# RL low pass filter, Voltage across inductor at 100hz

#### Jamie8286

Apr 14, 2016
8
Hi guys

Iv been set a frequency response report to complete iv pretty much got to the bottom of all of the circuits and have nearly completed it.

There is one thing iv not been able to find in all of my textbooks or the internet im not sure why.

It states in the lab script to set up a RL circuit with a 100ohm resistor and 25mH inductor and plot the voltage at a number of frequencies from 1Khz to 10Khz iv done this bit and plotted the frequency response of the circuit.

It then says also measure the voltage across the inductor at 100hz and asks why do you need this measurement? Thats the bit im stuck on i cant seem to fins the answer anywhere.

Is it to check the inductor is operating correctly?

allowing the low frequency content to pass giving the same voltage as the input voltage?

Any help would be great thanks!

#### LvW

Apr 12, 2014
604
As I can read - you have an RL circuit, whatever this may be.
Perhaps you have rough drawing, because there is not only one single alternative to connect both parts.

#### Jamie8286

Apr 14, 2016
8
Hi there yes heres an LTspice simulation.

Also i forgot to mention the circuit is supplied via signal generator (in the real experiment) simulating AC

thanks

#### Attachments

• rl high pass.png
19.3 KB · Views: 107

#### Jamie8286

Apr 14, 2016
8
Just to add to this I have noted that the voltage across either end of the inductor was 58mV. But i guess it is not to clear if they want a voltage reading at Vout which would be 1v as the supply voltage magnitude = 1v

thanks

#### Arouse1973

Dec 18, 2013
5,177
But if you measure across the resistor what do you get? This is where it gets interesting... If you try it with another voltage say 10 Volts, what do you get? And if you measure across the inductor what do you get? This should confuse you initially. Well it might do

#### LvW

Apr 12, 2014
604
I really dont know what your problem is.
What is the question?
How can a voltage exist "across either end of the inductor"? What do you mean ? Voltage across the inductor?
In the headline you speak about a lowpass. How do you know if it is a lowpass?

#### Jamie8286

Apr 14, 2016
8
I really dont know what your problem is.
What is the question?
How can a voltage exist "across either end of the inductor"? What do you mean ? Voltage across the inductor?
In the headline you speak about a lowpass. How do you know if it is a lowpass?

Iv just realised the above picture in incorrect and shows a high pass filter i will attach the low pass picture on this post.

The question is with the circuit of an "RL lowpass filter" With measurements taken over the frequency range of 1kHz to 10kHz to enable plotting of the frequncy responce on a graph

Why would you need to take a measurement "across" the inductor at 100hz.

(If this refers to applying probes to each of its connecting wires or measuring Vout to Ground it is unclear)

Yes when probes where attached to either side of the inductor a small voltage was detected on the digital multimeter? This may have just been stray voltage but it was there. I recorded this value as it wasnt clear what the question was asking.

If it is refering to the measurement of Vout the voltage equals 1v

Thanks

#### Attachments

• RL Low Pass.png
22.9 KB · Views: 120

#### Jamie8286

Apr 14, 2016
8
But if you measure across the resistor what do you get? This is where it gets interesting... If you try it with another voltage say 10 Volts, what do you get? And if you measure across the inductor what do you get? This should confuse you initially. Well it might do

Hi measurement across the resistor at this frequncy wasnt in the lab script for this experiment so is not needed in my report 1v was also specifed as reference voltage.

Im assuming the measurement at 100hz is for something trivial.

Thanks

#### Arouse1973

Dec 18, 2013
5,177
Oooops sorry I thought it said resistor... I'll shut up now

#### LvW

Apr 12, 2014
604
Why would you need to take a measurement "across" the inductor at 100hz.

Yes when probes where attached to either side of the inductor a small voltage was detected on the digital multimeter? This may have just been stray voltage but it was there. I recorded this value as it wasnt clear what the question was asking.

A voltage is to be measured BETWEEN two points - how can you detect a voltage at one side of the inductor? Referenced to which node?
More than that - if you are asking me why I would "need to take a measurement "across" the inductor at 100hz" my answer is: I do not need such a measurement. The only important test would be to find the 3dB-cut-off frequency.

#### duke37

Jan 9, 2011
5,364
The output voltage will not be 1V since there is a 100Ω resistor to reduce it.
LT spice uses peak voltage not RMS so watch this.
58mV across the inductor seems strange. What is the frequency to get this?
Please show us a plot of the frequency response.
It would help if you checked your spelling and use of capital letters.

#### Arouse1973

Dec 18, 2013
5,177
I make it 154 mV peak across the inductor not 58 mV.

#### Jamie8286

Apr 14, 2016
8
Hi guys thanks for the replies i think i have worked out the answer to this myself.

Iv been plotting all the values of Xc, XL, etc against frequency for my report.

Looking at the RL low pass as mentioned is can be seen that for the frequency range 1kHz to 10kHz total circuit impedance Z is equal to XL for this entire range

On inputting the 100Hz signal into my calculations I get expected capacitive reactance (Xc) = 16ohm derived from (2.pie.100Hz.25mH)

Whilst measured value = 101ohm (derived ohms law for an inductive circuit Xc= Vrms/Irms)

Later in the lab script it mentions compare measured with expected results at this frequency so I'm assuming it is expecting you to get this result.

Am i correct in saying then that at this low frequency the opposition to current in the inductor is much higher then at higher frequency's and this could be used to check the inductor is behaving as it should be?

Thanks Jamie

#### Jamie8286

Apr 14, 2016
8
I make it 154 mV peak across the inductor not 58 mV.

Hi the 58mv we recorded with a RMS multi meter 100Hz a the signal generator checked via a digital counter.

It may be incorrect but thats just what we recorded om the day.

just looking at your calculation its looks like the peak to peak for 58mV not far off anyway i make it 164mV

0.058 x 2.8284

Thanks Jamie

#### Jamie8286

Apr 14, 2016
8
"Am i correct in saying then that at this low frequency the opposition to current in the inductor is much higher then at higher frequency's and this could be used to check the inductor is behaving as it should be?"

I think i may have explained that incorrectly. I mean is Xc higher then expected at this low frequency in relation to the rest of the circuit.

It may just be a dodgy reading though i guess.

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