Author Topic: Help with voltage dividers  (Read 851 times)

Offline nsled

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Help with voltage dividers
« on: May 12, 2023, 11:52:46 PM »
Are voltage dividers to measure the voltage of HV capacitors a thing? or is it done by other means?

I've tried to model it, seems ok until I realised there is only 5nA of current left to measure after 1:1000 division. Will voltmeters measure this low current?

I've used 5MOhm on R1 to keep it to a reasonable 5W.

Offline klugesmith

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Re: Help with voltage dividers
« Reply #1 on: May 13, 2023, 12:08:28 AM »
You set up a circuit simulator and picked a voltmeter from library.
A voltmeter with 1-gigaohm impedance.  What does your simulator show about voltage and power at the ammeter?

What you need is a realistic voltmeter, with input impedance substantially higher than 5 kiloohms.
Or put a realistic milliammeter in place of the 5k ohm resistor.

Consider changing the 5Meg resistor to 4.995Meg if you want a 1000:1 division instead of 1001:1. :)

It is common to measure HV using a multi-megohm resistor in series with analog or digital panel milliammeter.
Often the meter is shunted with a permanently-connected resistor or nonlinear device, to limit the voltage in case panel meter is accidentally disconnected or goes open circuit.
« Last Edit: May 13, 2023, 12:11:49 AM by klugesmith »

Offline nsled

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Re: Help with voltage dividers
« Reply #2 on: May 13, 2023, 01:45:15 AM »
Okay thankyou, I guess I know nothing of input impedance.

The ammeter shouldn't really be there and can be ignored it shows zero zero.

I'm pulling parts out of a shed from over a decade ago and you've made me realised what the string of 5 100Mohm 10W resistors was for. It had me baffled but no doubt it was to wire into an analogue meter.

Offline MRMILSTAR

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Re: Help with voltage dividers
« Reply #3 on: May 13, 2023, 05:39:17 AM »
The easiest way is to use a microammeter and a high voltage resistor. For example, I use a 25 microamp meter in series with a 1G ohm high voltage resistor to measure 0 to 25 KV. The ammeter scale reads directly in kilovolts.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline nsled

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Re: Help with voltage dividers
« Reply #4 on: May 14, 2023, 06:59:50 PM »
I think I'm understanding, the reason for using an ammeter instead of voltmeter is because it has very low resistance.

And this physically is the same thing https://www.aliexpress.com/item/1005002267936039.html?spm=a2g0o.cart.0.0.28d438daueGLXf&mp=1 I checked user reviews and it shows in a pic to come with 500Mohm resistor.

Offline MRMILSTAR

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Re: Help with voltage dividers
« Reply #5 on: May 14, 2023, 09:29:25 PM »
Correct. Those voltmeters are actually microammeters with a voltage scale reading in kilovolts. As the ad says, you must use a matching high voltage series resistor.
Steve White
Cedar Rapids, Iowa
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Re: Help with voltage dividers
« Reply #5 on: May 14, 2023, 09:29:25 PM »

 


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