seconds later sometimes but it's very inconsistent.
In my troubleshooting, I disassembled one of the relays and found a simple rectification circuit ahead of the relay coil (capacitor across line + bridge rectifier). That's not too unusual since the manufacturer can just use a DC coil and drive it with the ac signal. I checked the current waveform to ensure it was actually crossing zero and it was (waveform below).
This breaks my mental model of SCRs and seems to defeat the whole purpose of this device. Please help me to understand and remedy the issue.