Multimeter for metering HV

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Pablo Llaguno
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Multimeter for metering HV

Post by Pablo Llaguno » Wed Jan 23, 2019 12:29 am

As the topic implies, is there any reason why the FAQ for metering HV recommends a digital/analog voltmeter? For my setup, I would do a HV divider in the same way: by adding resistors in series. Using the right resistors, is there a reason why I couldn't get a multimeter to display 1v/1000v?

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Rich Feldman
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Re: Multimeter for metering HV

Post by Rich Feldman » Wed Jan 23, 2019 3:15 am

That would be fine, Pablo.
Just be careful that the multimeter's Mode and Range switch(es) are correct whenever you have high voltage connected. Some multimeters have a separate input jack for 1000 volt range. I once attached some resistors to the tail end of a banana plug, to get a 2500 or 5000 volt range.

Your divider design will depends on the multimeter's input resistance for the voltage range you will be using.
Usually very close to either 10 MΩ or 1 MΩ for digital multimeters. More data in this post:
viewtopic.php?f=11&t=12407&p=80759#p80759
(in which the plain old analog VOM had higher input resistance than any of the digital multimeters).

You would be less sensitive to meter's input resistance, if you make a divider with a low-ish low side resistor.
For example, 100 Meg over 100 K, for 1000:1 voltage attenuation.
Then connecting multimeter in parallel with the 100 K resistor would perturb the ratio by 1% to 10%.
If the divider were 20 Meg over 20 K, for same voltage attenuation,
then the change due to multimeter would be only 0.2% to 2%.
But that divider is 5 times heavier load upon the HV circuit under test, and receives 5 times more heat.
¿comprendes?
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

Pablo Llaguno
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Re: Multimeter for metering HV

Post by Pablo Llaguno » Wed Jan 23, 2019 3:29 am

Thanks Rich!

What exactly do you mean by being less sensitive to the meter's input resistance? Do you mean the error percentage?

Si es que comprendo, then ideally I should use a lower resistance value in the low side but at the cost of a heavier load and thus needing a higher power rating (>10W?)

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Rich Feldman
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Re: Multimeter for metering HV

Post by Rich Feldman » Wed Jan 23, 2019 3:47 am

I meant errors if you pretend that the multimeter has infinite resistance.
It's good to have a low side resistor built into the divider, to avoid HV at the meter connection even when meter is off or disconnected.
But there's no penalty in accuracy, if the meter resistance is figured into the voltage divider design.

Reminds me of joke after Arjen Robben flopped in a World Cup match a few years ago.
Knock knock!
Quien es?
Tu nuera.
Que nuera?
Nuera penal!

A popular real-world example: R_high 990M, R_low 1.1M. If my spreadsheet is correct, voltage ratio is
* 1.110 volts per kV when unloaded
* 1.000 volts per kV when a 10M meter is connected
* 0.529 volts per kV when a 1M meter is connected.

Or slightly adjusted example from earlier post: R_high 99.9M, R_low 101K. Voltage ratio is
* 1.010 volts per kV when unloaded
* 1.000 volts per kV when a 10M meter is connected
* 0.917 volts per kV when a 1M meter is connected.
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

Pablo Llaguno
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Re: Multimeter for metering HV

Post by Pablo Llaguno » Wed Jan 23, 2019 4:53 pm

Thanks!

Looking at the hassle of using the multimeter, I think I am going for a digital voltmeter. It would be a bit simpler and more suited for my whole setup.

Do you know where can I get one that reads 0-30V (most I've seen only go from 5-30V)?

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Rich Feldman
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Re: Multimeter for metering HV

Post by Rich Feldman » Thu Jan 24, 2019 6:36 am

I've got a few different kinds from an ebay seller called nyplatform, for around $3 each.
Keywords 0-30V (or 0~30V) 3 wire LED panel meter. One wire for voltage to be measured, one wire for power (often 3 to 30 volts), one common.
There are similar-looking units for sale with ranges like 0-100V.
panel.JPG
You can see some in service, in a thread here called Neon Sign Transformer Power.

I'm powering them with 9 volt batteries.
Have you considered analog panel meters, that need no power supply? There's one pictured in HV Divider thread here.
Mike echo oscar whisky! I repeat! Mike echo oscar whisky, how do you copy? Over.

Pablo Llaguno
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Re: Multimeter for metering HV

Post by Pablo Llaguno » Thu Jan 24, 2019 3:31 pm

I have indeed, but since I want to get this locally I actually think that digital meters are more common here. For example https://articulo.mercadolibre.com.mx/ML ... quantity=1

Translated technical details:
Measurement range 0,00 V-30,0 V
Alimentation voltage red DC4.50-30.0V, green DC5.00-30.0V, blue DC4.70-30.0V
1% error
Input impedance: 100K ohms
Consuming current. red <23mA, green<18mA, blue <13mA
300ms update speed

Would this meter need a resistor kit like the ones Richard modified in the Fusor Voltage II FAQ?

Also, due to the 100k impedance, if I understand right I would want the HV divider to be 20M to 20k or 10M to 10k.

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