Author Topic: Voltage divider for measuring HV  (Read 538 times)

Offline boom198

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Voltage divider for measuring HV
« on: July 11, 2022, 05:39:01 PM »
Greetings, I recently made a voltage divider for testing HV. I was able to successfully measure a low voltage with low power resistors (I'm using a variac powered by 110 V mains). after obtaining new resistors with higher power but same ratio, I can measure the full voltage across the load leads, but my voltage is half the input when measured between R2 and R3. I need to measure voltages up to 20kv so this won't do. I have 5 resistors of each resistor if there might be a way to run some in parallel?



Thanks

Offline davekni

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Re: Voltage divider for measuring HV
« Reply #1 on: July 12, 2022, 04:32:55 AM »
Presuming you are measuring voltage using a meter with 10M input impedance:  Recalculate what you expect to measure with meter's 10M in parallel with resistor(s) you are measuring across.  BTW, auto-ranging DMMs often have 10M input impedance at lower voltages rising to ~11M for higher ranges.

Also, which node if any is connected to earth ground?  If the meter is far above ground potential, corona discharge to hand holding meter or whatever will alter reading.
David Knierim

Offline klugesmith

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Re: Voltage divider for measuring HV
« Reply #2 on: July 13, 2022, 05:33:37 AM »
Nice schematic you drew there, Boom.
But it doesn't show your measuring instrument.   
It would help if your schematic shows the meter in circuit.
As Dave said, you should also indicate which (if any) place in the HV circuit is connected to earth ground, because it matters.

It's nice that you named the resistors, so you can say R2 instead of "the middle R" or "the 300M R".
You can also name electrical nodes, like GND or V1 or V2, to facilitate discussion with written words.
Just don't say "voltage measured at V1" without being explicit about where the other meter lead is connected.

[edit] If you are measuring AC, it's even more important to know details of the meter.   Many analog multimeters have very nonlinear input resistance  in AC Volt ranges .  Some digital multimeters have AC coupled inputs (infinite DC resistance ) in AC volts modes.
« Last Edit: July 13, 2022, 05:39:46 AM by klugesmith »

Offline boom198

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Re: Voltage divider for measuring HV
« Reply #3 on: July 14, 2022, 05:44:47 AM »
Thank you very much for the help. I am going to have to re-evaluate my question, because I was merely running tests to obtain results of a lower voltage, I have been using AC. I'm actually going to be reading the final HV circuit in DC. Will having the circuit in DC mitigate the effects of impedance?

at the moment I'm not able to go out and tinker (I mainly can't get details on the meters I'm using at the moment) I was able to redraw the schematic in DC. Is this the correct orientation of circuit polarity? and in reference to the "safety" ground wire, is that connected to the cathode?






Offline Twospoons

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Re: Voltage divider for measuring HV
« Reply #4 on: July 14, 2022, 07:33:33 AM »
Your result would make sense if r2 and r3 were 300k and 220k, not the 300M and 220M shown. Are you sure about your resistances?

Offline klugesmith

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Re: Voltage divider for measuring HV
« Reply #5 on: July 14, 2022, 04:39:00 PM »
Are you paying attention to the voltage ratings of your resistors?
A resistor can be over-volted even if power dissipation is within rating and there is no external flash-over or surface leakage current problem.

Voltage rating often becomes more constraining than power rating, when R values are many megohms.
Ordinary 1/4 watt axial-lead resistors are typically rated for a few hundred volts.  I have seen 1W carbon composition resistors have significantly lower resistance at 2000 V than at low voltage.

Offline boom198

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Re: Voltage divider for measuring HV
« Reply #6 on: July 15, 2022, 01:28:57 AM »
Your result would make sense if r2 and r3 were 300k and 220k, not the 300M and 220M shown. Are you sure about your resistances?

I have an image of the resistors


I got them off ebay from overseas. It does say the resistor value on the side and they were sold as 20kv rated resistors (i'm also only testing with low input power, max 110 V)
I don't think my meter can read over 2M ohms so I'm not entirely certain if they are mis labeled.

Offline klugesmith

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Re: Voltage divider for measuring HV
« Reply #7 on: July 15, 2022, 01:38:36 AM »
If your meter had infinite impedance, wouldn't you have a V2/V1 ratio of about 0.999 ?
And if meter had 10M impedance, wouldn't the V2/V1 ratio be about 0.95 ?
It would be different if meter measured across R1 instead of across R2 + R3.

And in a previous post you mentioned "the cathode".  Nothing in your schematic identifies a cathode.   Some kinds of Load might have a cathode and anode. That would be independent of where earth ground is connected.  MWO magnetrons are practically and safely operated with grounded anode.

Your circuit might make more sense if the meter Common wire and earth GND were connected to top of R1 and to the positive side of HV power supply.  Then you would have a negative high voltage applied to load, while divided by (R2 + R3) / R1 for measurement.
« Last Edit: July 15, 2022, 01:51:03 AM by klugesmith »

Offline boom198

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Re: Voltage divider for measuring HV
« Reply #8 on: July 15, 2022, 02:55:19 AM »

It would be different if meter measured across R1 instead of across R2 + R3.

And in a previous post you mentioned "the cathode".  Nothing in your schematic identifies a cathode.   Some kinds of Load might have a cathode and anode. That would be independent of where earth ground is connected.  MWO magnetrons are practically and safely operated with grounded anode.

Your circuit might make more sense if the meter Common wire and earth GND were connected to top of R1 and to the positive side of HV power supply.  Then you would have a negative high voltage applied to load, while divided by (R2 + R3) / R1 for measurement.

I had tried reading the voltage across R1 and was reading zero. I was fairly confused at that point because I had read half the input voltage across R2 + R3, which lead me here.
In terms of cathode, my limited understanding is that any electrode with negative polarity is a cathode.


Is this schematic what I should be going for?

I also was able to test all of my 510k Ohm resistors, and they check out. I'm assuming at this point that the 220M and 300M resistors are good.

Offline klugesmith

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Re: Voltage divider for measuring HV
« Reply #9 on: July 15, 2022, 04:44:11 AM »
Wikipedia has a good explanation of Cathode.

In an electrochemical "battery" cell, the positive terminal is cathode during normal use (discharging).  It's where conventional current is exiting from the device.
Negative terminal is cathode when current is reversed to recharge the cell.
Cathode was defined and named by Faraday in 1834, decades later than the convention for sign of electric charge.

Cylindrical alkaline cells have a cathode cup on positive terminal, to be physically interchangeable with "carbon zinc" cells where the metal cap is pressed onto the carbon rod cathode.
« Last Edit: July 15, 2022, 05:01:21 AM by klugesmith »

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Re: Voltage divider for measuring HV
« Reply #9 on: July 15, 2022, 04:44:11 AM »

 


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