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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
In order to preserve the 1000:1 ratio of the divider, I must take account of the 10M input impedance of the DVM module in parallel with the measuring resistor.
As I want to have 3M on the tail of my 3000M column, I must use a value of 4M286 - i.e. when resistances of 10M and 4M286 are connected in parallel, their combined resistance is 3M.
To arrive at 4M286, I have selected resistors with face values of 100M, 1G, 4M7, and 100M, whose nominal parallel resistance is 4M278. But by measuring the resistance of scores of resistors with these value, the nearest combination of values I could get resulted in a paralled total of 4M291, which I can live with.
Were all the other measurements completely accurate in this Best Of All Possible Worlds, , my effective tail resistance is 10M for the meter input impedance shunted by 4M291 for the measuring resistor, a grand total of 3M003, making my voltage divider ratio 999:1. (What wishful thinking! I'll be pleased if I can get it in under 5%. )
Soviet-style soldering with a 100W iron and a pot of pine resin from Russian forests was needed to wet the threaded brass Earth rod.
To keep the fierce heat of the 100W iron out of the resistors, I used a Spencer-Wells artery clamp as a heat sink.
You may well ask, why go to all this trouble trying to achieve a precise tail resistor value with fixed resistors, when a pre-set could have been used?
My answer is two-fold: firstly, there is safety: it is unlikely that all four resistors will fail open-circuit such that the high voltage being measured appears at the bottom end of the resistor column and gets up to mischief; secondly, four very generously rated fixed resistors are less likely to drift in comparison with a pre-set pot, even a good one.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Finishing The Meter Unit
After some thought, I decided to replace the LCD meter module with a red LED one so that the meter could be read in darkroom mode if necessary.
Having assured myself that all was well, and that no further modifications would be required, I secured the wiring, and painted the nuts with threadlock - clear acrylic nail varnish in this case!
There was now nothing left to do with this part of the apparatus than secure a bag of fresh silica gel dessicant in the empty upper right quadrant, paint the box flanges with Dow Corning 3140 RTV, and screw down the lid.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
hey Proud Mary... i got out my old 2 giga ohm, 1000:1 resistive voltage divider and connected it up too a 270vDC (+- 1.2 %)source then used a miliVolt meter to mearsure the low side but it wasnt stable it should have read 27mV but instead it would go down to 9mv and up to 41mv very slowly, no loose leads seemed to be the cause,i have no idea if its become and antenna or not? i need help. any ideas?
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A 75kV 1000:1 Voltage Divider With 3G Input Impedance
, my effective tail resistance is 10M for the meter input impedance shunted by 4M291 for the measuring resistor, a grand total of 3M003, making my voltage divider ratio 999:1. (What wishful thinking! I'll be pleased if I can get it in under 5%. 
