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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I was digging through my supplies, demonstrating some of my equipment to a friend. I have a 20kv DC voltage probe. I also have an ignition 1:10,000 probe and a 1:1000 voltage probe that I made in the past.
So I'm running my 15kv neon transformer and I want to show the voltage on any kind of meter. I used a high voltage diode/capacitor and I was able to measure the DC voltage from the transformer. So, I have one way.
I wanted to use my oscilloscope, and here come the question. Did I do the right thing here:
I grounded one end of the neon transfromer. I connected my 1:1000 divider probe onto the other end/ground and put my oscilloscope probe on the divider's hot end. I connected the oscilloscope probe (x10) to the other end of the transformer, which was connected to ground. I went as high as 2kv on the transformer, using a variac to control the input. The oscilloscope saw 2vac coming into it (one 0.2v division).
Is this acceptable, meaning I won't blow my oscilloscope? One end of the neon transformer had to go through a 1:1000 divider. The other end was grounded.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Neon sign transformers are center tap grounded. I think you goofed there and your variac felt excess current. is this why you stopped at 2kvac? or am i not understanding your question?
In any case, attach the probe's ground lead as close, electrically and mechanically as well as possible (NEVER LET IT COME LOOSE, EVER) to the HV generating devices ground lead. If you do it right your O-scope wont get killed so easy.
In your case, and in all of my cases, i had the ground lead of probe on the NST case screw, where I know inside there is a center tap wire held. Then your free to attach the HV end of the probe to various parts of HV circuits. Keep your BNC/O-scope grounded well too, ideally to the local ground. Take care to avoid ground loops as with any measurement system.
Somewhere here on the forum i have an example, both single and double ended NST measurments with my circuit and USB o-scope.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The way high voltages were measured in power stations is with a potential transformer (PT) that reduces the nameplate voltage to 120 volts. The meter is calibrated considering the ratio of the PT.
But a NST operates as a constant current transformer when connected to the glow tubes. The secondary voltage drops to about 50 % of the open circuit value. Open circuit voltage waveforms may not represent the current wave of the primary.
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The right way to measure high voltage
