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Multi-channel AC switch w/microcontroller

zboson

May 18, 2015
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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.
 

Harald Kapp

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Welcome to electronicspoint.

Also, the relays are significantly more expensive than triacs.
That depends on the type of solid state relay you use. At 30mA you can use small and comparatively inexpensive photomos relay, e.g. AQW610 (photomos is a Panasonc trademark, there are other makers of comparable components, too).


A friend recommended using a low-voltage serial to high-voltage parallel converter (like the HV9708).
This component is rated at 80V max. You would still need teh higgh voltage driver and optical isolation is a very good idea. In that case you can use any shift register to do the serial to parallel conversion, e.g. a 74x164 (8 bit) or 74x673 (16 bit) where "x" is HC, HCT, VHC depending on availability. These shift register can be cascaded for 16 bit, 24 bit, 32 bit length etc.
 

zboson

May 18, 2015
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Harold, thanks for the reply!
That depends on the type of solid state relay you use. At 30mA you can use small and comparatively inexpensive photomos relay, e.g. AQW610

I want to avoid floating the input to the liquid crystal device. This is why I chose the double throw (form C) relay, so that in the 'off' state my liquid crystal is shorted to ground instead of floating. If that wasn't a requirement I could stay with the much cheaper triacs (even with optical isolation). The relays I can find for $3-$5, which is cheap until I need to put 12 (or maybe 32 or more) on a single board. Then the cost becomes expensive quickly (and takes up a lot of space on a PCB).

you can use any shift register to do the serial to parallel conversion
Yes, I can use any shift register if I need to increase the number of channels, however, if the parallel (high voltage side) can't handle the power requirements, then I still need individual relays to switch each channel, which is something I'm trying to avoid (see above).
 

Harald Kapp

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An LCD per se operates at a few volts, not 120 V AC.
SO I assume you're controlling devices that operate from 120V but have an LCD for display? Can you find a point within these devices that would allow you to control the on and off state without swizching the 120 V AC mains?
 

zboson

May 18, 2015
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I'm not powering a typical LCD. My device is a PDLC film, a switchable privacy window. It requires a high voltage to switch, anywhere from 50 V to 150 V AC, depending on the film. I'm building the electronics control several films together from a single computer. The first version is simply using the 120 VAC mains power to drive the film. The next stage of development would to use a square wave with variable frequency control (50-5000 Hz). Square wave is preferable in this application, but sine will work.

I was hoping that I could find a single IC that has the capability to switch multiple AC signals, but so far, I have not found it.
 

Harald Kapp

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I was hoping that I could find a single IC that has the capability to switch multiple AC signals, but so far, I have not found it.
My guess is that you probably will not find a single devce that fulfills your requirements in terms of voltage and number of channels simultaneously. A solid state rely of photomos relay is the nearest you'll come.
 
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