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| High voltage probe and high voltage resistors |
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| TimFox:
I believe that some Caddock resistors are serpentine. |
| NiHaoMike:
Given that a high voltage probe is going to use resistors well into the megaohms, inductance is unlikely to cause any problems. Capacitance is going to be significant and that will have to be compensated for. Here's a video on DIY high voltage probes: |
| xzswq21:
--- Quote from: TimFox on July 31, 2021, 10:01:31 pm ---I would never use X, Y (such as Y5P), or Z dielectric ceramic capacitors in a measurement system, since they lose capacitance drastically with applied voltage. For ceramic, you need NP0 or C0G (basically synonymous). However, there are special capacitors for high voltage, such as reconstituted mica or high-voltage polypropylene film. You didn't give a country flag: to where would the freight charge get that high from Texas? --- End quote --- Before I come here, I had googled about high voltage capacitors, for low voltage filters I always use C0G capacitors but for high voltage capacitor I couldn't find anything in Digikey or Mouser except Y5P, N4700, Z5T, . I found several large size and unusual and very expensive capacitors. Can I use 1kV C0G capacitors in series? In theory there is a problem, for example, the voltage drops by 1kV per capacitor but there is 20kV and 19kV on the first capacitor! 20kV -||- 19kV -||- 18kV -||- 17kV -||- 16kV -||- .... I think I can't use 1kV capacitors I wanted to buy from Digikey.com, the shipping cost for other components is 30$ but for the Ohmite resistors is 150$. It's better for me to find the the components from other distributors. |
| PartialDischarge:
The inductance in hv helix resistors is not a problem due to the extremely low currents handled. The real reason for serpentine construction is to avoid magnetic field coupling from external sources into the resistor, this magnetic field is not filtered by most shield cans, specially at low freqs. Capacitors used for hv are tricky, as their rating should be set according to partial discharges, not Ac or dc rating. Capacitors in series do not distribute the voltage linearly unless compensated by resistors. This is a well studied problem in hv ceramic/glass insulators for distribution power lines |
| Gyro:
Here's a 10kV peak R-C attenuator I built for my Gay MTA Memory (peak capture) Voltmeter. I included as much detail as possible in the post, including how I chose parts and calculated the capacitances for the input impedance of the Voltmeter and connecting cable strays... https://www.eevblog.com/forum/testgear/high-voltage-x10-attenuator-for-my-gay-mta-memory-voltmeter/ An important note that this is not intended or suitable for high energy circuits, or to handle 10KV DC continuously (resistor wattage ratings). But it may help you in the construction of a higher voltage probe (more stages). Due to the nature of the termination and the input characteristics of the meter, I wasn't able to directly look at the square wave response on a scope - I was more interested in the overall frequency response of the probe and meter combination, so I had to check the roll-off with a signal generator. It made it to 5MHz though. It might be of some help anyway. |
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