Measuring voltages with Ammeters and Voltmeters

[michela02]

Hi everyone,
have done an experiment on electricity.
We had an AC function generator, set on a PP voltage of 3V. We measured the output voltage both with an Analogue Multimeter and with a Voltmeter at two different frequencies.
Everything went right with the voltmeter, and for a frequency of 1 Hz we got an output PP voltage of 0V, while for a freq of 1kHz we got an output PP voltage of 2.97V.
Then we used the AMM, and both for a freq of 1Hz and for a freq of 1kHz it gave an output PP voltage of 2.55V. Is that right? I can't understand why it doesn't change according to different frequencies.
Please help, it's urgent!
Thank you in advance

 

[sophiecentaur]

Hi and welcome.
It isn't clear what the experiment was, what you were measuring and where.
You don't connect Voltmeters and Ammeters the same way, do you (? look in your notes or textbook). Also, an Ammeter doesn't read in Volts, does it? I suggest that you were looking at the input and output volts for a filter, which was passing 1kHz signals but not 1Hz signals.
If you could post a diagram of what you did, that might help me to give a more useful answer. It may be easiest to post a photo of the diagram in your book than to try to produce a picture with a drawing package. Attachments are easy on this forum.

 

[Delphi51]

The meters measure voltage, not frequency so there should not be any change in the voltage when the frequency is changed, unless there is some limitation in the meters' ability to respond to some frequencies.

 

[michela02]

The AMM is the analogue multimeter, but it's not set on amperes but on volts for this particular experiment.They are the only elements of the circuit: the aim of the experiment was just to compare the readings from the analogue multimeter and the digital one(dmm), which should be more precise. (the dmm actually has an error of 0.005 while the amm of 0.05).The PP voltage is set on the ac frequency generator, together with the frequencies of 1hz and 1khz, and the output voltage was to be measured on the two different devices (the amm and the dmm).
For 1khz everything seems to be working fine (dmm measures 2.97v while amm measures 2.55v, it's less accurate and has got a higher resistance), but for 1hz, I understand why I get 0 on the dmm but I don't get why I got 2.5 with the amm for the same frequency.Shouldn't I get 0?

 

[Delphi51]

I don't understand why you think it should be zero! Maybe we can put our minds together and figure it out. If you looked at the input to the meters on an oscilloscope, you would see a sinusoidal wave. The voltmeters are measuring the vertical height of the signal. The horizontal distance between the peaks varies with the frequency. Frequency does not affect voltage.

Any difference between the meters is showing up the deficiencies in their design, not differences in the voltage. Oh, and the signal generator may not provide the same voltage at all frequencies.

 

[CWatters]

Everything went right with the [digital] voltmeter, and for a frequency of 1 Hz we got an output PP voltage of 0V, while for a freq of 1kHz we got an output PP voltage of 2.97V.

but is that right? An ideal meter would read the same at all frequencies.

Got a specification for the digital meter you used?

 

[michela02]

I read on some website that the output voltage decreases with the frequency, but it could be wrong. Everybody's saying something different and I'm getting confused! So was the amm right and the dmm wrong in getting 0v for 1hz?

 

[bsbs]

Hz only means flow of electron, 1 Hz means 1 time per second to and fro,

can't see any relation with the output voltage. but it does effect higher current when the Hz is higher on a capacitive load, and lesser current on an inductive load

 

[CWatters]

If both meters were "ideal" volt meters with a pk-pk setting then they would both read 3V regardless of the frequency.

If they don't read 3V then it will be because they aren't ideal meters. I would expect both to read roughly 3V when the input is 1kHz because that's a common test frequency for fault finding.

Without some sort of specification for the meters it's impossible to tell what they will read at 1Hz on the pk-pk setting. 1Hz is practically DC. Some might not be designed to work below say 20Hz. I would expect most meters to work down to at least 50 or 60Hz which is the mains frequency.

 

[sophiecentaur]

1Hz is a 'very low frequency'. It is, of course, not DC and an analogue DC meter would actually respond by waving its needle about at a rate of 1Hz. Few AC meters would be expected to respond to 1HZ - hence my original comment about a "high pass filter". I am surprised that the Analogue meter showed any reading at all at 1Hz and even more that it was not lower than the 1kHz reading. Are you absolutely sure of that reading and was it in the same place in circuit as the DVM? Could it have been switched to read 'peak value', for instance?

 

[sophiecentaur]

I read on some website that the output voltage decreases with the frequency, but it could be wrong. Everybody's saying something different and I'm getting confused! So was the amm right and the dmm wrong in getting 0v for 1hz?

What website was it? Did it describe the instrument you were using? Most actual 'waveform synthesisers' can produce any frequency of waveform - down to small fractions of 1Hz. Conventional 'signal generators' cannot do that.

The reason we're all giving different responses is because we're trying to cope with data that isn't making sense and I don't think you can help us or yourself - if you can't re-do the experiment. You ask which is wrong and which is right. Because you are discussing the performance at 1Hz, neither can be assumed to be correct. What did the waveform synthesiser say it was doing?

Why don't you just ask your teacher? If you say you've been on line about it, he/she will be pleased that you have made an effort, I'm sure. Have any of your fellow students any ideas and did they get the same results?

 

[michela02]

I know it doesn't make much sense, but I wrote it last night after having been on the internet all day trying to solve the problem, and I probably had to write everything more specifically.. the easiest thing for me to do was to re-do the experiment, but the report was due in today at 5pm and I didn't have time as I've been attending lectures all day. I tried and "mask" the problem a bit saying I got a systematic error, but probably my partner and I set the circuit up wrongly.
I tried to ask the teacher but he wasn't there all day either. Well, I'll just hope for the lecturer to understand that we're human and we make mistakes, and as I can't redo the experiment I just wrote I encountered some problems.
Thank you very much for your help, it was nice of everybody to take some time and answer me.

 

[sophiecentaur]

You'll have to go online whilst you're actually doing your next experiment.

 

[CWatters]

Pity you don't have any data for the meters handy. I would have been happy to try and look up the specs for you.

One of my meters is an analog meter but it also has some electronics in it. For example it has FET front end to give a ≈10M input impedance on the voltage ranges.