HV Meter tutorial

For posts specifically relating to fusor design, construction, and operation.
Post Reply
User avatar
Richard Hull
Moderator
Posts: 12434
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

HV Meter tutorial

Post by Richard Hull » Tue Apr 23, 2002 6:13 pm

First, Analog meters are +/- 5% at best!! PERIOD

Second, check all meters out prior to use.

Third, never use a meter to measure HV that is less sensitive than 100ua full scale. The following meter movements are OK 100ua (10,000 ohms/volt), 50ua (20,000 ohms/volt and 20ua (50,000 ohms/volt.

Fourth, Never use a plastic meter for HV work. All acceptable meters should have a bakelite case with glass face. Never mount a meter for HV work in a plastic panel.

Methodolgy

Testing:

100ua meter demands a good 1% 100kohm resistor for test.
50ua demands a 1% 100kohm resistor
20ua demands a 1% 1 megohm resisitor

Place the required resistor in series with its mated meter and a 0-20 vdc variable supply. Take the supply to 5 volts output. The 100 ua meter should read half scale (50ua), the 50ua meter should read full scale, the 20ua meter should read 1/4 scale (5ua). move the supply slowly over the range and see if the readings are smoothly moving between points and that the needle indicates proportional to the input voltages.

Once you are sure that the meter is reading correctly to within about 5% over the entire range and moving without sticking, you are ready to scale it for HV.

100ua meter full scale with 100 megohm resistor is 10kvdc, 200megohm is 20kvdc, etc.

50ua meter full scale with 100megohms is 5kvdc, 200 megohms is 10kvdc, etc.

20ua meter full scale with 100megohms is 2kvdc, 200megohms is 4kvdc, etc.
*******************************************************
Beware:

You must use special HV resistors and not a string of junky little carbon film resistors to make the meter resistor.

Always chassis and electrical ground the return lead of the meter be it positive or negative (depends on supply polarity desired).

To the non-grounded meter lug, attach the HV resistor and to the other end of the resistor, hook the HV hot lead. Any other arrangement can, if a failure occurs, leave the guts of the meter and its needle at full lethal potential!

Even with this safest arrangement, should the meter itself burn out, the guts can go high. However, the resistor is still there to limit the current to the human body to well under 100ua, even dead short to a person in a pool of water soaking wet.

A good rule of thumb, don't point to and touch the meter face of any HV meter while under power.

************************************************

Plastic meters may or may not accept and hold an electrostatic charge on their cases or faces from the HV and cause needle sticking problems! The same may be true of plastic or polymer meter panel mounting arraangements.

Analog meters are simple quick and easy to use once you pay attention to the above details. I have collected over 250 meter movements from hamfests over the last 25 years. very few are defective and prices range from $4.00-$15.00 each normally, untested, unwarranted, never see the guy again, so buyer beware. You can rest assured that if the meter has a face with a 0-10 kilovolt scale it is not a 0-10kv meter!! The actual movement value is usually way down to the extreme lower left or right of the face plate below the level of eyesight in a tiny printed legend like "F.S.50ua"

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

guest

Re: HV Meter tutorial

Post by guest » Tue Apr 23, 2002 6:22 pm

Thanx Richard:

I posted the note on meters to let the new people know about the rash of really junky stuff out on Ebay.
Beware the caselot. How I got so many dogs. Meters don't like rough handling at all. Ups is a crap shoot at
best on damage. I got a case of 45 meters. Paid about a dollar fifty a meter.
Only a few were hv. I managed to get a third of them running.
Makes hamfests look positively saintly.

Larry Leins
Physics Teacher

Tom Dressel
Posts: 287
Joined: Fri Jun 22, 2001 8:44 pm
Real name:

Re: HV Meter tutorial

Post by Tom Dressel » Wed Apr 24, 2002 1:02 am

Richard:

Why should you not mount a meter in a plastic panel?

My power supply consists of a 1/2" thick plexiglass box with 1/4" thick plexiglass baffels to prevent arc over and a 1/4" thick clear plexiglass face for the panel mount. I am using plastic meters from Radio Shack and running only 10 KV. It seems to work just fine.

Am I playing with an electricution hazard?

Tom Dressel

User avatar
Richard Hull
Moderator
Posts: 12434
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: HV Meter tutorial

Post by Richard Hull » Wed Apr 24, 2002 2:36 pm

Tom,

As I noted in my post. Some, NOT ALL plastic meters can become electrostatically charged and stick. Top brand, large panel meters by Simpson, Weston and others are specifically designed to NOT have any such problems. This is accomplished by careful selection and treatment of the plastics in a special manner. You can tell if you have a crummy plastic meter by rubbing it with a dry cotton or silk cloth vigorously on the face. If the needle floats upscale or moves about a lot then the meter is suseptable to static charging during HV useage.

Plastic panels can also pick up HV electrostatic image charges either directly from the wiring or from the metering itself. A plastic panel can also pick up charge triboelectrically by impact or rubbing by operating personnel. All of these can also create false readings or stuck needles. I see this stuff happen all the time.

Metal panels, firmly grounded to a driven electrical ground system with bakelite meter bodies are the safest bet for the most dangerous of HV systems. For ultrahigh voltages above 30kv only a specially engineered remote, isolated electronic system should be used.

Some of the 60s Farnsworth stuff can be seen with meters recessed in the hollow HV shrouds atop the fusor assemblies. This is a neat old trick (before electronic solutions) which required no heroics other than reading the things with a telescope while the system was hot and live. The ultimate isolation scheme!

A grounded panel also helps protect against shock hazards by hopefully arcing to the panel in a failure mode rather than a hand. Still, direct metering is always hazardous to a more of less limited degree, even if properly engineered.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: HV Meter Tutorial

Post by DaveC » Sun May 05, 2002 5:50 am

Couple of Safety points here.
1. When using analog meters for HV measurements, it is always a good idea to connect an MOV (Metal Oxide Varistor) across the meter terminals. The MOV is essentially an open circuit below its breakdown voltage and a reasonably low resistance to voltages above its breakdown value. This will protect you and the circuit in the event the meter coil opens. Use the lowest voltage you can find... A second choice would be a pair of nose to nose Zener diodes, again with the lowest possible voltage rating.. A third possibility is a two foward biased silicon diodes connected across the meter. Since the 50 uA meter movements are typically about 2K ohms, full scale deflection occurs with 50 uA x 2,000 volts or 100 mV. Below the 500 to 700 mV foward barrier voltage, silicon diodes have an exponential current versus voltage relationship, and thus will not conduct "much" current until you get a few tenths of a volt across the meter... then they conduct well and provide a solid shunt to ground.

2. If you have the opportunity, get meters that are labelled - "Taut Band" . These use a fine conducting band to suspend the meter coil, rather than the needle point shaft and bearings. They are much more robust and nearly immune to vibration damage.

3. Another good safety practice is to use a resistor in parallel with the analog meter to provide an alternate path in the event of a meter coil burnout. If you use a resistor value equal to the meter's resistance, this will halve the meter deflection for any given applied voltage.

3. I personally have not had any problems with static charging of plastic case meters beyond the usual effect that comes from wiping the face and statically charging it that way. But this occurs with bakelite case and glass face meters, too.

4. One caution I would add here, concerns the calibration of the meter with high voltage. Be aware of the effects of corona discharges from sharp points and other high stressed areas of the circuit. Corona discharges will usually alter the calibration, by providing other leakage paths to ground. Even with electronic metering - i.e. digital, or buffer amplier/ analog systems, corona leakage currents will change the voltage divider ratios as much as 100 % sometimes even more.

5. HV resistors are available from several sources, normally with accuracies to 1% and up to 100 Tera Ohms. Power and voltage ratings under oil are about 2 -3 times higher than in air. Carbon Composition resistors are usually conservatively rated at 250 volts for the 1/8th and 1/4 watt sizes. Under oil, you can usually bump this up substantially, but you should test the resistors you plan to use, to be sure.

Dave Cooper

User avatar
Richard Hull
Moderator
Posts: 12434
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: HV Meter Tutorial

Post by Richard Hull » Mon May 06, 2002 5:49 pm

Dave's points are all valid. Here are some further thoughts...

Never ever use the proper meter value for the job!!!

Some of you might find the perfect, ready to use 50ma or even 25ma current meter at a hamfest. DO NOT BUY THEM FOR FUSOR WORK!

You must use a 100ua or 50ua meter movement. This will force you to use a wirewound .5 to .05 ohm resistance to across the meter making it a totally safe system to use.

You will remember that the ground of the fusor case shouldbe connected through the current meter. If this meter were to blow in a proper 25ma meter, the entire case of the fusor could rise to lethal potentials! With a sturdy, hand made shunt out of nickel or nichrome wire, rigidly attached both physically and electrically, the 50ua meter can be destroyed and you still have a good solid ground.

Needless to say, never place the current meter in the hot or hv circuit. Place it only in the grounding circuit, ALWAYS!

Following the above advice, there is no way that the meter or any of its connections can rise above a fraction of a volt even if the meter burns out and catches fire!

The voltmeter must, unfortunately be across the supply and one leg will go to the hot lead via an HV resistor divider. To prevent corona, I just cover any exposed connection in the voltage divider resistor chain with hot glue or silicone HV putty to exclude air and corona points. This is all just part of good HV common sense technique.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Post Reply