High Voltage Probe with Voltage Devider

High Voltage Probe with Voltage Devider

Description

Starting with warnings: Dealing with high voltage is dangerous. So use this design on your own risk only! Only use this design if you can assess risks by your professional qualification. This design is not suitable for high voltage power grid application, because it is too short to provide sufficient safety distance. It is only suitable for high impedance sorces. Since I fond no information about the isolation properties of PLA, I assume PLA ist not suitable for isolation purpose. In consequence it is not suitable for this project. Beside the design provides high creepage distance on its surface, fdm prints tend to have fine fissures witch might pry out the isolation. Especially if the part is wetted with conductive dirt. Resistors have a separate voltage rating. The voltage they can withstand is not only limited by the power dissipation. They are commonly rated between 300 and 500V. That's why I'm using 30 resistors in series to achieve 10000V There are maybe more considerations are to be taken into account. Lets continue with known limitations. Limitations As explained above the practical usage is limited to 10000V non power grid usage. The extremely high (100MOhm) input resistance makes it very sensitive for emi (elector magnetic interference). The helical shape of the tunnel for the resistors may add to this problem. Measuring dc is quite reliable, but ac is a problem. Grounding the probe helps a little. Usage Inside the shaft is an helical tunnel to take a long series of resistors. The simplest usage is to install an 90MOhm series resistor to the 10MOhm (commonly) internal Resistor of voltmeter. This divides the readings by 10. An additional parallel Resistor can be installed to set other dividing factors. To minimize the influence of emi, the configuration as voltage divider is preferable, but you can leave the ground terminal floating to go back to use it as simple preresistor. To minimize emi, route the ground wire from the subject to the ground terminal of the probe first and than to the common terminal of your voltmeter. Assembly For installing the resistors inside the body of the shaft feed a folded nylon string through the helical cavity. If this doesn't work, use the vac to suck a thread through. Solder the resisters in series and pull the string of resistors through by gently pushing from the other side. I fond this surprisingly easy. Use shrinking tube to isolate the remaining resistors. Solder the first resistor to the tip. The tip can be made from 2mm diameter round stock. I used brass, because it is easy to solder and not that sensitive to corrosion. Complete the internal wiring like shown on the pictures. Use wires with dielectric strength of at least 1000V. The ones I used are too skinny. The plugs I used are certified for 1000V. To improve the isolation, fill the thread between the shaft and the handle with epoxy. Be care full (not only) when dealing with high voltage!

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