Placement of busbars on smartfilm

October 21, 2022 11:06 AM

I'm investigating smart film for a project and I'm having trouble finding recommendations for the placement of busbars. For small samples putting them next to each other on the same edge appears to be preferred, but there are references to putting one at each end for longer pieces. I've also seen two busbars at each end. Is there a formula for this?
posted by Tell Me No Lies to Technology (1 answer total)

I didn't see a manual (which would very likely be sufficiently incomprehensible on several crucial points anyway) but just looking at this stuff with my technical cap on: it consists of five layers, a PLCD film sandwiched between two conductive films, and then a protective sheet each side. Applying a voltage to the two conductive layers turns the PLCD film transparent.

Now this takes a bit of power, 5W for a square meter. Which means that there will be current flowing through the conductive films, and as those films have a certain electrical resistance there will be a voltage drop the further a point on the film is from the bus bar. Though without knowing the actual area resistance of the film you can't calculate how much drop will occur over a particular distance. Apparently it's low enough that for a small-ish piece of 'smart glass' you can just put the bus bars on one edge next to each other. As you start using larger pieces that voltage drop will cause parts of the glass to not become fully transparent if you have the bus bars on the same edge. But putting them on opposite edges will result in the voltage across the PLCD layer to be as good as constant: the further a point is from the top layer bus bar, the closer it is to the bottom bus bar, and as the sum of these distances is a constant, so is the total resistance a current encounters going through the top conductive film, then through the PLCD and through the bottom conductive layer.

So, opposite edges is just the best configuration anyway, but for a small piece of smart glass you can get by with having them on one edge which can be more convenient from a construction point of view. There's also the matter that theoretically you would have to have the bus bars the same length as the edge they're on, but the demos show they don't have to be. Half as long seems to be OK for small-ish films again, for larger areas you will likely have to go up to 80% or so.
posted by Stoneshop at 2:12 PM on October 21, 2022