On Fri, Jul 14, 2023 at 09:34 AM, eewiz wrote:

All due to the failure of one little air blaster.

Great story :)??
Arcs don't need 138kV btw - I had an automotive 6-layer PCB with just 12V (at about 1000A, car battery) that got wet, formed a cathodic short (took 6 hours), then struck an arc.? The arc then whizzed all round the board like a Pacman tornado - greatly helped by the power planes, until everything caught fire and a million pound prototype car was destroyed.?

Why only the ONE little air blaster?? ?I know it's easy to be wise after the event, but an FMEA should have picked-up the catastrophic consequences.?
It's relevant to LTSpice in that simulation is a tool of "best practise" - where good design process can eliminate future disasters, and FMEA is a similar thing.? ?

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On 14/07/2023 12:17, Tony Casey wrote:

changing the feedback resistor to 5p6 seemed to be about the optimum

...changing the feedback *capacitor* to 5p6 seemed to be about the optimum

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On 14/07/2023 12:17, Tony Casey wrote:

Next, you have defined a -2.5V supply (Vn), but used it.

Next, you have defined a -2.5V supply (Vn), but *not* used it.

This schematic does not run, as uploaded. Andy explained why (.options lines split up). The "+" character at the start of a line, tells LTspice to append this line to the previous one. When these are not within the same directive block, they will be not be associated with the intended line, and be appended to some other line that they are not supposed to be associated with.

The .options... were put in the the original fixed schematic because of issues with the opamp model used in that schematic. They should not be used as a default in others, where it isn't necessary.

You also used the "startup" option in the .TRAN directive, but there seems to be no reason why it would be needed. Only use these things where necessary and for good reason. Here, there isn't one.

Next, you have defined a -2.5V supply (Vn), but used not it. Is it, by any chance, intended to be the opamp's negative supply? If so, you can't use +5V for the positive supply, because the datasheet says in the absolute maximum ratings section:

Supply Voltage V+ to V– ...........................................5.5V

With the opamp's V- supply pin grounded, the pulsed source the input voltage falls below the inverting input's minimum level and the ESD protection diodes are forced into conduction.

I reconfigured the circuit to use ±2.5V supplies, including biassing the photodetector from -2.5V.

The photodetector model is producing 5mA pulses. You feedback resistor is 1k, so potentially the output should reach 5V, which is, of course, is beyond the supply rail. Therefore, it can't. So, assuming you need the output voltage to be a linear function of the input current, we need to reduce the feedback resistor, so the opamp isn't clipping - I reduced it to 390 for a little margin. But...

The LTC6268-10 opamp is only stable for closed loop gains in excess of 10x. That's what the "-10" suffix means. I would refer you to page 13 of the datasheet for some stability considerations. In summary, at high frequencies, the feedback gain is approximately Cin/Cfb. Your photodetector seems to have a junction capacitance of 2p, requiring a feedback capacitor of <200f. This is probably lower than the internal capacitance of the opamp. I don't know which PD you have in mind. I checked out some very high speed InGaAs PDs (infra-red for LIDAR etc) from Marktech of the sort of speed (2Gbps) you seem to be looking for. They seem to have a higher junction capacitance than 2p - more like 12p...

Anyway, without treatment this TIA oscillates. It will take a quite a bit of tweaking to make it properly stable. With your original arrangement, it appears not to oscillate, but that is misleading because most of the time it is saturated on the rails.

What exactly is your requirement? Just picking the fastest opamp you can find isn't necessarily the right approach. FYI, substituting an LTC6268 (unity gain stable version) made the TIA stable without further tweaking, but it still had some ringing - changing the feedback capacitor to 5p6 seemed to be about the optimum, but on a real circuit that might not quite be so. It isn't, of course, as fast.

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The spark plug in a spark-ignition engine has a gap of about
0.020 inches, and the voltage required to initiate a spark across this gap is
about 20,000 volts.

Hello,

I am trying to make overvoltage switch using ACST from STmicroelectronics but seems can't simulate its spice model. Any help will be appreciated. Please find all the files over here,

/g/LTspice/files/Temp/ACST_Files.zip

Regards,
jagdish

FYI -

The LTC6268-10's input voltage range just reaches the V- supply pin but does not go below it even a little.? So it's taking a risk, having the negative supply pin grounded, with the +IN pin also grounded.? In fact the voltage on the -IN pin reaches -0.66 V in the simulation, which is below the allowed input voltage range.

Andy

When you have a command such as this on a schematic:

? ??.options cshunt=1e-14
? ? + noopiter
it is essential to have all three lines in the same SPICE Directive.? But in the schematic you uploaded, the second and third lines and the first line are in separate SPICE Directives.? You can't control how the lines are ordered in the SPICE Netlist (well, actually you can, but it is not how you'd expect, it depends on the order they were added to the schematic).? As a result, the "+ noopiter" and "+ gminsteps=0" come after and add to your ".tran" line, and that doesn't work.

That causes an error, and your simulation can't be run.

Remedy: put the "+ noopiter" and "+ gminsteps=0" lines in the ".options" line's SPICE Directive.

Andy

Tom wrote, "One should never use a gas tube protector across an AC or DC line. Once they fire they will not shut off unless the voltage/current drop below the sustaining level. "

But with AC, doesn't that happen twice every cycle?? Or does the discharge tube take a few seconds to recover?

Andy

They are basically the same principle except the neon gas both lowers the trigger voltage and makes it predictable. Neon bulbs must operate within power limits while spark gap power is only limited by physical size and material.

Spark gaps are etched into PCBs all the time to provide some protection from surges. Unfortunately PCB and air spark gaps exhibit widely varying breakover voltage dependent on gap, temperature, humidity, contamination and altitude.?Spark gaps should never be placed across an AC or DC line. If the line voltage is high enough, they effectively become a short once triggered.

Spark gaps are widely used for protection. The most common is the ceramic gas tube protectors for Telecom. They come in 2 and 3 lead versions. 3 lead used for balanced lines.

In the old days, the gas could have radioactive isotope added but that is very rare today except for certain military and industrial applications. Waveguide Radar receive protectors back in the day were filled with a radioactive isotope gas and a HV priming voltage.

One should never use a gas tube protector across an AC or DC line. Once they fire they will not shut off unless the voltage/current drop below the sustaining level.?

Thanks so much for the detailed explanation!

I actually ended up switching to the LTC6268-10 and found that I'm able to actually get some semblance of a response from that opamp at my smallest pulse width. I am not trying to exactly duplicate the pulse; I am trying to detect that there is a pulse at all so I am thinking (hoping) that this is a sufficient response.

Here's the circuit?if you are interested in continuing to follow this saga. There shouldn't be any other files required except for the LTSpice circuit itself.

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The difference is gas pressure. Obviously, the spark gap has atmospheric pressure, about 100 000 Pa, but the pressure in a neon lamp is much lower: 100 Pa to 3000 Pa.

One thing I was never clear on, is the difference between a spark gaps and neon discharge lamps.? Are they the same principle?? They seem so widely different.? Spark gap discharge is very hot, leading to eventual destruction of the spark's electrodes.? Neon lamps are "cold" discharge.? Are they the same principle, just orders of magnitude apart?? Or different like night and day, which coincidentally end up with similar electrical properties?

The physics aside, it might end up making a difference for electrical modeling.? Or then again, maybe not.

Andy

Maybe off topic, but there is some discussion of gas mixture in different neon lamps (many no longer made) in the 1965 GE Glow Lamp? manual I mis-named earlier. There is some discussion of purpose of radioactivity (maybe altering ignition/extinction characteristics).

--

Thanks John.

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That law is about hot-cathode diodes and probably doesn't apply at all well to spark gaps.

In my studies I found this:The Child-Langmuir law is a differential equation that describes the current through a spark gap as a function of the voltage and the gap spacing.

Thanks for the further clarification of purpose.?
It's not enough to get any real purchase on the problem, but OK that's your business not mine.
I'm just saying that the tolerances on real spark gaps, and the many unknowns that a good model might erm, model, do suggest that it's not the easiest of approaches.?
You might get 20% repeatability.??
There are many established methods for measuring very small currents at very high voltages -? stick to the path if you can, it avoids solving inessential problems.?