Piezo protection question

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Another piezo thread!

I've got a couple of the twin driver CTS ksn 1177 tweeters. They are very efficient and nicely designed with a different damper on each half to smooth the output.

They don't have the power line protection and I'd like to add something like that.

I have a handful of ptcs from the bay and am thinking something like 27R with a tripping current in the tens or low hundreds of ma.

I'm thinking the calculation should be around 18V RMS over 27+@100R at say 10khz. Based on 75w at 8R.say 140ma.

Any better thoughts?

Where to start with sizing a bulb in parallel?
I found the original patent on line and that gives enough clues to work out component values I think.

The bulb is described as having 50 to 500R impedance. This ties up with a standard 50mA 24V Grain of wheat bulb.

It also matches with the 400W peak related to 8R. Probably dropping around half the 50 odd volts at 2000hz which is highest useful impedance. Dropping more of the volts as the frequency rises.

The only suspect info there is the use of a 1R thermistor. That implies 800mA trip for a typical 0.8W disc and I reckon the piezo would have blown well before that toggled. I think something in the 10s of Ohms and hundred ish mA is more like it.

Any thoughts?
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I measured the combined resistance of the protection on a Power line tweeter and it is 15R cold.

That says 20 ish Ohms for the PTC.

I guess 27R for my smaller discs in the power horn and x2 in the twin drive means my calc is about right.

508 lorry bulbs look exactly like the bulb in the Power line :)


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Well, a bulb or PTC works on conventional drivers as protection but they won't save the piezo at all because both are current/power driven while piezos are almost completely voltage depenent. Therfore power reduction with these won't work, the piezo will be destroyed from over-voltage. If you want to protect the piezos, you'll have to limit the maximum voltage, i.e. with a (or more two) zener diode or something similar.
Agreed. Instant overvoltage will blow the device by flashing over or via mechanical over travel I guess. However, the filter network of resistors and capacitor will take out DC or the big peaks at below 2khz. So it's just fast transients at high frequency, in which case the lower impedance has less voltage to drop.

How much transient energy the unit will take isn't stated. A fast parallel transorb would be a simple addition I suppose?

I think the Motorola circuit is very good for what it is and does. Add say 40V transorb for belt and braces?
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I do like the CTS piezos.

I've been playing with some old KLH 38 speakers with cone tweeters with new caps, some Morel dome tweeters and hexatech mid/bass with various crossovers and faital pro compression drivers with fane 10 300 bass/kids.

The piezos are quite flexible and should not be discounted for good quality home sound.
Use a pair of zener diodes near the needed voltage cut off, plus speaker fuse in line with tweeter. Fuse should be in series with tweeter, and before diodes. Zener's should be in parallel with tweeter. Make sure the fuse is relatively small. Should be much less than the amplifier could provide without damage. 1 Amp may even be enough assuming impedance is high. I can't find any info on this unit.

Um, or not. I personally never use piezo's or protection circuits but I don't use my speakers for party environments.


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