Electronics > Projects, Designs, and Technical Stuff
DIY High voltage probe - safety concerns
floobydust:
I'm using Ohmite Slim-Mox SM108 (20kV) HV resistors, they are low cost and small. I also use the cheap chinese red HV resistors, like 5GIG 10W 100mm, but their coating is paint thin. No comp cap for DC work.
sourcecharge:
--- Quote from: AlienRelics on October 30, 2019, 03:27:22 am ---I don't think I'd use PVC. PVC absorbs moisture. I recall long ago seeing in something like 73, CQ, or some other ham radio magazine that coil forms made from PVC tended to arc across the surface and form carbonized tracks.
--- End quote ---
https://omnexus.specialchem.com/polymer-properties/properties/water-absorption-24-hours#PM-PP
Water Absorption 24 hours
Absorption Properties of Polymer
Water Absorption Values of Several Plastics
Polymer Name-----------------------------------------------------------Min Value (°C) Max Value (°C)
PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced-----------------------0.01 0.20
PVC, Plasticized----------------------------------------------------------------0.20 1.00
PVC Rigid-----------------------------------------------------------------------0.04 0.40
and as a comparison:
PTFE - Polytetrafluoroethylene------------------------------------------------0.005 0.010
PI - Polyimide------------------------------------------------------------------1.34 1.43
PETG - Polyethylene Terephthalate Glycol------------------------------------0.10 0.10
ABS - Acrylonitrile Butadiene Styrene----------------------------------------0.05 1.80
ABS Flame Retardant----------------------------------------------------------0.10 0.80
ABS High Heat-----------------------------------------------------------------0.10 0.80
ABS High Impact-------------------------------------------------------------- 0.10 0.80
(No PLA listed)
Well the plasticized PVC "Water Absorption Values" seems to be much higher than most plastics, but the rigid PVC look pretty good.
I also used super corona dope on the resistor chain, and electrical tape all around the outside of the PVC pipe.
Unfortunately, it's been so long since I used this at high voltage, I would verify it was working correctly at high voltages before touching it with a hot circuit.
Siwastaja:
Putting resistors in series is the way to go. I would use ones with axial leads, so much easier to deal with than the ones with the leads coming out sideways.
Use physically large enough, brand name resistors so that you can trust their ratings are real. Derate, add your own margin - then put in series to build up the full rating.
PVC heat shrink tubing is your friend. Ones with hot melt glue exist, sealing the connections. Use several layers of tubing to get the physical thickness. Then you can add a 3D printed case for decorative purposes, but I wouldn't trust it as a reliable insulator.
The idea of putting resistors in series is you will create a physically long chain. If you divide your 16kV to, say, the length of 32cm, you'll only have 500V/cm, which is much easier to deal with than, say, several kV per cm you get with optimistically rated single HV resistors. High voltage gradients require much more understanding of insulation materials: avoiding trapped air to avoid corona discharge carbonizing the materials slowly, avoiding materials that absorb moisture, and so on.
Example:
HVA12FA50M0 is 0.37€ at Digikey; 20pcs is 7.40€
https://www.seielect.com/catalog/sei-hva.pdf
20 in series is 1 GOhm, total 160kV(!) rated working voltage.
Body length is 11mm, voltage per resistor is 16kV/20 = 800V, voltage gradient is 727 V/cm. Creepage distance is actually a bit longer.
Allowing 7mm for soldering the leads, total solution is 360mm in length.
Gyro:
--- Quote from: Siwastaja on October 30, 2019, 07:01:33 am ---...
PVC heat shrink tubing is your friend. Ones with hot melt glue exist, sealing the connections. Use several layers of tubing to get the physical thickness. Then you can add a 3D printed case for decorative purposes, but I wouldn't trust it as a reliable insulator
....
--- End quote ---
Not necessarily. In my Picoammeter experiments I've found heatshrink to be surprisingly leaky, even at low voltages...
https://www.eevblog.com/forum/projects/picoammeter-design/msg790280/#msg790280
At the very least, I would say that not all heatshrinks are created equal and that a sample of the one you intend to use should be specifically tested for resistivity.
tooki:
--- Quote from: sourcecharge on October 30, 2019, 04:40:47 am ---https://omnexus.specialchem.com/polymer-properties/properties/water-absorption-24-hours#PM-PP
Water Absorption 24 hours
Absorption Properties of Polymer
Water Absorption Values of Several Plastics
Polymer Name-----------------------------------------------------------Min Value (°C) Max Value (°C)
PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced-----------------------0.01 0.20
PVC, Plasticized----------------------------------------------------------------0.20 1.00
PVC Rigid-----------------------------------------------------------------------0.04 0.40
and as a comparison:
PTFE - Polytetrafluoroethylene------------------------------------------------0.005 0.010
PI - Polyimide------------------------------------------------------------------1.34 1.43
PETG - Polyethylene Terephthalate Glycol------------------------------------0.10 0.10
ABS - Acrylonitrile Butadiene Styrene----------------------------------------0.05 1.80
ABS Flame Retardant----------------------------------------------------------0.10 0.80
ABS High Heat-----------------------------------------------------------------0.10 0.80
ABS High Impact-------------------------------------------------------------- 0.10 0.80
(No PLA listed)
Well the plasticized PVC "Water Absorption Values" seems to be much higher than most plastics, but the rigid PVC look pretty good.
--- End quote ---
Well, is that surprising, given that we use PVC for water pipes? It wouldn't really work if it absorbed tons of water, I would think.
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