The goal of this project is to make a simple HV probe to measure voltages for some up coming HV electronic projects.
The probe is based off the EHT Stick featured in the April 2010 Issue of Silicon Chip. Housed inside a PVC pipe, 80 x 10 Meg HV resistors and an adjustable 750K to 850K output section divide the voltage in 1000:1 ratio.
Due to lack of unclad prototyping board of suitable size available at Jaycar, I’ve chosen to construct the resistor network using point to point construction. The long string of 10M resistors will be wound up to fit them inside the PVC tubing and possibly potted.
The 10M resistors are 3.5kV rated Vishay - HVR37 series resistors. I didn’t notice they were larger 0411 (“1/2W”) size resistors when I ordered, smaller 0207 (“1/4W”) resistors would have been preferable. The output section is a regular 750k metal film resistor and a 100k 25 turn trim pot.
I bent a loop on the ends of each resistor
And chained them together
After 2 hours, alI 80x10M resistors + a 750K resistor chained together.
The chain is flexible and can be easily wound
I wound the chain around a 15mm PVC pipe fixing one end through a hole and blue taking the other end down. Winding the resistors in a coil will introduce inductance into the voltage division network but hopefully this does not reduce the bandwidth below the required ~100kHz.
The loops were soldered together with a generous amount of solder to form a ball are each joint which will hopefully limit potential corona discharge.
The loops had to be soldered upside down in order for gravity to pull the solder into a ball on the outside.
An attempt was made to clean off flux using the leaky can of isopropyl alchol in the drawer and a fresh 20cm section of 15mm PVC pipe cut and the resistor coil remounted onto it.
Trim pot and leads soldered on
Testing to ensure everything works. The lab power supply is calibrated to output 20V (some arbitary relatively high voltage) then the output of the divider measured and calibrated to ensure error is within calibration range.
Resistor cleaned and given a coat of Scotch 1601 insulative spray. The trim pot was fixed with a piece of double sided foam tape, this dodn’t work well and had to later be replaced.
After left, before right. Sanded down the end caps to have a flat surface to mount the terminal and cable glands
Terminal and cable glands mounted to end caps.
Terminal wired to input resistor. The wire and terminal sit inside the inner PVC pipe which insulates it from the rest of the circuit.
The double sided tape didn’t work very well, the trim pot kept breaking off so I mixed a bit of 5 min epoxy and glued it on. It isn’t electrical grade epoxy but isn’t much risk at the low voltage output section.
Fit into the outer PVC pipe
But it needs to be opened up again for testing and recalibration
Was planning to melt some paraffin wax candles to pot the resistors up but the canister didn’t have any gas
Banana plugs and aligator clip for ground return connected
This is the output when it was hooked up to a frequency generator running some 1-2KHz 10 Vpp sine as you can see the signal is overwhelmed by mains interference. Hopefully the signal to noise ratio is better at the intended operating voltages ~30kV corresponding to 30V output.
I cut up a tin tray and put it around the tube and grounded it to act as makeshift shielding which resulted in this output. The mains interference is reduced but so is the signal. This is likely due to increased parasitic capacitance with the shielding.
Here is a badly drawn diagram of what likely what is happening based off my understanding from Dave and Doug’s talk on probe design: https://www.youtube.com/watch?v=jUvSP3BQpvs The joints between the resistors have parasitic capacitance to earth which attenuate high frequency signals by shunting them to earth. The parasitic inductance of the resistor string also may act as a high pass filter but this should be compensated by parasitic inductance in the output resistor section.
The standard practice is to add capacitance parallel to the resistors to allow additional high frequency bypass to compensate for signal lost due to parasitic capacitance to earth. An adjustable trimming capacitor allows tuning of capacitive division ratio (i think). Additional inductance on second thought is probably uneccesary and will only add to parasitic oscillations
End of Day 2
High voltage test setup. The high voltage is generated using a flyback transformer with an internal diode from a CRT display driven by a Mazzilli ZVS circuit (http://adammunich.com/zvs-driver/). Power is supplied by a home-made unregulated 36VDC power supply. The DMM had to later be switch to a higher quality one that would accept the shrouded banana plugs. I have some videos of testing which I might edit and upload later if there is interest.
Testing appeared to be successful. The DMM reading peaked at 18v corresponding to an output across the flyback transformer of 18kV. Testing with an old oscilloscope, didn’t give interesting results, the input impedance of the scope is much lower and so appears reduce the measured voltage, the output signal is also nearly completely flat; a very low bandwidth may be the issue here.
Some discharge into the concrete appears to have scorched it a bit.
The primary windings became very hot and the glue holding the previously broken ferrite core fell apart. Got it rewound with thicker wire and glued back together again, this time with proper super glue.
Also finished building the gate drive transformer driven H-bridge output stage for my plasma speaker (http://www.instructables.com/id/A-reliable-plasma-speaker/?ALLSTEPS the orignal design is by Jan Martis which is not credited here) might make a seperate project post for this too.
- Read: http://dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf
- Test with oscilloscope and HV signal