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

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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.

Use a scope instead.
 

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
Most Helpful Member
Are you measuring the voltage directly across the resistor?
 

Fahime

New Member
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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 I already posted a whole thread about true RMS voltage, apparrent power, real power....

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
Most Helpful 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
Most Helpful 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.
 
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