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Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040
you might consider getting an HV probe at some point - you can get them for a decent price on eBay, but I always see them dirt cheap at Ham Fests - I picked up a 35 kV meter for $20 at a hamfest here in Texas, and the one I went to yesterday had a couple as well
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Connecting a moving coil meter in series with a dropper resistor is not a good idea where high voltage is involved, and one that it is potentially very dangerous. If the meter movement goes short circuit, or the meter connection to earth goes open circuit, you will have a situation where high voltages will appear on your front panel.
The usual way of doing this is to make a 1000:1 or (or higher ratio) potential divider, so you have a string of 0G999 in series with a 1M measuring resistor across which you take your reading. If you intend to use a moving coil meter, you must connect it across the measuring resistor via a 99% source/kathode follower, to reduce the shunting loss of connecting your measuring instrument in parallel with the measuring resistor.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Or just stick a neon lamp or a couple of diodes across the meter. This whole thread isabout why you don't need to use a divider.
Meter protection in HV circuits is a serious matter, especially if they have capacitors storing a lot of energy. You should read ham websites that deal with metering in tube RF amplifiers for more details. People have been killed by touching live meters on broadcast transmitters, and if the protection circuit isn't up to scratch, a tube arc can blow the front of the meter off, shooting pieces of glass in your face.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
One thing about electrostatic voltmeters, which are just advanced electrometers (anyone remember pith ball electroscopes), is that the range is extended by connecting capacitors in series with them.
The one in the picture goes to 5000 volts. if it were extended to 25,000 volts, the result would be the same. It takes no current except when the value is changing just like any capacitor on DC.That instrument is no too ancient. It was last calibrated in 1988.
Metrology information in the electrical field has many great papers from the Bureau of Standards. Those folks spend years developing instruments for calibrating things.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello, what if I put it on the cathode end? (Cathode is grounded). I saw a website with guy who built a 120kV VM for an x-ray tube that had a mA meter on the cathode end. He claims this makes it safe. I dont see anything else besides the meter going from cathode to ground. Matt
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Radiotech: I don't believe that you can extend the range of an electrostatic voltmeter with capacitors. Maybe in theory, but in practice, everything has leakage, so the reading will wander away.
Matt: As I explained in my previous post, putting the meter in the cathode end is perfectly safe, until the tube arcs. The arc is a dead short, so the 120kV power supply ends up dumped straight across the meter, which instantly goes bang. Also, if the HV terminal arcs to ground rather than cathode, the cathode jumps 120kV below ground, taking your filament transformer out.
If you look at schematics for X-ray machines, RF power amps and the like, you'll see that a meter in the cathode circuit has a gas discharge tube, or a couple of hefty diodes, shunted across it right inside the HV tank.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Steve McConner wrote ...
Radiotech: I don't believe that you can extend the range of an electrostatic voltmeter with capacitors. Maybe in theory, but in practice, everything has leakage, so the reading will wander away.
Matt: As I explained in my previous post, putting the meter in the cathode end is perfectly safe, until the tube arcs. The arc is a dead short, so the 120kV power supply ends up dumped straight across the meter, which instantly goes bang. Also, if the HV terminal arcs to ground rather than cathode, the cathode jumps 120kV below ground, taking your filament transformer out.
If you look at schematics for X-ray machines, RF power amps and the like, you'll see that a meter in the cathode circuit has a gas discharge tube, or a couple of hefty diodes, shunted across it right inside the HV tank.
Hello, So you think a neon lamp would be okay for protection? I am quite certain that an arc may occur since I have seen it happen already. The HV+ will occasionally hit the lead shield and then to the cathode (bypassing the x-ray tube). This is with the lead wrapped directly on top of the tube. I am having trouble getting an enclosure planned for this tube. PVC pipe and fittings just isnt getting it. (sizes just arent working out). I may have to opt for lower voltages to keep from having the HV jump the tube. I like having the lead on the glass as it requires less lead to wrap and it acts somewhat as a heatsink.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Gas discarge tube (neon lights) are found everywhere HV arc-overs may happen-even in TV sets on the sockets assembly. As for protecting a sensitive meter, it wont protect the movement, what it will do is keep the circuit closed. If the meter coil opens it prevents a fire/arc in the meter. This limits damage because a wildcat arc generates lots of other circuit damage/problems.
In some old power distribution gear, the secondary of C/Ts had a large gas tube across them in case the loop opened when an instrument was withdrawn and the contact shunter didn't work.
Re: extending range of electrostatic meters: I agree on the leakage errors. My other e/s meter is a multiple range meter and I am going to try and photograph it, inside and out. It has terminals for the various voltage.
(The other thing about this technique with those meters is they can measure the dielectric constant at the voltage in question.) If I were to make a range multiplier for the 5000 volt meter, it would be exaclty the same form,(ie air dielectric) so whatever leakage the meter had due to air, so too would the multiplier.
The multi range meter was used for experiments making electrets.
[MOD EDIT] - Corrected Double Post Please refrain from double posting in the future. Thank you
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
radiotech wrote ...
If I were to make a range multiplier for the 5000 volt meter, it would be exaclty the same form,(ie air dielectric) so whatever leakage the meter had due to air, so too would the multiplier.
That won't work. (I'll leave you guys to debate why)
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so I got my circuit hooked up. (minus the x-ray tube for now). I have two 240Mohm resistors in parallel in mineral oil and the 1mA meter in series with the resistor and HV+ from the tripler (the other end of the meter goes to the circuit ground). I can get it up to 600uA before the resistor arcs over (the contacts that are out of the oil each have a 1in diameter brass sphere). The distance between the two brass spheres would also suggest im in the 60kV+ area. Does this sound reasonable?
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Measuring HV with mA meter?
