HV Meter tutorial
Posted: Tue Apr 23, 2002 2:13 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
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