Multimeter measuring full-wave rectified DC current?

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Homework Help Overview

The discussion revolves around understanding how a multimeter measures voltage in the context of a full-wave rectified DC current, particularly in a physics electricity and magnetism lab setting. The original poster expresses uncertainty about the multimeter's internal workings and its measurement capabilities when connected to a rectified AC source.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the nature of multimeter readings, considering different types of meters (analog vs. digital) and their measurement methods. Questions arise about whether the multimeter is set to measure AC or DC, and what specific value it would display when connected to a full-wave rectified voltage source.

Discussion Status

The discussion includes various interpretations of how the multimeter operates and what it measures. Some participants suggest that an analog meter would read the average value of the rectified signal, while others question the specifics of the setup and the characteristics of the voltage source. There is no explicit consensus on the multimeter's behavior, but several productive lines of inquiry are being explored.

Contextual Notes

Participants note the lack of clarity regarding the multimeter's settings (AC or DC) and the nature of the voltage source, which is described as a 7.8 V, 60 Hz signal processed through a full-wave rectifier. There is also mention of ideal conditions for the rectifier and potential diode drops that may affect measurements.

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I have this question as a pre-lab for a physics electricity and magnetism lab. (I attached the question as an image.)
I am having difficulty answering the second part:

A full-wave rectified DC current is just the absolute value of the sinusoidal AC current: http://en.wikipedia.org/wiki/Image:Gratz.rectifier.en.png.

I can not answer the this question because I do not know the internal workings of the multimeter.
I can see three possible outcomes:
1) the multimeter is analog, it uses a galvanometer with the needle having some inertia, so the reading would be some sort of average of the maximum amplitude.
2) the multimeter is digital, and thus might measure the rms voltage.
3) the multimeter has no 'inertia', and thus would show the actual time varying voltage V(t).

Any suggestions?
 

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  • Picture 001.jpg
    Picture 001.jpg
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The way analog meters work when in AC mode is that they will always read the average value. If you're measuring an AC signal, the meter first rectifies it(either half or full wave), then takes the average value of the rectified signal. It then multiples it by a multiplication (form) factor to display the RMS value.

There is always inertia with these analog meters.
 
I have no idea if in the question the multimeter is set to DC or AC.
 
What is the question? I don't see a question in the OP! :confused:
 
The question is attached in the Picture 001.jpg attachment.

As I understand it now, the question is that you switch a Voltmeter to measure DC voltage. You connect it to a power source that produces a 7.8 V 60 Hz voltage that has been put through a full-wave rectifier.

What will this Voltmeter measure? Will it give the amplitude of 7.8V or will it measure something else?
Basically in the question the guy is trying to measure a voltage tht varies with time V(t) with a DC voltmeter.
 
It turn out that the attachment is "pending" for some reason...

Here is the link to the question that I uploaded:
http://e.271828.p.31415.googlepages.com/Picture001.jpg
 
Last edited by a moderator:
The peak is 7.8V assuming ideal conditions for the rectifier (no diode drop). A dc voltmeter would meter the average value (as mentioned in my initial post). Can you find the average value of this signal?

Hint: Use calculus to find the average vale of a full wave rectified signal. The wave has a period of pi radians.
 
Is this a full wave bridge? Silicon diodes?
 

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