What is the 555 configured as in this schematic?

[betwixt]

The input of a CMOS gate like the 74HC14 is voltage driven, not current driven so the only effect the 10K resistor would have is to form an RC filter where C is the capacitance of the gate and diodes. This will only be a few pF. possibly less than your wiring to reach it and your frequency is fairly low at a few 10s of KHz so it shouldn't cause a problem. The reason for fitting it is to limit the current into the diodes in case you get a flash-over from the arc. Ideally you would use a resistor rated at several KV but they are very expensive, if you want to be asolutely sure about protecting the gate, use say 10 x 1K resistors in series. Most small (0.25W) resistors are rated to around 250V. The neon should conduct most of a flash-over to ground anyway so it should be fairly safe.

It is difficult to test it without lab equipment. A mobile phone might cause a few mV pick up if you are lucky so forget that idea. For testing for overload protection in commercial equipment I used to use a sealed steel room, even the entrance door had RF seals around it. Inside was a dipole antenna and 200W transmitter, the dipole was typically 1m away from the equipment being tested and the frequencies between 100KHz and 100MHz were fired at it. Even that would only induce a volt or two. When you consider your mobile phone has less than 1W ouput, and uses 1.8GHz, you see it will have no noticable effect at all.

If you want to test it, I suggest a car battery and ignition coil. wire it like a spark plug and shoot a jolt of a few KV at it by momentarily connecting 12V across t's primary. You should get a very healthy flash from the neon and the IC should survive it.

Brian.

 

[boylesg]

I was actualy trying to figure out a way to test the effect of the 10k resistor on the current coming through the aerial due to the RF rather than the flash over.

But it sounds as though I needn't bother.

Oh well I have implemented a little common collector amplifier anyway. I have the option of passing the aerial input through it or not.

What about the wire wound 3W+ resistors? I have salvaged tonnes of those from old TVs and 10K is one of the common values used.
However I have used a 6.8K and a 4.7K (these are reasonably common in TVs too) in series based upon your recommendation.....some fairly beefy ones......5W I think.

I just was reading a bit about these and found references that state they can sustain of the order of kV in short pulses.

By the way, how can you tell the hex inverter is a voltage triggered device rather than a current triggered one from the datasheet? Is it just long experience or is there something blindly obvious in the datasheet that I have missed?

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It's worth a try. The gate driver can handle 9A peak and the gate current will be mostly dynamic currents and much smaller so you can probably use 1A fuse or even less. Make sure it's a "fast blow" type and not "anti-surge". You could add extra protection by adding a Zener diode from the drive signal to ground on the IC side.

Brian.

I take it a zener voltage of a slightly larger than than the Vcc driving gate driver chip would be appropriate?

 

[betwixt]

The antenna series resistor problem is one of voltage rating rather than power. Most small resistors have a voltage limit because of their construction, in particular ones with tight tolerance because they are trimmed by cutting narrow slits in them with a laser and it makes an easy gap for a spark to jump across. Similarly, some use a spiral of carbon around a core, rather like a wire-wound resistor but using a carbon track instread, these also have a tendency to flash between turns if the voltage is too high. The power rating is almost irrelevant as the duration of overload may only be a few microsconds. I wouldn't recommend using wire-wound resistors in any antenna application though becasue they are also inductive.

All CMOS IC's are voltage driven, they use the static charge on the semiconductor to displace electrons in the conduction channel so apart from current to charge up and discharge the capacitance of the junction, they draw no current at all. That's the reason they need protection, there's no route to discharge any build up of voltage on the antenna. In fact it's entirely possible the gates in your schematic will be operating when they shouldn't anyway, a resistor from the input to ground will ensure the logic state prior to receiving any signal. Even a resistor of 1M or more to ground will ensure the gate is 'low' until a signal lifts it higher.

Regarding FET gate protection, yes that's the Zener voltage to use but keep it as close to VCC as you can.

Brian.

 

[boylesg]

OK I have mostly implemented my twin monostable 555, with changeable timing capacitors and a double gang potentiometer for the resistors. I am just waiting on the gate driver ICs to arrive.

I have previously implemented twin astable 555s with 50% duty cycle and changeable timing capacitors but still need to implement an adjustable voltage divider to ensure the output voltage is at 50-60% of the supply for the monostable 555s. I was intending to use these as twin flyback drivers for stereo plasma speakers. So they also have the original gate driver chips, without the enable pins, that I was using. But that is OK because I can re-direct the output of the 555s to the monostable 555s on the other circuit board.

I almost have an ajustable non-inverting amplifier with two channels, as soon as I figure out why opamp 2,3,4 wont work.

 

[betwixt]

I agree. They should be good for 250V so they are suitable.

Brian.