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help on DC-AC inverter for Public art Project. Thks

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federico muelas

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Hi Everyone,
My name is Federico Muelas, I run a New Media/Public Art studio in New York. I’m currently working on a Commission for the George Pearl Hall building, the new School of Architecture and Planning at the University of New Mexico.

You can find further info about the project at:
www.federicomuelas.com

For this project I’m designing a large format display incorporating 3,700 pixels of 5’x5’ each made out of NPD-LCD. I’m using RS422 serial communication to control the units and most likely I will use a MAX335 chip to locally multiplex the signal on the screen.

The problem I have is that NPD-LCD technology requires AC current (although the required voltage is quite low, only 15V ac @ 7.3mA per pixel). So I need to find a reliable and inexpensive way to convert the 15 V DC that I’m getting out of the MAX335 chips into 15V AC voltage at 60 Hz (or closer) to control each pixel.

I’m afraid that if I use triacs with optoisolators or Solid State relays to convert the DC current coming out of the MAX335 into AC current I will end having too much electrical noise and ultimately an instable system.

Any Ideas?
Thank you so so much
Best wishes
Federico Muelas
 
Woww! More than two acres of LCDs? You'll probably want a lot of 15VAC supplies because your total requirement is more than 27 amperes and your distances will be in hundreds of feet.

I would suggest locally generating 15VAC maybe half an ampere peer location (50-60 places) and supplying the MAX335's with 15VAC to be switched. (The MAX335 would need a +/- DC supply slightly higher than the AC peaks.) That way you can distribute 120VAC 60 Hz to 50-60 individual 15V transformers (maybe 8-10A total).
 
Thanks a lot mneary,
they are a lot of amps in fact, the screen is divided en 28 panels of 4 by 8 feet and each panel contains around 200 cells.
but I'm still confuse on how you feed the AC into the MAX335, I thought this chip only works with DC.
thanks a lot
Fede
 
Thanks a lot mneary,
they are a lot of amps in fact, the screen is divided en 28 panels of 4 by 8 feet and each panel contains around 200 cells.
but I'm still confuse on how you feed the AC into the MAX335, I thought this chip only works with DC.
thanks a lot
Fede

hi,
When you say the panels require 15Vac do you mean a 15V square wave signal.?,
which is much easier to produce from a 15Vdc source.

Do you have any techno data on these panels, I've looked but cannot find anything realy helpful.:)
 
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Hi Eric,

Actually it should be a Sine wave, this LCD film seems to be pretty sensitive to current, I tried PWM but this would also destroy the film.

What I’m exactly using for the screen are 5 by 5 inches individual squares of NPD-LCD film. This is the film used in privacy windows or Smart Glass , it’s also called 3G Privacy Film and it’s made by Scienstry, Inc.
specs:

Driving Voltage: 50 ± 5 V_AC, 50 – 60 Hz_
Current: 0.1 ( A/ m2 )
Energy Consumption: 5 ( Watt/m2 )
Response Time: < 0.2 ( Seconds )
Operational Lifetime: > 80,000,000 (Switching Times)

The Generation I’m using can be switched with 15v AC, but there is a newer generation that uses
0.05 ( A/ m2 ) of current and only 10V AC.

Thanks

F
 
5 inches is a little bit smaller!

The MAX335 is general purpose switch that you can use for any voltage inside the rails. And it can be used with rails up to +20V and -20V.

I would get a 15V center tapped transformer and connect the center tap to the common (GND) for the section. Each end of the winding is 7.5V in opposite phases. I would rectify the 7.5VCT into + and - approximately 11VDC (unregulated). These go to the +V and -V pins on the MAX335. One end of the 7.5VAC winding goes to one terminal of each (all) pixels. The other end of the 7.5VAC goes to each (all) of the COM pins of the MAX335. Each of the NO pins on the MAX335 goes to the other terminal on each pixel.

Provide a logic supply, clock, data, etc. to each MAX335 and you're done.

If the pixels turn out to be accidentally driven by capacitive leakage, you would drain that off with maybe 10K across each pixel. Maybe a 100nF capacitor in series with every pixel just to block all incidental DC.
 
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one question

Hi mneary,
One question, if the LCD requieres 15v AC shouldn't I get a 30v center tapped transformer so I get -/+ 15 v?
thanks a lot

Fede
 
No, when the switch is on, the total potential across the LCD is 15VAC. Remember that one side of the LCD is on one side of 15VCT and the switch is on the other side. This allows you to switch 15VAC while only applying 7.5VAC to the switch.
 
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