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# How to measure the current of a resistor with a special kind of voltage

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#### Fahime

##### New Member
Hi,
How to measure the current of a resistor that its voltage is as the attached file.

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#### Noggin

##### Member
Calculate RMS voltage, divide by resistor value.

#### Fahime

##### New Member
Calculate RMS voltage, divide by resistor value.
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?

#### kpatz

##### New Member
Most multimeter's AC modes are designed to read RMS for 50/60 Hz AC. Other frequencies and odd waveforms like yours won't be measured accurately by a multimeter.

#### transistor495

##### Member
Forum Supporter
Is that waveform produced from an electric chair in operation?

#### Nigel Goodwin

##### Super Moderator
Most multimeter's AC modes are designed to read RMS for 50/60 Hz AC.

No, they don't measure RMS, they measure average - and this is then scaled to read RMS for a 50/60Hz sinewave only.

True-RMS meters are available though, and you could use one of those.

#### Roff

##### Well-Known Member
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?
You are obviously doing something wrong. I don't see how we can determine what that is. Tell us more about the circuit that the resistor is in.

#### Noggin

##### Member
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?

As nigel suggested, you could try a true RMS meter, however, some of them wont work at high frequencies. If you have access to a digital scope it can do the calculation for you. If you don't need it to be highly accurate, you could try to measure the temperature rise of the resistor and determine the power consumption (this is how some RMS meters work). If you can collect enough data points then you can calculate RMS voltage yourself.

#### crutschow

##### Well-Known Member
Are you measuring the voltage directly across the resistor?

#### Willbe

##### New Member
Is that waveform produced from an electric chair in operation?
The elec. chair voltage is 2400 vac with a current of 4A to 8A, so I doubt it.

#### Fahime

##### New Member
Let me answer the questions at a time:
1.
Are you measuring the voltage directly across the resistor?
yes.
3.
Noggin said:
If you have access to a digital scope it can do the calculation for you.
How does it do that?

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#### Noggin

##### Member
I assume it uses the definition of RMS voltage to calculate the RMS voltage It doesn't know the resistance, so you'd still have to convert from voltage to current.

#### Roff

##### Well-Known Member
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?
How do you know what the peak-to-peak voltage is?
If this is true, it doesn't matter what kind of meter or scope you use. Crest factor can cause a meter to read low when the peak-to-peak is high, but I don't see how the opposite can occur. I suspect that your peak-to-peak voltage is actually higher than 160VAC, or you are doing something wrong.

#### bountyhunter

##### Well-Known Member
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?

I think we need a tutorial with a sticky.

#### bountyhunter

##### Well-Known Member
Let me answer the questions at a time:
1. No it is from an electronic ballast. The ferequency is about 45kHz.
Then you need to borrow or rent a very good (like HP) AC voltmeter which has wide bandwidth capability on it's true RMS function for AC voltage. Lower cost TRMS meters do not have enough bandwidth to get a decent reading at a frequency that high.

There were also some very wideband AC meters with analog (needle) face from years back which use thermal measurement which would work. Insert a low value resistor in series with the current and measure the voltage across it.

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#### The Electrician

##### Active Member
If you mean I measure the voltage by the multimeter on Vac mode, it doesn't show the correct voltage. it shows 160Vac while the peak to peak voltage is a few volts. why is that?

Did you capture this waveform on an oscilloscope? If so, then post a photograph of the scope display, showing the scale factors.

#### audioguru

##### Well-Known Member
I am laughing so much at these stupid questions that it hurts.

There should be a website for NOOBs who know nothing about simple electricity.
I thought it was taught in junior high school.

#### Fahime

##### New Member
I am laughing so much at these stupid questions that it hurts.

There should be a website for NOOBs who know nothing about simple electricity.
I thought it was taught in junior high school.
It's more stupid to laugh at people trying to learn and figure out problems and whome trying to help others.
Does it say anywhere in this website that it's only a place for expereinced and knowlagable people?!

#### audioguru

##### Well-Known Member
This is a site for experienced and knowledgeable people.
There should be another site somewhere for * little school kids who are not learning anything in school.

We help many hobbiests understand a few complicated electronic theories here.
But the simplest of the basics of simple electricity are much different.

A multimeter is made to measure sine-waves at 50Hz and 60Hz only.
An oscilloscope can accurately measure the 45kHz unusual waveform across your resistor.

#### Roff

##### Well-Known Member
This is a site for experienced and knowledgeable people.
That's a new one on me. I have searched the site, and can't find such a restriction.

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