Zener diode clamp not working for fast spikes?

[SeanB]

Most fence energisers use a LED in the sense circuit, with a long path to the photodiode used to sample the power. The IR LED will provide a light pulse, and the photodiode can be used with an integrator, probably leaky, to provide an idea of the power level, as the fence energiser uses this to provide a rough power level, and also to detect touch as it damps the pulse energy down a lot.

[twospoons]

First line of defense should be a lighting diverter on the fence itself. This consists of a choke between energiser and fence (just a big stretched out steel spring), a spark gap of about 20mm made from some fat bolts, and a number of earth rods.


Second line of defense would be a GDT.  I'd choose this over a MOV as  MOV's degrade every time they get hit (and they can explode if hit too hard). Either GDT or  MOV needs some impedance to work against, so your divider (be it resistive or capacitive or both) needs to be able to handle a substantial overvoltage - I'd rate it for at least 50kV in this case.

[T3sl4co1l]

First line of defense should be a lighting diverter on the fence itself. This consists of a choke between energiser and fence (just a big stretched out steel spring), a spark gap of about 20mm made from some fat bolts, and a number of earth rods.
(Attachment Link)

Second line of defense would be a GDT.  I'd choose this over a MOV as  MOV's degrade every time they get hit (and they can explode if hit too hard). Either GDT or  MOV needs some impedance to work against, so your divider (be it resistive or capacitive or both) needs to be able to handle a substantial overvoltage - I'd rate it for at least 50kV in this case.

MOV degradation is a popular misconception; in fact, they do not wear below a threshold.

But MOVs and GDTs are wildly overkill here.  For direct electrical measurement, large-value resistors are the best protection and filtering (as RC filters).  The resulting energy at the circuit level is trivially handled by clamp diodes.  You may need to use high-voltage resistors to stand off direct or induced lightning surge, and an air spark gap would do just fine.  The use of a spark gap implies a two-stage (or more) protection system, i.e. series resistor (expected to arc over or otherwise fail during lightning), spark gap, another series resistor (can't arc over before spark gap does), then clamp diode/TVS.  And probably one more resistor to the MCU/acq/detector/etc. system.

Or maybe you mean to protect the energizer, but considering its output is already many kV, I don't see what good a typical MOV or GDT would do there, that wouldn't immediately defeat its whole purpose.  You'd need one of those MOV stacks from the power company, and the clamping voltage is still many times nominal operating voltage.

Tim

[twospoons]

I personally wouldn't bother with the MOV or GDT either, just that MOVs were mentioned further up the thread. Diverter on the fence + HV divider + TVS or similar should be enough. 

I've also witnessed first hand some weird behavior around HV resistors and fence energisers - notably they are just fine when directly connected, but sometimes will blow their guts out when connected via a small arc gap to the resistor lead wire. Something about the arc must set up a resonance or something that creates an internal overvoltage in the resistor. Next thing your 25kV (expensive) resistor has a hole in the side even though the energiser only has a peak output of 8kV.  I never did find an explanation, but the effect was quite repeatable with any kind of film resistor. Wirewound ones didn't appear to have the same issue.

Also a note to the OP: don't underestimate how far a high voltage will track across a surface. I've seen 25kV travel 20cm across a sheet of mylar, instead of punching through.  This is another reason to use a string of resistors - you can greatly increase the total creepage distance.

[T3sl4co1l]

Not familiar with that effect myself, but not to say it can't happen.  Sparks in air can have extremely short risetimes; sparking to the resistor lead basically sets up that lead as a crude transmission line element, resonance of which is some 1GHz, ballpark.  That, and perhaps the resistor element and its ceramic body makes something of a waveguide structure, and combined with the extreme peak power (say 10kV in fractional ns), you could very well end up with the effect of microwaving a CD, except it's the thin metal film of the resistor.

Put another way: the voltage along the resistor element does not distribute evenly and instantaneously, and the high-frequency voltage limit can be much lower than you would otherwise expect.  This is why oscilloscope probes are rated as such, for example.

You would want to use pulse-rated, wirewound, or perhaps thick-film, types for this.  Or good old carbon comp

Tim

[Berni]

The MOV shouldn't ever really see any activation in any sort of normal operation. It would just be there in the case of lightning or similar that could arc over the high value divider resistor. You can never know what to expect. Sure a direct lightning strike into the input would turn the MOV and all the rest of the electronics (and possibly enclosure) into plasma and charcoal. But most strikes would happen just near by and the long fence wire might pick up a huge voltage spike from that. Having an extra layer of protection rater for handling large energies would help you survive that. Hopefully the high voltage divider resistor survived the arc over and can continue working.

If you want to also help protect the HV resistor you could add a spark gap directly to the input. This is quite common in telecom equipment where the PCB has exposed metal close together right at the input so that huge overloads would arc over in that spot and possibly save the protection circuitry down the line. But since your project is going to live outside you can't use a PCB pad as a free sparkgap since condensation is going to screw it up.

It all depends on how robust you want to make it. It would work just fine even with 0 protection, just direct divider into MCU pin (there is some weak ESD protection inside the pin after all). And if you are lucky it might take years before it sees a transient big enough to blow up the MCU. But protection circuitry is so cheap that it is worth the time it saves you in having to fix blown electronics later.