I am working on a project to produce a pulsed detonation engine. I am using a spark plug to ignite the fuel mixture and measuring the wave speed using capacitive discharge microphones sampled at 25 kHz. I am concerned that the high voltage signal produced by the spark plug may travel down the I/O, 5 volt, and ground lines running from the microphones to the DAQ. I have done what I can to shield the microphones from the spark but there is the chance that the engine temperature could erode the insulation and form a closed circuit with the mics.

I originally looked into TVS diodes but found that they cause signal degradation on high sample rate sensors. Do TVS diodes exist that would suit my experiment?

A colleague has suggested using capacitors in the pico-Farad range on all of my lines into the DAQ. I have tried 222pF, 15pF, and 1pF capacitors and found that the microphones appear to be unresponsive with the 15pF and 1pF capacitors when I observe the voltage signal produced. How do I go about finding the proper sized capacitor for my application?

Are there alternatives for ESD protection on high sample rate signals? I am worried that the spark duration will overload the capacitors if there is a closed circuit. I am working on a very small budget since it is being funded out of my own pocket.

Thank you very much for your assistance.

 

Have you thought about input protection diodes like this?

 

I have very limited understanding of ESD protection equipment. What allows this to not interfere with my high sample rate I/O line when TVS diodes cause signal degradation?

 

I have tried 222pF, 15pF, and 1pF capacitors and found that the microphones appear to be unresponsive with the 15pF and 1pF capacitors.

If the signal completely collapses with 1pf in parallel with it, this is a foolish suggestion. Sorry.

 

Just out of curiosity, what is the engine temperature?

 

Pulsed Detonation Engines typically melt aluminum after a few minutes of operation. I'm trying to avoid this during testing by limiting it to short duration runs. I am not certain how rapidly the temperature will climb per pulse. There is a thermo-couple attached and I intend to avoid going over 4-500 degrees.

I put the 1pf capacitor in series with the signal line. If it needs to be in parallel for ESD protection I will test the signal input again. It just seems to me that the capacitor in parallel wouldn't stop the ESD from reaching the DAQ.