Voltage and current measurement in a simple circuit with LED

Thread Starter

JohnyCage

Joined Dec 1, 2021
4
Hi, because of my son's new interest, I started playing virtual circuits. I created a simple flashlight with a saltpeter. I created this circuit in three different simulators. What was my surprise when I wanted to measure electrical quantities, voltages and currents and I found that the measured values differed significantly. I enclose a picture with a measurement. Where is the mistake please?all circuits.jpg
 

bertus

Joined Apr 5, 2008
21,763
Hello,

In the upper part of the picture, the multimeter is on 12 V AC.
Switch it to 3 V DC and see what happens.
Also, you are using a 1200 Ohm resistor in stead of a 220 Ohms resistor as in the other pictures.

Is this only in simulation or did you also test in real live?

Bertus
 

crutschow

Joined Mar 14, 2008
28,538
Depending upon the model characteristics, the LED may have slightly different forward voltage drops at the same current in the different simulators.

You should build a real circuit and see how it compares with the simulations.
Note that a 5 to 10% difference between a simulation and a real circuit is not unusual.
 

ApacheKid

Joined Jan 12, 2015
590
The upper large image seems consistent, as bertus says you actually have a 1.2K resistor and with a current of 2.94 mA that would drop about 3.53v meaning the LED must be dropping 5 - 3.528 which is 1.472v.

I any of these circuits the LED voltage will always be 5 - (i x R) where's i is the measured current and R is the resistor.

In the bottom left image the current reading 22.73 mA is exactly what we'd expect if the LED was a short circuit, for that current to flow through that resistor (220 Ohm) means the entire 5v is across the resistor so perhaps you set that up wrong? did you somehow put the ammeter across the LED? (thereby short circuiting it).

The bottom right image like the first image, seems correct, consistent. So the first image is correct (but the resistor is not what you thought it was) and the bottom right image is correct, but the bottom left image implies you did something wrong.
 
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bertus

Joined Apr 5, 2008
21,763
Hello,

Also a current is in SERIES with the elements.
You have the meter PARALLEL with the led, wich will give you a wrong measurement.

Bertus
 

Thread Starter

JohnyCage

Joined Dec 1, 2021
4
Hello,

In the upper part of the picture, the multimeter is on 12 V AC.
Switch it to 3 V DC and see what happens.
Also, you are using a 1200 Ohm resistor in stead of a 220 Ohms resistor as in the other pictures.

Is this only in simulation or did you also test in real live?

Bertus
Hi, I've made adjustments and attached a picture. The difference in values is still large. So where else can there be a mistake?
 

Attachments

ApacheKid

Joined Jan 12, 2015
590
Hi, I've made adjustments and attached a picture. The difference in values is still large. So where else can there be a mistake?
No mistake, that voltage on the LED of 1.56 v is pretty much what we'd expect, the resistor must have 3.44v across it given a current of 15.65 mA.

I think you're expecting the new image to show the very same readings as the bottom right image, after all the voltage is the same and so is the resistance, so why the difference?

The difference will be caused by the simulators assuming different characteristics for the LED in each case. You can buy a mix of red LEDs and experiment, you'll find that although the look the same and may have the same rated forward current, they are not the same, there are many differences and although small they will certainly lead to the kind of thing you're seeing.

LED's also are not linear, the current through them vs voltage across is not a straight line (as it is for a resistor) this too will play a role.

How did you tell the simulator what kind, brand, model of LED to use? In reality (except for common hobby use) LEDs have well defined characteristics and the model number will let you see them. Most LEDs I see are unlabeled and we know next to nothing about them.
 
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crutschow

Joined Mar 14, 2008
28,538
The difference in values is still large
That's because, as bertus noted, in the other two measurements you put the ammeter in parallel with the LED instead of in series with the LED.
With it in parallel, you are measuring the current through the resistor with the LED current being shorted out by the low resistance of the ammeter (thus 5V / 220Ω = 22.7mA).
Remember that a perfect voltmeter has infinite input impedance, and a perfect ammeter has zero input impedance (obtainable in a simulator but not quite in the real world).

Measuring rule no. 1: Voltmeters connect in parallel, ammeters connect in series.
Mixing the two can cause interesting (and perhaps magic smoke) results in the real world.
 
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bertus

Joined Apr 5, 2008
21,763
Hello,

The forward voltages of the leds depend on the materials the leds are made of, wich also creates the different colors.
Here is a picture with an example V-I curve of leds with different colors:
IV-curves-all-colours.png
Here is also a list of colors with the uesed matarials:
led color guide.png

Bertus
 

dl324

Joined Mar 30, 2015
13,836
Welcome to AAC!
The difference in values is still large.
The 3 simulations have different operating currents. Even at the same current, a half volt difference among LEDs of the same type isn't unusual. You appear to be using a different color in each simulator.
OSRAM
1638386497861.png
1638386477378.png
1638387115572.png
because of my son's new interest, I started playing virtual circuits.
Ditch the simulator and buy some components, batteries, and a DVM. You and your son will learn more. That's why you learn how to do arithmetic in school before they let you use calculators.

While I don't condone destroying components intentionally, the lessons from doing that accidentally will stay with you longer (the awful smell, maybe a burned finger, and a small monetary cost will be incentive to be careful). Just use 9V batteries that can't source much current.
 
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ElectricSpidey

Joined Dec 2, 2017
2,024
Where is the mistake please?

The mistake is coming to an electronics help forum expecting to learn the basics instead of doing that first.

Now go take a proper course before you embarrass yourself in front of your son, or worse.
 

Thread Starter

JohnyCage

Joined Dec 1, 2021
4
No mistake, that voltage on the LED of 1.56 v is pretty much what we'd expect, the resistor must have 3.44v across it given a current of 15.65 mA.

I think you're expecting the new image to show the very same readings as the bottom right image, after all the voltage is the same and so is the resistance, so why the difference?

The difference will be caused by the simulators assuming different characteristics for the LED in each case. You can buy a mix of red LEDs and experiment, you'll find that although the look the same and may have the same rated forward current, they are not the same, there are many differences and although small they will certainly lead to the kind of thing you're seeing.

LED's also are not linear, the current through them vs voltage across is not a straight line (as it is for a resistor) this too will play a role.

How did you tell the simulator what kind, brand, model of LED to use? In reality (except for common hobby use) LEDs have well defined characteristics and the model number will let you see them. Most LEDs I see are unlabeled and we know next to nothing about them.
[/CITÁT]

Thank you so much for your tips and other discussants. I have corrected what you are writing about and I am attaching an updated image. If something is still wrong, please advise what and possibly how to fix it.
View attachment 254000
 

ApacheKid

Joined Jan 12, 2015
590
Nothing looks wrong, each of the three diagrams is consistent with reasonable expectations, the reason the readings differ in each case is the LED itself, each simulation likely assumes slightly different characteristics, no doubt very similar but different and that's why we see the differences in each simulation.

You can see from Bertus's image:

1638394461872.png

How all diodes (not only LEDS) have what appears like infinite resistance until a small voltage around 0.6 v is present at that point conduction begins, resistance falls. As voltage then rises further the diode's resistance rapidly falls and it starts to behave more like a resistor, that's what the straight angled lines are showing above.

The reason we mention this is that the specific readings one will get reflect this behavior, the LED assumed in one simulator may begin to conduct at 0.600 v but in another simulator that might be 0.675 v and so on.

In order for a simulator to truly simulate a diode it will need to do what's shown in that graph, it will need to be able to calculate the forward current for any applied voltage, that might be some mathematical function or it might be some table of values but it will do this, it has to do this to truly simulate a diode.

How did you tell each simulator what kind of LED to use? did it just offer you "red LED" or did it offer more options for choosing this? perhaps there's a way to specify the LED's characteristics, I'm not familiar with these tools myself.
 
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