Author Topic: "Proving" safe voltage (detecting a very small % of a high voltage)  (Read 27448 times)

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Offline tookiTopic starter

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Hey everyone,

<background>
I'm working on an experimental setup at work that involves a quite high voltage power supply. (The experiment requires the feedstock chemicals, in gaseous form, to flow through an electric field to catalyze a reaction somehow. We designed the electrodes as two cylinders separated by a gap. The lower electrode has the outflow in the center, so that the gas mixture cannot exit the reactor without passing between the electrodes.) This project was started before I was hired, and a high voltage power supply had already been ordered with a maximum voltage likely far higher than needed: 40kV DC. So even though they'll likely never run the experiment at that voltage, the entire setup has to be designed to protect human safety even if someone were to crank up the voltage.

The power supply is current-limited to 1mA in terms of regulation, but the peak current as its caps discharge can reach up to 500 times that, according to the manual.

Additionally, the 3 meter long HV cable, being the typical coaxial construction, has about 140pF of capacitance of its own, so on the advice of my old boss (who has experience with HV stuff) I'll be adding a discharge circuit (with analog moving-coil meter) and a series resistor to the electrode to limit current.  Between my discharge circuit, and the discharge circuit built into the power supply, any voltage should be discharged within 1 second, which is faster than one could physically open the barriers between the user and the electrodes.

There will also be interlocks, interfacing with the fume hood door, as well as the door to the metal cabinet that will house the reactor itself. (And inside that, everything HV is insulated.)
</background>

So the actual question:
One of the things that the users would like is a light (like the typical industrial beacons) that indicates whether it's safe to touch or not. A theoretically simple one would be a light that simply signals when the HV output of the power supply is turned on (regardless of what voltage it's set to), but unfortunately the power supply does not have an output for that, only a 0-10V proportional analog output, so a safe-to-touch output voltage of 50V DC, for example, would mean an analog output of just 20mV -- and I can't be sure if that output would even operate if the power supply's mains input is turned off.* The even better option would be something that actually indicates a voltage of 50V or less at the electrodes, but I am not sure how I would go about measuring that in any halfway reliable way.

So how would one go about detecting that the voltage is 0-50V when the maximum is 40kV?

Or is there some entirely different way of sensing this, especially something entirely passive?

I'll be using a safety relay (probably something from Phoenix Contact) to monitor the interlocks and cut off power to the HV supply, and to control the beacon, if used.

What we don't want to do is a light that doesn't actually indicate anything useful, or that could even be misleading. So I don't want the beacon to simply illuminate when the interlocks are closed (=ready to operate), but the HV supply isn't on, since that's signaling "danger" even when it's not, which could lead to complacency.

I'm currently waiting on a reply from the power supply manufacturer to see if they can offer a solution. The power supply's own HV output on/off button illuminates exactly as I want, but I'm not opening and modifying an expensive, dangerous HV power supply without their say-so.

Thanks for any ideas!


*I did lash together a quick comparator circuit as a test, but all I had laying around was an LM339, whose input offset is too big for comparing 20mV to 0V. Even with hysteresis it flutters around.
« Last Edit: March 07, 2024, 08:41:40 am by tooki »
 

Offline Gyro

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #1 on: February 24, 2024, 11:18:08 am »
You probably want to go as passive as possible for safety, and maybe double up too. My first thought is a simple resistor chain and indicator, either a neon or very high efficiency LED. You could then rely on this to discharge the capacitances that you mentioned - for a neon you would need a high value shunt resistor across it to ensure that you get below 50V. The problem is that you have a relatively small amount of current to play with in terms of illumination. With the loading of the indicator, you could probably add a label saying something like 'Wait 10 seconds after the indicator is extinguished before opening'.

As an alternative, you could use a MOSFET with gate clamp and high value shunt resistor in place of the LED / Neon to drive a larger indicator - as long as you can ensure that the supply to it is failsafe.

The comparator method is reliant on an independent supply and other factors that might make it more prone to failure, not to mention the difficulty in handling such a wide input range without possible noise problems at the low end.
Best Regards, Chris
 

Offline PartialDischarge

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #2 on: February 24, 2024, 11:21:11 am »
For that kind of voltage span, 50V to 40kV a passive method is not possible, because resistors will create a large voltage drop or the power dissipation will be too high.
Maybe they can help https://saker-mv.com/voltage-detector-mvd30-mvd15/

 

Offline Gyro

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #3 on: February 24, 2024, 11:53:17 am »
What's going to discharge the output capacitances down to <50V? The PSU? - one for tooki to check. I'm not saying a 40kV resistive divider is easy, but it is doable with proper insulation and layout. For a few 10s of uA dissipation isn't a problem, eg. 800mW @20uA. If it turns out that a discharge load is needed anyway to bring the voltage down in a sensible time, then you may as well make use of it.
Best Regards, Chris
 

Online DavidAlfa

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #4 on: February 24, 2024, 11:59:29 am »
HV resistors are not that expensive. RS Pro 500Mohm rated for 20KV:
https://uk.rs-online.com/web/p/through-hole-resistors/1754074?gb=s

Use 4 of them to make 2Gohm 80KV + a final 200K sensing resistor. (1KV=100mV).
2Gohm load (20uA @ 40KV, 800mW), giving a 10000:1 ratio or 4V ouput for 40KV input. Adjust the final ratio as required.

Feed this to a  instrumentation op-amp/comparator, these usually have GigaOhm input impedances.
It doesn't need to come out for anything, just put it in a earthed metal enclosure. Use a small secondary psu for running the detection circuitry and turning the led on/off.

Make the circuit redundand just in case, but the interlock should be the true safety mechanism, disconnecting the HV supply and applying a bleeding resistor unless the HV PSU automatically does this when off.

As long as everything is referenced to ground (Protective earth), it shouldn't be dangerous.
Dip everything in HV silicone, perform degassing if required, should perform great.

The power supply's own HV output on/off button illuminates exactly as I want, but I'm not opening and modifying an expensive, dangerous HV power supply without their say-so.
Or just a phototransistor reading this!
But this probably isn't a real HV indicator, but simply turned on by the driving circuitry, assuming there will be HV output unless something is wrong.
« Last Edit: February 24, 2024, 12:26:45 pm by DavidAlfa »
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Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #5 on: February 24, 2024, 12:15:22 pm »
You probably want to go as passive as possible for safety, and maybe double up too. My first thought is a simple resistor chain and indicator, either a neon or very high efficiency LED. You could then rely on this to discharge the capacitances that you mentioned - for a neon you would need a high value shunt resistor across it to ensure that you get below 50V. The problem is that you have a relatively small amount of current to play with in terms of illumination. With the loading of the indicator, you could probably add a label saying something like 'Wait 10 seconds after the indicator is extinguished before opening'.

As an alternative, you could use a MOSFET with gate clamp and high value shunt resistor in place of the LED / Neon to drive a larger indicator - as long as you can ensure that the supply to it is failsafe.

The comparator method is reliant on an independent supply and other factors that might make it more prone to failure, not to mention the difficulty in handling such a wide input range without possible noise problems at the low end.
I am already planning to have a discharge circuit using a 100uA meter through 400Mohms, which will thus display the actual voltage, but also discharge. But of course once we're down to a few hundred volts, one won't have any useful resolution.

The problem with lamps and whatnot is that this thing is never going to be run at 40kV -- I actually expect they'll be below 5kV -- so an indicator may not even light up with the series resistors that have to handle 40kV. :/
 

Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #6 on: February 24, 2024, 12:16:31 pm »
For that kind of voltage span, 50V to 40kV a passive method is not possible, because resistors will create a large voltage drop or the power dissipation will be too high.
Maybe they can help https://saker-mv.com/voltage-detector-mvd30-mvd15/
Thanks, that's a great tip. Maybe they can do a higher-voltage model or can point me to one.
 

Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #7 on: February 24, 2024, 12:31:59 pm »
What's going to discharge the output capacitances down to <50V? The PSU? - one for tooki to check. I'm not saying a 40kV resistive divider is easy, but it is doable with proper insulation and layout. For a few 10s of uA dissipation isn't a problem, eg. 800mW @20uA. If it turns out that a discharge load is needed anyway to bring the voltage down in a sensible time, then you may as well make use of it.
The power supply was ordered with the "quick discharge" option ("discharges to 10% of Vout within 2 seconds"), which does discharge any external capacitance as well, but of course in more time.

The 400Mohm discharge and meter circuit described above should be able to discharge the cable alone to essentially zero in well under a second, and will help discharge the PSU's output cap as well, so between the two it should discharge fully within a second or so, plus I like having the two redundant discharge circuits.

My current plan is to use four 100Mohm, 10W resistors in series (something from Ohmite, I don't have the model handy), mounted on solder eyelets screwed to nylon standoffs screwed into a thick plastic baseplate in threaded blind holes. Another branch will put one or two of those resistors in series with the output to the HV electrode to limit current in case of a discharge or arc. At around 8-10 cm between eyelets, this means the entire voltage is across a distance of around 35cm, far more than should be necessary for both clearance and creepage. There will be ample of both around the edges, too. I hadn't planned on potting or coating, since there's ample room in the setup to just keep things physically separated.

The setup is a big aluminum box that will be grounded. The internal wire to the HV electrode is Daburn #2477/22-50, a 50kV wire that can handle an operating temperature of up to 250C (since the electrodes must be heated).
 

Offline Wolfram

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #8 on: February 24, 2024, 12:46:49 pm »
If you only need to detect dangerous voltage, and not accurately measure it, you can use a lower divider ratio than planned, and clamp the measurement voltage under normal operating conditions. This means you can make your threshold voltage some hundreds of millivolts for example, instead of a few millivolts. This will greatly simplify the task of designing a threshold detector after the divider.
 
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Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #9 on: February 24, 2024, 12:52:42 pm »
HV resistors are not that expensive. RS Pro 500Mohm rated for 20KV:
https://uk.rs-online.com/web/p/through-hole-resistors/1754074?gb=s

Use 4 of them to make 2Gohm 80KV + a final 200K sensing resistor. (1KV=100mV).
2Gohm load (20uA @ 40KV, 800mW), giving a 10000:1 ratio or 4V ouput for 40KV input. Adjust the final ratio as required.

Feed this to a  instrumentation op-amp/comparator, these usually have GigaOhm input impedances.
It doesn't need to come out for anything, just put it in a earthed metal enclosure. Use a small secondary psu for running the detection circuitry and turning the led on/off.
This is sorta what I was envisioning/wondering about. But from what I gather, I would need to choose components fairly carefully to ensure it's "clean" given the tiny output voltage we'd be comparing, right?

There will already be a 24V PSU to power the safety relay and the PID temperature controller that heats the electrodes. That could be used to power detection circuitry and the beacon lamp.

(Cost isn't a concern, really. I'm certainly not gonna dick around with unknown-quality RS Pro crap when I can get Ohmite for a few bucks a piece... Heck, getting that 50kV Daburn wire ended up costing something like $200 after shipping and customs, for 12 feet of it. The electrode housing alone is made of hundreds of $ of PEEK plastic.)

Make the circuit redundand just in case, but the interlock should be the true safety mechanism, disconnecting the HV supply and applying a bleeding resistor unless the HV PSU automatically does this when off.
Agreed. As mentioned above, the PSU has the quick discharge option (which does operate passively if mains voltage is lost), and since our actual load shouldn't have any current flow at all -- there should be no current flowing between the electrodes -- I can afford to leave my own discharge/meter circuit (also described above) connected at all times.

I did float the idea of having a mechanically-operated crowbar circuit (some kind of mechanical linkage to the device door, such that when open, the HV output is shorted to ground via a bleeder resistor) but our safety official said that was overkill.

As long as everything is referenced to ground (Protective earth), it shouldn't be dangerous.
It will be, for sure!

The reactor will be housed in a heavy aluminum enclosure with a polycarbonate front door. All exposed metal will be connected to PE. This enclosure will be bolted to the back of the fume hood. The enclosure's door will have an end switch going to the safety relay. The fume hood's existing door slider sensors will also interface to the safety relay. The safety relay in turn will interface with the PSU's interlock circuit, as well as switching the PSU's 230V supply via a contactor with monitored contacts.

The power supply's output cable's outer conductor is PE, but the PSU has an additional binding post for PE which I intend to use, plus my old boss (one building down in a different department, so he's come to look in person) suggested a solid PE connection within the fume hood. I can easily use the ground pin of an electrical socket within the fume hood (which is where the HV power supply will be plugged in), but he said that if possible, I should PE directly to a PE bus bar. I'm looking into whether I can gain access to one.

Dip everything in HV silicone, perform degassing if required, should perform great.
How important is that, given the clearance/creepage distances described in the prior reply?

Bear in mind that as it stands now, the electrodes are only about 5mm apart, so it's essentially a spark gap far, far smaller than any of the clearances.
 

Offline NiHaoMike

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #10 on: February 24, 2024, 12:54:35 pm »
Use a diac and capacitor to flash the LED using the current from the sense resistor, that way it would remain visible even as the current drops to very low levels.
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Online jpanhalt

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #11 on: February 24, 2024, 12:57:18 pm »
I am not into HV, but when you mention using a light to indicate safe, be sure to use two lights (one safe;other unsafe).  If you have only one light, "on" should be safe; off unsafe.  That's probably obvious, but maybe is worth repeating.
 
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Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #12 on: February 24, 2024, 01:04:56 pm »
If you only need to detect dangerous voltage, and not accurately measure it, you can use a lower divider ratio than planned, and clamp the measurement voltage under normal operating conditions. This means you can make your threshold voltage some hundreds of millivolts for example, instead of a few millivolts. This will greatly simplify the task of designing a threshold detector after the divider.
Indeed, there are already two voltmeters planned: the digital readout of the PSU, and the analog meter I plan to put directly in the enclosure. The idea of the lamp is indeed an "idiot light", e.g. red lamp = dangerous voltage present, green lamp = voltage is safe to touch.

I don't have enough experience with clamping circuits running on tiny currents.

Do you mean something like this? Suppose a threshold of 0.5V. So for Vthr=0.5V when Vin = 50V, I'd need a 1:100 divider. That in turn means that at 40kV, Vthr=400V. So then you'd have a diode clamp or something to limit that to e.g. 5V for a comparator? 

The PSU is capable of 1mA, but of course I'd rather run it with as low a current limit as possible. Let's suppose 100uA. How would that affect the clamping?
« Last Edit: March 07, 2024, 06:29:58 pm by tooki »
 

Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #13 on: February 24, 2024, 01:05:48 pm »
Use a diac and capacitor to flash the LED using the current from the sense resistor, that way it would remain visible even as the current drops to very low levels.
How would that work? I have absolutely zero experience with diacs.
 

Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #14 on: February 24, 2024, 01:10:35 pm »
The power supply's own HV output on/off button illuminates exactly as I want, but I'm not opening and modifying an expensive, dangerous HV power supply without their say-so.
Or just a phototransistor reading this!
But this probably isn't a real HV indicator, but simply turned on by the driving circuitry, assuming there will be HV output unless something is wrong.
Indeed, it's simply the "output enable" status. Of all the signals/indications currently available, without me actually building my own circuits, this is the closest to the users' desired function as possible.

But of course then they could just glance over at the HV PSU itself and look at the button directly, since it'll be right next to the fume hood. Given changing ambient light (and the theoretical failure of that lamp itself!) I'm not sure that optically monitoring it is a good idea. (From everything I've seen, this is why real-world light curtains and free-space optocouplers always use a carrier frequency, just like IR remote controls.)
 

Offline tookiTopic starter

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #15 on: February 24, 2024, 01:20:34 pm »
I am not into HV, but when you mention using a light to indicate safe, be sure to use two lights (one safe;other unsafe).  If you have only one light, "on" should be safe; off unsafe.  That's probably obvious, but maybe is worth repeating.
I did think that, but thank you -- it's definitely worth repeating when safety is involved.

As I see it, the most essential component of the safety design is the discharge circuit that ensures that when the fume hood slider is opened, opening the interlock, the voltage will discharge to safe levels by the time you manage to open the device enclosure. That ensures it's safe even if something has gone wrong within the enclosure, allowing exposed wiring or electrodes. Normally, once the enclosure is opened, you'd still need to unscrew the entire reactor assembly within (which takes way more than a few seconds) until the electrodes are exposed. Within that amount of time, either one of the two discharge circuits (the one in the PSU, and my discharge/meter circuit) alone would have long since discharged it to zero.

In a way I know that a lot of this is belt-and-suspenders design. But frankly, I don't want anyone to get injured or killed with something I built, even if someone else is legally responsible, and even if a user does something really, really dumb. So even though I've been told that training is part of the safety design, I want it to be as inherently safe as possible.

So I'm a bit skeptical of relying on DIY monitoring circuits that could potentially fail, creating false safety.
 

Offline tggzzz

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #16 on: February 24, 2024, 03:59:06 pm »
As I see it, the most essential component of the safety design is the discharge circuit that ensures that when the fume hood slider is opened, opening the interlock, the voltage will discharge to safe levels by the time you manage to open the device enclosure.

Making sure it is very difficult to defeat the interlock is too important :( That's one advantage of moulded mains plugs over plugs that can be dismantled. (I have an example of that which I discovered on a scope. You can guess what a previous user had done.)

Interesting thread :)
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Online joeqsmith

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #17 on: February 24, 2024, 04:14:58 pm »
1mA DC max, 40kV max, 150pF added capacitance.  Well under 0.2J.  You never supply a PN# for the supply.  I assume you have checked with the manufacture about their internal filter and are working at much higher levels.   I also assume you have followed some standard like the following to asses the risk.   

https://www.standards.doe.gov/standards-documents/1000/1092-BHdbk-2013

There are products approved by safety agencies.   If your design injures someone and you have chosen to make your own safety devices which have not been certified,  you may be on the hook. 

Offline mag_therm

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #18 on: February 24, 2024, 04:43:39 pm »
Yes is likely that in the EU lab workplace, there will be regulations for this.
There will surely be IEC or Swiss standards that Tooki can search for  "Proving" safe voltage ...in R&D research lab
In some jurisdictions, I read that the legal definition of high voltage equipment is now defined by  bounds of [ kV, s/c A, stored mJ ]

There is the USA OSHA "Lab Safety Guidance" which is not in itself regulatory.
However, it points to the various ordinary workplace safety rules including electrical, which are mandated in labs, including employee training and qualification.

In my experience (now dated) HV access permits reqired a time limited document with the procedure, signed on and off by all workers who were accessing.
The procedure involved a group master padlock box, isolation lockout sub procedure, followed by use of  approved and calibrated live line tester. (On a long pole)
Then before worker access, a phase shorting/grounding bar was connected to the HV parts, with sufficient rating to clear the incoming breaker.
And with DC:
Capacitors and cables with high performance dielectrics that have been polarized for a long time can have a charge "rebound" if left open circuit after discharge.
I did not find ref to this today. With the oil dielectric capacitors I was involved with, they had to be shorted with lock wire during storage and after removal from service.
 

Offline David Hess

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #19 on: February 24, 2024, 05:27:06 pm »
Use a diac and capacitor to flash the LED using the current from the sense resistor, that way it would remain visible even as the current drops to very low levels.

Old high voltage stuff sometimes used the same concept with neon bulbs acting as the DIAC.  Capacitive discharge through the bulb gives good intensity where microamps would not.  This is probably what I would do although it does not indicate voltages below the neon bulb's strike voltage.
 

Offline bdunham7

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #20 on: February 24, 2024, 05:32:54 pm »
I don't have enough experience with clamping circuits running on tiny currents.

Either learn or sub that part out.   :)

When I have issues like this my way of thinking starts with "how could I do that using regular lab (test) equipment" and then that helps me think of a way to design an actual specialized circuit.  I can think of three ways of doing it that would all work quite reliably and I can demonstrate them all with regular stuff I have on my bench.

1.  Use a regular 1G 40kV voltage 1000X divider probe (e.g. Fluke 80K-40) and a 5.5-digit or better DMM set to the 100V range.  You'll be able to clearly see the voltage with a resolution of 1V, so 50V is very easy to spot.

2.  Use a Picoammeter, set to (as an example) 100pA full range and a 500G resistor in series.  50V will show full-scale, but beyond that the meter will be pegged (I have an analog Keithley 414A) but that won't bother it due to the nature of picoammeter amplifier designs.

3.  Use a regular handheld DMM set to the 50mV or 100mV range and the same 1G 40kV 1000X divider, you'll get a 50mV indication at 50V and for higher voltages it will indicate overload.

The last one utilizes simple clamping with a high-imedance input amplifier and a large resistance in series, which is probably the easiest reliable method to implement.   However, I'd suggest at least setting up the first method for testing if for no other reason than to verify that your discharge circuits are working as rapidly as you expect them to.  An exponential decay from 40kV can have a long tail if your lower threshold is 50V, especially if you pick up some extra capacitance unexpectedly.
« Last Edit: February 24, 2024, 09:13:21 pm by bdunham7 »
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Zero999

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #21 on: February 24, 2024, 05:49:22 pm »
Use a diac and capacitor to flash the LED using the current from the sense resistor, that way it would remain visible even as the current drops to very low levels.
I was going to suggest the same thing. The frequency will also increase with the voltage. The only thing is the sense resistor must pass more current than the leakage of the DIAC, but that shouldn't be an issue.

I would also use more than one, so there's redundancy.
 

Offline D Straney

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #22 on: February 24, 2024, 05:54:07 pm »
Another simple way to look at low voltages while blocking high voltages, stolen from common gate-driver-desaturation-detector circuits, is to have an HV-capable diode or diode string with its cathode at the output voltage, and its anode with a pull-up resistor to a lower DC supply.  When high voltage is present on the output, the diodes block in reverse and the anode is at the lower DC supply.  When the output voltage gets low, the diodes conduct and the anode voltage will be (Vout + Vf).  Vf can be in the 10s of V in this case, but if you're not relying on a precise measurement and you find the right diodes it might do what you want.  Maybe stick a large-value fusible HV resistor (value < pull-up resistor / 10) in series with the diode(s) to limit current just in case something arcs over.

Online DavidAlfa

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #23 on: February 24, 2024, 06:08:03 pm »
Paper about Non-Contact DC Electric Field Sensors:

https://www.osti.gov/servlets/purl/971778
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Offline jonpaul

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Re: "Proving" safe voltage (detecting a very small % of a high voltage)
« Reply #24 on: February 24, 2024, 08:50:47 pm »
neon bulb, connect 1 end to HV or divider with seriers R.

Leave other end disconnected.

E filed will lihgt it when HV is ON.

Check power lineman's non-contacting HV detectors.
 
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Jean-Paul  the Internet Dinosaur
 


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