"simple" 0-50 kilo volt meter ? - Page 1
Forum section: electronics / projects

author: kyndal
author_rank: Regular Contributor
timestamp: December 03, 2012, 10:01:29 am
content: before i ask the question id like to assure everybody.that i am perfectly aware that working with such high voltage, especiallywith a large capacitor is very dangerous i forget how many joules i figured a good shockwould discharge across my center body mass... but it was plentySoone of the steps for me to avoid said cause of instant death with my latest project is toadd a dedicated KV meter to my "air gap flash"my plan is to build the whole unit inside a plexiglass case.. and i would like to add adedicated KV meter inside the box to show me the voltage across the pulse capacitor / charging circuitbut im not sure how to best / simplest accomplice this..my plan so far is an analog or digital voltmeter, and a large voltage divider circuitcomposed of multiple xxMohm High voltage taper wound resistors (to avoid flashovers).1:1000.dropping my 0-50kv desired range to a more reasonable 0-50v for measuring?since the whole measuring circuit and indicator will remain inside the "safe box" im not too concernedabout cat ratings etc.. i wont be anywhere near the thingjust dont want it to blow up eitherso ya.. im thinking an analog or even one of those 8 segment ebay voltmetersand a large voltage divider ?precision requirements are pretty low.. i just want to know if there is 35.000 volts across the cap+/- 10% or somethingany experienced HV guys on here with a good proven design ?, idea?, suggestions?/Kyndal

author: vk6zgo
author_rank: Super Contributor
timestamp: December 03, 2012, 12:43:57 pm
content: Such things are made,& used,in Radio & TV Transmitters,although If I remember correctly,the highest rated one I've actually seen had an FSD of about 15kV!I'd go with alargeanalog meter & a really large high resistance series resistor network.I think you may be able pick up surplus analog meters movements with a bit higher FSD than 50V,which would save a bit on the series R.Be very cautious about the plexiglass case,as butt connections of this stuff may break down.Your joins may need to have an increased tracking path.

author: alanb
author_rank: Frequent Contributor
timestamp: December 03, 2012, 02:30:25 pm
content: It may be worth investigating Electrostatic voltmeters. These are designed for very high voltages and are non-contact devices and do not use series resistors.Try googling Electrostatic Voltmeters.Alan

author: kyndal
author_rank: Regular Contributor
timestamp: December 03, 2012, 07:02:19 pm
content: staring blindly at my various resistor divider network ideas. i never thought about it.but a suggestion was made to just measure the current through a 1GOhm resistor with a uA meter.so say at 50.000 volts 1Gohm the uA meter should read 50uA (full scale)and the resistor will dissipate 2.5w while in circuitmaby use a resistor like this ohmiteMOX94021007FTE 1 GOhms 1% 10W 45kvhttp://ca.mouser.com/ProductDetail/Ohmite/MOX94021007FTE/?qs=%2fha2pyFaduiys0xz3Gjg2kW78mP07N8wM7mGUeF26qI%3dhttp://www.ebay.com/itm/50-A-DC-AMP-Analog-microampere-Panel-Meter-Current-Ammeter-0-50-A-/130802672553?pt=LH_DefaultDomain_0&hash=item1e747263a9wouldn't bet a the farm on the precision of this thing.but should indicate within my precision requirements ?any idea why this won't work?/Kyndal

author: SeanB
author_rank: Super Contributor
timestamp: December 03, 2012, 07:15:07 pm
content: 45kV rating when encapsulated or in very dry oil.......... You probably will want to use 5 lower value ones in series, and after soldering them together degrease and then pot in a long acrylic tube with a high voltage clear epoxy.

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2012, 07:23:05 pm
content: Be aware that the charge stored in high voltage generator/capacitors is typically very small, and using a resistor divider network to feed a voltmeter may simply end up being a discharge resistor that drains the charge away in moments. So indeed you may need some kind of electrostatic non-contact measuring device to avoid bleeding off the accumulated charge.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 03, 2012, 09:44:34 pm
content: All the measuring of HV I have done were with regular meters and probes that step down the voltage and you just multiply the reading to get the HV values. One probe I used was designed for doing CRT repairs and stepped down 40KV to 40V then you just multiply x1000. Not very accurate but it worked. Really the hardest part is sourcing the parts designed for the volts you need without breaking the bank.A resistor rated for the voltage can cost as much as $50 so it might be cheaper to just buy a HV probe and take the parts out.

author: kyndal
author_rank: Regular Contributor
timestamp: December 04, 2012, 04:37:19 am
content: according to the datasheet.http://www.ohmite.com/cat/res_supermox.pdfthe resistor is rated for 45kv.. i dont see anything about putting it oil or epoxy?was going to mount it free standingthe Resistor is about 20$ from mouser.i know thet my 35kv pulse capacitor only holds 0.03uF so i was thinking about maby swiching in / outthe meter to check the that the circuit is charging properly etc.?will also have a "safe" shorted position. possible with a discharge resistormaby a Ghm resistor voltage divider will dissipate less powerid hate to spend 100$on a couple of resistors though

author: SeanB
author_rank: Super Contributor
timestamp: December 04, 2012, 04:00:47 pm
content: Specs are for a perfectly clean resistor. Leave it in the air, let it get humid, get a film of dust on it and it will arc over. Only way to keep it clean is in a hermetically sealed environment. Easiest way is to either fill the box with dielectric oil or to pot in epoxy.

author: IanB
author_rank: Super Contributor
timestamp: December 04, 2012, 09:30:21 pm
content: Quote from: kyndal on December 04, 2012, 04:37:19 amJust to give you an idea about kV, the insulators in this picture are isolating 25 kV:It's hard to tell exactly, but it looks like they are several inches long.Now those insulators are sized with a decent margin of course, but that should give you a feeling for the wanderlust of electricity in the 20-50 kV range. It really doesn't like to stay where it is confined and will leak away and escape or jump to ground given the merest opportunity to escape. So if you are attempting to hold and store 35 kV, let alone measure it, you will have to pay considerable attention to leakage paths including corona discharge in free air.

author: Neilm
author_rank: Super Contributor
timestamp: December 04, 2012, 10:13:46 pm
content: Quote from: SeanB on December 04, 2012, 04:00:47 pmWithout doing the calculations, it looks OK. I have seen smaller resistors mounted in a box used as calibration tests. These were reasonably stable up to the rated voltage (5kV). One thing that does raise a concern is that the Voltage Co-efficient seems a bit vague. For instance, it says it is 0.1 on the 1k-2G5 range but it does not give units. It that ppm or percent?The real problem is contamination if you have a very high resistance (>100G). This then provides a leakage path that is a resistor in parallel which will cause inaccuracies in the reading. If you have similar issues, wrap a couple of turns of wire around the resistor near the cold end. Connect this to 0V and you will guard out this leakage. Leakage across a resistor could easily be very significant. Remember, if you want good accuracy to use cotton gloves and wipe the outside of the resistor with IPA to clean it.Also remember that at these voltages, every thing is a conductor. I had issues once when I had 10kV one end of a 3 inch plastic pillar and an op amp the other end. There was enough conduction up the pillar to introduce a significant offset in the readings. The only way I had to fix it was to ground the top of the plastic pillar.Neil

author: kyndal
author_rank: Regular Contributor
timestamp: December 04, 2012, 10:49:22 pm
content: the resistor is 77mm long.. minus a bit for the endsso it should not flash over at the rated 45kv.i suppose i can make a small cylinder full of Dielectric oil to hold the resistor..to keep down leakage.and the insulators in the picture.. are meant to insulate still when covered in rain, snow etc) i dont think im going to "all weather" rate my air gap flash

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2012, 12:21:29 am
content: No doubt you have done these calculations, but for my own amusement this is what I did.Charge stored by a 0.03 uF capacitor at 30 kV:Q = CV = 0.03e-6 * 30e3 = 0.0009 coulombsCurrent in a 1 Gohm resistor at 30 kV:I = 30e3 / 1e9 = 30 uADischarge time constant of capacitor at 30 uA:t = Q / I = 0.0009 / 30e-6 = 30 sSo you would not be able to leave the voltmeter connected permanently unless the capacitor was under continuous charge. It would drain too quickly.

author: kyndal
author_rank: Regular Contributor
timestamp: December 05, 2012, 02:18:01 am
content: Thanks IanB good looking math.i didn't do the math quite like that. but i figured it would be something in that order..as i know I'm discharging 2.5w through the resistor...from the capacitor / Charger (at 50kv)the capacitor will be under continuous charge "idling". I'm not sure of the charger yet though2.5W might be too much additional load. "reload speed" for my flash isnt all that criticaleach shot takes some time to set up anyway.i really just need an indication of weather the system is working / charging correctlyid measure the peak and leave it to charge back up before discharging..probably making a "blade" switch of sorts.should make for a nice discharging "safety" circuit aswell i suppose ;o)currently looking into some of the no contact ideas.../Kyndal

author: ptricks
author_rank: Frequent Contributor
timestamp: December 05, 2012, 01:14:35 pm
content: Reading up on some HV measurement methods I came across a no contact method that appears to be safe as well. The idea is to build a high gain volt meter. The measurement lead for the meter is a wire wrapped many times around the insulated wire you are wanting to measure. The idea is that even the best insulators still leak some voltage, an insulator that leaks just a little could even be chosen for the purpose. With enough gain and shielding to protect against outside sources it is possible to measure the voltage at any level through the insulator while not providing a path to drain the voltage or to provide a target for voltages arcing.

author: alanb
author_rank: Frequent Contributor
timestamp: December 05, 2012, 02:34:54 pm
content: Ptricks - are you able to provide any references to the method you have read about, I’m very interested.If there is leakage through the insulator it sounds as if it’s a conventional technology voltmeter with the insulation of the wire providing part of a very high series resistance. If there is no leakage and you are relying on the electric field then you have a form of electrostatic voltmeter.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 05, 2012, 09:19:45 pm
content: Quote from: alanb on December 05, 2012, 02:34:54 pmI was reading through some old issues of the CQ magazine and will need to see if I can find the article and will post the info as soon as I can. I think it was from 1990-1991.It definitely wasn't electrostatic because the author was talking about how he got the idea from devices that measure the breakdown voltages for insulators.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: December 06, 2012, 03:26:29 am
content: Hallo guys, first post here,Well, I have been thinking about it for a while and well... Practically thinking of what a contact-less method of measuring would beis placing capacitors as resistors for the voltage divider. Of course you are going to use capacitances as resistor values.I don´t know much about the dielectric materials at that range, but use one with the lowest "parallel" resistor and you should befine. And of course using capacitors of the same dielectric is heavily recommend.I don´t really think a resistor would be a good idea, Schrödinger´s cat the reason, plus the need for a very expensive component.And case you are afraid of tolerances, put caps in parallel, every four in parallel you halve the tolerance.Then with a fast sample and hold circuit you should be able to play safe and nice and maybealso with an appropriate bleed resistor for the lowside capacitor.That´s all from me, hope that helps :-)

author: robrenz
author_rank: Super Contributor
timestamp: December 06, 2012, 02:52:16 pm
content: Quote from: TrickyNekro on December 06, 2012, 03:26:29 amWelcome to the forum.How do you calculate the tolerance improvement in parallel capacitors? I never heard of that.

author: AlienRelics
author_rank: Unknown
timestamp: August 26, 2022, 03:31:56 pm
content: That's because it is not true.100pF +-10% is 90 to 110pF.Two in parallel is:90+90 = 180pF110+110 = 220pF200pF +- 20pF is still +-10%.

author: Warpspeed
author_rank: Contributor
timestamp: August 27, 2022, 01:00:03 am
content: Just buy a Radiospares 610-281 high voltage probe, either new or secondhand.40Kv, its safe, its calibrated, and it will work.How about this one on flea bay, brand new and only sixty dollars ?https://www.ebay.com/itm/123956804594?_trkparms=amclksrc%3DITM%26aid%3D1110018%26algo%3DHOMESPLICE.COMPLISTINGS%26ao%3D1%26asc%3D240657%26meid%3D5ae580141215466bbef87d507e7d99fc%26pid%3D101196%26rk%3D5%26rkt%3D12%26sd%3D393325078490%26itm%3D123956804594%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DPopularItemsWithSimpleRanker&_trksid=p2047675.c101196.m2219&amdata=cksum%3A1239568045945ae580141215466bbef87d507e7d99fc%7Cenc%3AAQAHAAAA8A7MsZABwLuaVPSMs%252FTholPj6Yl%252BKviZkjBwf4Ot%252FuQX58w6Ax3db0QRImOMryL25uvVO5RvUV7XbKbQhVLy9enmGjiwK3xvR7KOHRqAAyF7ncNyyeeizQn9ZBSOwgaJ6StAwgi5RVcN2h1LTZQh5gPnHTg6fiIqC4Z4nQZ6dXJQX4bi4L24oHpbCrIpXgJpOvInhaT039g2ZmCb88J%252FH%252F%252Bo5l6HX6jIZCCDqPc7BfqC7CPWeKIo9N8ctIyfmZdHfKc53UAZLFnle8KNoSO63XhCFh4IFd4xj762PjV%252Fi1rweGVscKIlDNnfIENXlNZOag%253D%253D%7Campid%3APL_CLK%7Cclp%3A2047675&epid=13045739344

author: joeqsmith
author_rank: Super Contributor
timestamp: August 27, 2022, 01:24:07 am
content: I think I used the MOX series in my larger home made high voltage probe. Be aware of their load life.My application is normally pulsed and I needed a decent frequency response so I use some magnet wire to form the capacitance. You can see what it looks like here, or search this website as I posted a few photos of it.

author: Marco
author_rank: Super Contributor
timestamp: August 27, 2022, 01:46:26 pm
content: Beckman HV-211 seem to be available pretty cheap.

author: cepsbow
author_rank: Newbie
timestamp: August 27, 2022, 04:04:30 pm
content: What he is describing is the basis of how megaohmmeters work. Any megaohmmeter could easily be converted to do the basis of these measurements, less any attenuation or gain factors that may need to be addressed.Quote from: ptricks on December 05, 2012, 01:14:35 pm

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2022, 08:03:58 pm
content: Quote from: kyndal on December 03, 2012, 07:02:19 pmThat is how I would do it, but it may require several resistors in series to get a safely high breakdown voltage.I have a Caddock MG815-3 right here which came out of a high voltage probe and is 999 megohms, 15 watts, 30 kilovolts, and 6 inches long. I checked its resistance by applying a voltage and measuring the current into the 10 megohm input of a digital voltmeter at 991.1 picoamps per volt.Mechanical considerations are important. A good insulator should be used to hold a string of high voltage resistors in place.

author: Warpspeed
author_rank: Contributor
timestamp: August 27, 2022, 09:48:02 pm
content: The mechanical part is probably the most difficult, and as a proper HV probe is not that expensive anyway, why even bother ?

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2022, 10:51:49 pm
content: Quote from: Warpspeed on August 27, 2022, 09:48:02 pmI think he wants the high voltage meter built in as part of a larger instrument.

author: Marco
author_rank: Super Contributor
timestamp: August 27, 2022, 10:54:26 pm
content: No reason you can't just clamp a hand held probe to use for that.