I've been working on this project. I have a solution that works, but I want to know if there is a more elegant/simple solution. Let me describe it to you.
Simply, I have a dozen channels on a liquid crystal device running on 120 VAC (60 Hz) that I need to switch on and off independently, controlled by a computer or microcontroller. Each channel draws no more than 30 mA each. The first iteration of my design involved an Arduino-type microcontroller with 12 digital outputs controlling 12 triac circuits that fed the 12 channels with the 120 VAC (all channels share a common neutral/return). The problem with this design is that in the 'off' state the triacs leave their channel floating. These liquid crystal devices can hold charge for up to a second, making it appear to turn of very slowly. In the next design I chose to replace the triacs with double throw solid-state relays (LCC110), so switching to 'off' connected the channel to neutral, immediately draining the charge and giving a clean visual turn off. Great!
This is where I need advice. The circuit works, but with 12 individual relay chips the PCB is not pretty. Also, the relays are significantly more expensive than triacs. Is there a better way to do this? What if I want to go up to 32 channels or more?
A friend recommended using a low-voltage serial to high-voltage parallel converter (like the HV9708). This doesn't appear to be able to handle that much current, as evident from the data sheet and the fried HV9708 that resulted from a test run on a small test device. It may also be that I didn't use the chip correctly. I need an AC signal to drive my device, but the only way I could figure out to get an AC signal from the HV9708 is to use one channel as the driving voltage and another channel as the return, and drive the polarity pin with the frequency that I need (giving the Vpp pin something around 60-80 VDC). If the output channels are both in the same state, i.e., 'high' or 'low' then the LC device is off. If the two channels are in opposite states then the LC is turned on. (These liquid crystal devices are fairly forgiving when it comes to driving signal. A 60V square wave is perfectly fine).
Regardless, the concept of this serial to parallel converter is attractive. One IC that can do all the switching that I need. It also has frequency control which is a feature I plan to add in the future. I just wish I could find one that can handle some power.
Let me know your thoughts.
Simply, I have a dozen channels on a liquid crystal device running on 120 VAC (60 Hz) that I need to switch on and off independently, controlled by a computer or microcontroller. Each channel draws no more than 30 mA each. The first iteration of my design involved an Arduino-type microcontroller with 12 digital outputs controlling 12 triac circuits that fed the 12 channels with the 120 VAC (all channels share a common neutral/return). The problem with this design is that in the 'off' state the triacs leave their channel floating. These liquid crystal devices can hold charge for up to a second, making it appear to turn of very slowly. In the next design I chose to replace the triacs with double throw solid-state relays (LCC110), so switching to 'off' connected the channel to neutral, immediately draining the charge and giving a clean visual turn off. Great!
This is where I need advice. The circuit works, but with 12 individual relay chips the PCB is not pretty. Also, the relays are significantly more expensive than triacs. Is there a better way to do this? What if I want to go up to 32 channels or more?
A friend recommended using a low-voltage serial to high-voltage parallel converter (like the HV9708). This doesn't appear to be able to handle that much current, as evident from the data sheet and the fried HV9708 that resulted from a test run on a small test device. It may also be that I didn't use the chip correctly. I need an AC signal to drive my device, but the only way I could figure out to get an AC signal from the HV9708 is to use one channel as the driving voltage and another channel as the return, and drive the polarity pin with the frequency that I need (giving the Vpp pin something around 60-80 VDC). If the output channels are both in the same state, i.e., 'high' or 'low' then the LC device is off. If the two channels are in opposite states then the LC is turned on. (These liquid crystal devices are fairly forgiving when it comes to driving signal. A 60V square wave is perfectly fine).
Regardless, the concept of this serial to parallel converter is attractive. One IC that can do all the switching that I need. It also has frequency control which is a feature I plan to add in the future. I just wish I could find one that can handle some power.
Let me know your thoughts.