IMPORTANT NOTE: The following post has been superceded with better information in a subsequent posting. Get the updated version here.
I’ve been making an effort lately to get useful information on piezo films into these pages for the benefit of anyone who may want to use them. Here’s one more posting along those lines. The question I get most frequently from customers regarding the piezo film is, is it OK to trim a piezo film pickup to a smaller size?
The quick and safe answer to this question is “Beware: cutting a piezo film pickup may ruin it.” But a more complete answer is, sometimes it’s OK to cut the film, sometimes it’s not; it depends on how you cut it. Read on for details.
The piezo film pickups we sell are made up of a very thin film of piezoelectric material protected by a layer of clear plastic lamination on both sides. It’s hard to see this unless you look quite carefully, but the piezo under the plastic is actually a double layer, in the form of a single piece of very thin piezo film folded back on itself. The two terminals for the wire connections, located alongside one another at one end of the pickup, are actually at opposite ends of this underlying piezo film, brought next to each other by the folding. (Think of a towel folded in half: the folding brings the opposite ends together.) Now imagine that you decide to shorten the pickup by snipping off the end. By cutting off the point where it folds, you are (unknowingly) cutting the piezo film into two separate sections; it’s then no longer a single sheet. The electrical continuity between the two terminals is broken, and the pickup will not function
So that’s the big negative: you can’t shorten the pickup by snipping off the end; that will destroy the pickup
However, you can trim in other ways, and as long as you do so without compromising the electrical continuity, the pickup will still function. First of all, you can always trim off the clear plastic borders; there’ll be no ill effects because there’s no piezo material in those clear portions anyway. Second of all, if you want a narrower pickup, you can usually get away with trimming the sides, as long as you make sure that the folded end isn’t snipped off
Keep in mind, too, that the piezo film pickups are quite sturdy. If it’s necessary to give the pickup a sharp bend or something of a fold to make it fit in the intended application, that’s ok.