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Static Discharge Latching Issue
In General Discussions
Nov 27, 2021
@Harry Zachrisson Like I said, its more complex but I think about them that way for a basic mental model. If you need to actually design proper protection systems it is a lot more complex and a GDT is just one stage. The 90V rating is a nominal DC arc over voltage (it is normally a wide tolerance +/- 20%). GDT should NOT be used at DC where the voltage will get anywhere near that level. If it fires it will stay on and basically self-destruct. You are right about the higher trigger voltage for transients. For instance the datasheet I'm looking at for a TDK (Epcos) unit lists spark over voltage at about 500V for a 100V/us pulse. However once transitioned into conduction, the voltage across the unit is held quite low. They specify 15V for an arc current of 1A, higher currents would have a higher voltage. This low arc voltage is why care has to be taken in the presence of DC. Transient pulses are a lot harder to reason about and simulation is probably the best option without access to specialised testing equipment. The low pass filters on the output of the transmitter will help suppress any pulse getting into the actual transmitter until the GDT kicks in. But simulating it with some of the standard pulse models (8/20us for instance), would be an interesting exercise. Parasitic inductance and capacitance is important to model for this, and is also why testing with high power pulse generators is important for verification. For use as an antenna transient suppressor, basically just ensure the RF power output can't trigger the GDT by a wide margin. High power RF becomes a lot more tricky, but for lower power things this is fine. I've seen these used a lot in cellular phone systems when I worked for a company that built tower mounted amplifiers and filters. We did significant testing with lab ESD pulse generators to verify our protection circuits. However in these applications, we were building bias-Ts into the device, the GDT was placed after the RF choke so would operate at DC only. It was there to protect against induced voltage between the inner and outer conductor of the coax. We also had specially designed power supplies powering the devices which would detect current spikes like an arc and totally disconnect the power for a period before re-powering to clear any arc faults - a bit like re-closers on the electrical grid. They would retry a small number of times before locking out that power feed up the tower and raising an alarm. 73s, Ash. edited: typos


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