I had an enquiry about using LED’s to protect the input of the Phantom Piezo Preamp (available to buy here) which puzzled me slightly, as I didn’t initially see the point. However it was noted that if a piezo transducer is dropped onto a hard surface, the voltage spike generated could be high enough to damage downstream electronics.
Here’s the schematic of the Phantom piezo preamp:
The preamp has a fully balanced input topology so the ground/earth/shield connection is not required at the input for normal operation. (The ground may be used at the input to shield against common-mode noise and interference. The ground is required at the output to act as a return for the power supply).
The obvious and correct place therefore to fit input over-voltage protection is between pins two and three of the input.
A pair of LED’s connected back-to-back have been suggested – and this is a good choice. The input voltage will be limited to the forward voltage drop of the LED chosen. This gives us a range of about 1.7V to 3.4V depending on the Vf of the particular LED chosen. There will not be enough current available to burn out the LED’s and – because there are two LED’s connected back-to-back – the LED’s will protect each other against excessive inverse voltage.If you wanted a lower maximum input voltage, you could use two signal diodes (e.g. 1N4148) back-to-back which would limit the maximum input voltage to 0.7V or so.
If you want a higher limit (or a less expensive solution than LED’s), then you could use zener diodes. However, if you use zener diodes, you need to connect them in series not parallel and also you will have to add on the forward voltage to the zener voltage.
For example, if you chose BZX79C2V4 or 1N5221B zener diodes, you would add the 2.4V zener breakdown voltage of one diode to the 0.6V forward voltage of the other diode to give an input-protection voltage of 3.0V. You will note that it doesn’t matter whether you connect the zener diodes cathode-to-cathode or anode-to-anode – the effect is the same.
The notion of connecting two piezo transducers back-to-back to give a balanced output has also been brought to my attention (thanks Lee!) and I cover that subject here.