OMG.... omg, you beautiful man, you... How could I have missed that...? I must have skipped that entirely. Thank you. I feel silly.
okay, follow up... Since the resistor is placed before the LED, if we're observing (via serial monitor) the voltage at the point right after the resistor, isn't that the remaining voltage that will be dropped across the LED? and wait a second... if we can make observations like this, why would we even need to make calculations at all? sigh...

 

why would we even need to make calculations at all?

To demonstrate that you have understood Ohm's Law. Extra points for that.

 

My "theoretical" value of Vf would come from the rule-of-thumb that Vf is relatively constant for typical values of If, and is stated in the datasheet for at least one If.

From there I'd compute the theoretical If using ohms law (using a constant Vf).

There will then be a difference between the theoretical and actual for 5 reasons:

1. The actual Vf vs If diode curve that was not taken into account;

2. The difference between the actual and typical values of Vf at a given If;

3. The tolerance of the series resistors;

4. The voltage droop due to the non-zero output impedance of the Arduino I/O pins.

5. Temperature.

We are engineers. Good enough is good enough.

 

OMG.... omg, you beautiful man, you... How could I have missed that...? I must have skipped that entirely. Thank you. I feel silly.
okay, follow up... Since the resistor is placed before the LED, if we're observing (via serial monitor) the voltage at the point right after the resistor, isn't that the remaining voltage that will be dropped across the LED? and wait a second... if we can make observations like this, why would we even need to make calculations at all? sigh...

The trouble with that interpretation is that the calculation is simply:

Vled = 5 - Vr

There is no Ohm’s law involved, so that cannot be what he meant. Also, theoretical means without measuring, by math and physics only. A theoretical estimate should not invilve measurements.

I think the TS was right originally. It cannot be done from the information given.

It could be done by experimentally plotting the IV graph, then using that to predict the Vled for each resistor.

 

There is no Ohm’s law involved

It depends on the approach. A calculation of 5V-i(R)xR gives a theoretical answer for Vled.
(I'm playing devil's advocate here. The assignment could have been worded more clearly).

 

To demonstrate that you have understood Ohm's Law. Extra points for that.

I suppose so. I mean, now I get the feeling that we're not supposed to already understand that the test point, and the data on the serial monitor already tells us exactly what the voltage across the LED is... I mean, I knew what to expect when I saw the schematic. I've been running around assuming that we can't look at that data because that data tells us the answer that we're seeking in the first place. So I've been trying to calculate all of this without that information which is seemingly impossible. Now, I've just extrapolated every possible value for everything and threw it all in there. It just doesn't feel right. If it is that simple, then alright... It's an intro class. I think I liked the idea of an impossible solution better than this.
Thank you for your assistance.
- Hopeful EE

 

The trouble with that interpretation is that the calculation is simply:

Vled = 5 - Vr

There is no Ohm’s law involved, so that cannot be what he meant. Also, theoretical means without measuring, by math and physics only. A theoretical estimate should not invilve measurements.

I think the TS was right originally. It cannot be done from the information given.

It could be done by experimentally plotting the IV graph, then using that to predict the Vled for each resistor.

YES, SIR!!!!! This is exactly how I feel and I cannot stress enough my frustration with it... If we can make observations BEFORE calculations, WHAT'S THE POINT?! As mentioned, from the schematic, we already know that the data on the serial monitor should tell us what the voltage across the LED is... Which is why I'm so stuck on exactly what you said, we're supposed to make theoretical calculations BEFORE observations or measurements, WHICH WE CANNOT DO WITH THE INFORMATION GIVEN!
I just want to speak to my professor and have an actual conversation about it, but he's either unavailable or unwilling.
Oh well...

 

I think whomever set up this lab might have been trying to make a tweak to an earlier lab and messed things up.

The schematic that is provided does not measure the voltage across the resistor, but rather the voltage across the LED.

This would allow the person to calculate the voltage across the resistor and then calculated the current in the LED.

Note how this is consistent with the comments at the top of the supplied code:

// This code demonstrates the application of Ohm's law by measuring
// the voltage drop across each LED to infer the current flowing through them.
// Code Written by: Dr. Satinder Gill

The person that wrote the lab probably had something in mind but didn't think it through very carefully and probably never actually did the lab themselves, they just assumed that they thought it through correctly.

 

Ah, now it begins to make sense. I think the assignment was intended to have the student calculate the LED current from it’s Vf and the resistor and then compare it to the current predicted by using a fixed Vf for the LED. And somebody botched it.

 

Good day, everyone!

I have a question about a lab I'm doing for Intro to EE. So... We've set up an Arduino to power 4 LED circuits. Each with a different value resistor: 100Ω, 220Ω, 330Ω, and 1KΩ. All with a 5Vdc supply voltage. That's not the problem... That's simple. HOWEVER... we're asked to make a "theoretical" calculation of the voltage across the LED... That's the problem... This is impossible, is it not...? First off, I've been building circuits (especially LED circuits) and using Arduino for years... Typically, we take our LED and either read the data sheet or test with a meter to find the forward voltage, and then we use that and our desired current to calculate which resistor we should use for the circuit.

In this case, how can we calculate the voltage dropped across the LED? Let me be clear... the only information we are given is the respective values of the resistors and the supply voltage...

Our standard formula for voltage drop cannot be applied, right?


I emailed my professor to ask and this was his reply:

"Every LED is connected in series with a resistor. Some of the voltage will be dropped across each resistor and some across LED. Remember LED is a resistor as well.

Hope this helps."

SINCE WHEN DO WE CONSIDER AN LED A RESISTOR?!

Even if we could consider it a resistor, we aren't given its resistance. We aren't given forward voltage, or current... just the value of the resistor in the circuit...

Am I missing something? Am I over or underthinking this?

Thank you all for your help with this.

Respectfully,

Hopeful EE

Hi,

I can add a little here which might help.

First, the LED has basic characteristics similar to a diode, and both of these are taken to be voltage sources in the simplest models, as long as they are forward biased. That would mean you would first determine if they are forward biased or not. If they are not forward biased, then they are considered open circuit. That is the simplest model.
When forward biased they have a characteristic nominal voltage, like 2.2 volts, and since forward biased that means they are absorbing energy. This voltage is also considered to be constant. So we have a constant voltage absorbing power and so the equation could be written as:
R=v/i
where 'v' is the forward voltage and 'i' is the current. The current is determined from the other components in the system as well as this 'voltage source'.
This would make it work similar to a resistor, although it would be rare to call it a resistor. It might be better to call it a 'resistance' because that is exactly what it acts like in this kind of simple circuit.

As to what you do first and second and third, I suppose you have to follow the directions, but the best way I think is to first do the calculations, then do the lab measurements, then compare, and then calculate the differences you found. Then try to explain why there were differences between the original calculations and the measurements. You should be able to explain that at some point as that would mean you understand not only how to analyze circuits, but also what kind of variations we might see when going from the theoretical circuit to the practical circuit. A likely difference would be in the LED voltage, which might be 2.1 volts or even 2.3 volts, and that would mean the current would be different too. You could explore why that happened.

 

A better lab, and very possibly what was intended originally, would be to plot the four data points to show, roughly, how LED voltage and current are related. We assume a constant Vf, and such a lab would show to what degree that assumption is valid.

 

A better lab, and very possibly what was intended originally, would be to plot the four data points to show, roughly, how LED voltage and current are related. We assume a constant Vf, and such a lab would show to what degree that assumption is valid.

Far more educational.

Good way to teach "good enough is good enough".

 

A better lab, and very possibly what was intended originally, would be to plot the four data points to show, roughly, how LED voltage and current are related. We assume a constant Vf, and such a lab would show to what degree that assumption is valid.

I think. said that in post #29.

 

I think whomever set up this lab might have been trying to make a tweak to an earlier lab and messed things up.

The schematic that is provided does not measure the voltage across the resistor, but rather the voltage across the LED.

This would allow the person to calculate the voltage across the resistor and then calculated the current in the LED.

Note how this is consistent with the comments at the top of the supplied code:

// This code demonstrates the application of Ohm's law by measuring
// the voltage drop across each LED to infer the current flowing through them.
// Code Written by: Dr. Satinder Gill

The person that wrote the lab probably had something in mind but didn't think it through very carefully and probably never actually did the lab themselves, they just assumed that they thought it through correctly.

YES! This is exactly what I wrote to the professor! But he never responded to that. It really makes the entire lab completely pointless and I don't understand the point of it at all. We cannot make theoretical calculations without observing the data and if we observe the data, WE ALREADY HAVE THE ANSWER!!!

 

Ah, now it begins to make sense. I think the assignment was intended to have the student calculate the LED current from it’s Vf and the resistor and then compare it to the current predicted by using a fixed Vf for the LED. And somebody botched it.

You know... that makes about as much sense as anything else I can think of... Per the instructions, we're certainly supposed to "theoretically" calculate the voltage across the LED.... it's just stupid... a ridiculous lab for a ridiculous "professor" for a ridiculous class for a ridiculous school. all for a ridiculous piece of paper...

 

You know... that makes about as much sense as anything else I can think of... Per the instructions, we're certainly supposed to "theoretically" calculate the voltage across the LED.... it's just stupid... a ridiculous lab for a ridiculous "professor" for a ridiculous class for a ridiculous school. all for a ridiculous piece of paper...

And probably paying a ridiculous amount of money. So... why are you there?

I have no idea what school you are going to, so I have no idea how legitimate, or not, it is. But I have seen lots of students go $60k or more in debt to these diploma-mill schools and learn nothing, only to discover that having a "degree" from that school actually hurt their job prospects.

 

YES! This is exactly what I wrote to the professor! But he never responded to that. It really makes the entire lab completely pointless and I don't understand the point of it at all. We cannot make theoretical calculations without observing the data and if we observe the data, WE ALREADY HAVE THE ANSWER!!!

Hello again,

Sorry but that does not make sense to me.

We can do theoretical calculations without observing the data, and this is actually quite a norm in electrical theory.
We can also do measurements to observe that data, and that is actually quite a norm in practical electrical theory.

So we can do BOTH, and we very often do just that.

Maybe you are trying to meld theoretical calculations with measured data. Theoretical calculations are different than measured data by a long shot.

Example:
You have a circuit with two resistors and a voltage source. You do an analysis and come up with the expression:
Vout=Vin*R2/(R1+R2)
That's a theoretical expression not "data" in any way shape or form, and that is for a simple resistive voltage divider.

Next we do a measurement. Say Vin is 10 volts and R1 and R2 are both 1k each. We measure the voltage and we get:
Vout=5.05 volts. This is the 'data'.
We then use the theoretical expression to get Vout and we get:
Vout=5.00 volts. This is the theoretical result, not the data.

Now we investigate why we got two different answers.
One of the reasons we might be measuring 5.05 volts vs 5.00 volts is because the lower resistor R2 was actually 1020 Ohms not exactly 1000 Ohms. We can offer that as an explanation, or we can look for more possibilities, or we can come up with an expression or two that explains how we get different practical results than what we get from pure theory.

So you can see the difference between a theoretical result and measured data. Very often we need them both and it helps explain what we really have vs what we think we have, and could help solve a problem that came up in the real life circuit.

Another way the theoretical part is done is to do a sensitivity analysis. That's where we calculate the possible output voltages given the tolerance of the two resistors. In the above example I made one resistor higher by 2 percent, so a sensitivity analysis would show that if the resistors are 0.1 percent tolerance then something might not be right with one or both of them. If that analysis confirmed 2 percent was ok because we had 5 percent resistors, then everything is fine.

So you see the difference, and it could be very important to know how this all works. We might have to decide whether to use 5 percent, 2 percent, 1 percent, or even better quality resistors.

 

Another source to include is the accuracy of the measuring instruments, in this case, the digital volt meter.
How accurate is the DMM? It turns out that the absolute calibration of the DMM does not contribute to any error.

For example, suppose that you had calibrated the 10 V source using the DMM.
How accurate is the measurement at mid-point of 5 V? If the response of the DMM was linear across the measurement range, the results would still be valid even if the DMM was off calibration. If the DMM was non-linear or had a zero error, then you can anticipate an error while measuring at the mid-point.