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Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
Lets say we have a 100 stage 1 MV divider with 1 uH self inductance with 10 nF capacitors per stage, for total capacitance of 100 pF. For critical damping the total series resistance has to be around 200 Ohm, so 2 Ohm per stage.
PS. hmm, this would be a huge load on something like a Tesla Coil though. With 100 pF per stage we would get 20 Ohm per stage ... which would in total burn 250 Watt for a 100 KHz 1 MV Tesla. Much more civilized.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
Lets say we have a 100 stage 1 MV divider with 1 uH self inductance with 10 nF capacitors per stage, for total capacitance of 100 pF. For critical damping the total series resistance has to be around 200 Ohm, so 2 Ohm per stage.
PS. hmm, this would be a huge load on something like a Tesla Coil though. With 100 pF per stage we would get 20 Ohm per stage ... which would burn 250 Watt for a 100 KHz 1 MV Tesla. Much more civilized.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
This might not come as a surprise to anyone else ... but it surprised me.
Something I never really realized about high resistance probes until I read about it and ran some numbers. Lets say you are measuring a tesla coil and halfway along the probe the ground plane is far away but the tesla coil is close by ... the capacitively coupled voltage between the middle of the tesla coil and the probe will tend to overwhelm anything connected to the top for higher frequencies.
The capacitance between two 1 meter wires at 1 meter is ~3 pf ... and 1 pF at 100 KHz represents an impedance of only 10 MOhm.
Even when a high voltage source isn't nearby, the fact that the stray capacitance to ground represents a much lower impedance path than the rest of the divider will screw up the measurement (although this will just cause a high frequency fall off, which can be compensated for).
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
This might not come as a surprise to anyone else ... but it surprised me.
Something I never really realized about high resistance probes until I read about it and ran some numbers. Lets say you are measuring a tesla coil and halfway along the probe the ground plane is far away but the tesla coil is close by ... the capacitively coupled voltage between the middle of the tesla coil and the probe will tend to overwhelm anything connected to the top for higher frequencies.
The capacitance between two 1 meter wires at 1 meter is ~3 pf ... and 1 pF at 100 KHz represents an impedance of only 10 MOhm.
Even when a high voltage source isn't nearby, the fact that the stray capacitance to ground represents a much lower impedance path than the rest of the divider will screw up the measurement (although this will just cause a high frequency fall off, which can be compensated for).
OH CRAP! i thought you realized that! other wise i would have explained it. read here :
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
Why not use a resistive divider inside a capacitively divided coaxial shield?
the tektronix and NorthStars' are essentially capacitors with coaxial HV and ground plates which shield the resistors, once my finals are done for this semster i will have 4 weeks, to see if i can build the same.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
Come to think of it, discrete elements are actually kind of nasty for a capacitive divider ...
An arts and craft stack of capacitors from say rings of aluminum foil and cardboard or foam would have very low self inductance, might not need series resistance at all.
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DIY High voltage probe
