Attached is a picture of a schematic.
My question is: let's say the current rating for the LED is 20mA and the forward voltage is 2.7V (max.)
Looking at the schematic, there is 5V before the LED and 2.62V after the LED before the resistor. Which number should I go off of, if I'm trying to make sure I don't burn out the LED? Do I need 2.7V before I reach the LED, or is this scenario okay?

 

If you calculate with 2.7V, and the led actually has 2.6V when the current is 20 mA, then the current will increase above what you calculated, so better leave a margin of lower Vf.
Also note that most modern leds are uncomfortably bright at 20mA, for an indicator I´d shoot for roughly 5mA.

 

Welcome to AAC.

The simulator you're using has already calculated the voltages and current. What more do you need?

BTW, schematics are easier to read if you follow the convention of having them flow from left-to-right and top-to-bottom.

 

Attached is a picture of a schematic.
My question is: let's say the current rating for the LED is 20mA and the forward voltage is 2.7V (max.)
Looking at the schematic, there is 5V before the LED and 2.62V after the LED before the resistor. Which number should I go off of, if I'm trying to make sure I don't burn out the LED? Do I need 2.7V before I reach the LED, or is this scenario okay?

That is not quite how it works.

If you are to assume that 17.4mA is flowing through the circuit, then the voltage across the 150Ω resistor is 17.4mA x 150Ω = 2.61V.
It means that the LED has 5V - 2.61V = 2.39V across it.

In reality, an LED is highly non-linear, that is, the resistance is not the same for all currents and voltages. In other words, it does not obey Ohm's Law.

The best way to approach the problem is, given that the LED is rated at 2.7V @ 20mA, then the resistor has to take 2.3V @ 20mA.
Hence the resistor value = 2.3V/0.02A = 115Ω

Hence choosing 150Ω is not terribly wrong. In fact, experiment with higher resistances, 220, 330, 470, 1000Ω and see if you can live with less bright LED. It means there will be less power consumed and the LED will last longer (though it should still last forever till the sun stops shining).

 

Attached is a picture of a schematic.
My question is: let's say the current rating for the LED is 20mA and the forward voltage is 2.7V (max.)
Looking at the schematic, there is 5V before the LED and 2.62V after the LED before the resistor. Which number should I go off of, if I'm trying to make sure I don't burn out the LED? Do I need 2.7V before I reach the LED, or is this scenario okay?

View attachment 116343

The goal should be to keep current below the rated current of the LED. The forward voltage helps you calculate that current if you are using a resistor to limit current as in your schematic. Note that you'll need a regulated voltage to keep that current in compliance.

The other option is a constant current source circuit to keep current in compliance for the LED what ever current you pick (1 mA to 20 mA).

 

That is not quite how it works.

If you are to assume that 17.4mA is flowing through the circuit, then the voltage across the 150Ω resistor is 17.4mA x 150Ω = 2.61V.
It means that the LED has 5V - 2.61V = 2.39V across it.

In reality, an LED is highly non-linear, that is, the resistance is not the same for all currents and voltages. In other words, it does not obey Ohm's Law.

The best way to approach the problem is, given that the LED is rated at 2.7V @ 20mA, then the resistor has to take 2.3V @ 20mA.
Hence the resistor value = 2.3V/0.02A = 115Ω

Hence choosing 150Ω is not terribly wrong. In fact, experiment with higher resistances, 220, 330, 470, 1000Ω and see if you can live with less bright LED. It means there will be less power consumed and the LED will last longer (though it should still last forever till the sun stops shining).

Thanks, MrChips. Your explanation was helpful.

 

I think you are mixing factual data and experimental data.

The factual data that you have is the manufacturer provided specification for the led:
2.7 V
20 mA
The power supply:
5 V

Using this data you calculate the resistor that you will need:
(5-2.7)/0.020=115 Ohm resistor

But. 115 Ohm is not a standard resistor. So whoever came up with that circuit slapped a standard 150 Ohm resistor.

Also. Real life components have tolerances. Some simulated models include those tolerances, that translates into values that you see in simulation that do not match pure math answers.

 

I think you are mixing factual data and experimental data.

The factual data that you have is the manufacturer provided specification for the led:
2.7 V
20 mA
The power supply:
5 V

Using this data you calculate the resistor that you will need:
(5-2.7)/0.020=115 Ohm resistor

But. 115 Ohm is not a standard resistor. So whoever came up with that circuit slapped a standard 150 Ohm resistor.

Also. Real life components have tolerances. Some simulated models include those tolerances, that translates into values that you see in simulation that do not match pure math answers.

Define standard.
Mouser alone has 17 different 1/4 W through-hole metal film resistors at 115 ohms.

 

Mouser alone has 17 different 1/4 W through-hole metal film resistors at 115 ohms.

You're right. It's getting to the point where, "standard" doesn't mean much.
There are, "standard" values in 5%, 10% and 1% resistors, but you can find almost any value at a vendor website.

 

Define standard.
Mouser alone has 17 different 1/4 W through-hole metal film resistors at 115 ohms.

My apologies. I simply googled a table of "standard" resistors. Obviously the table needs updating.

 

My apologies. I simply googled a table of "standard" resistors. Obviously the table needs updating.

I agree that "standard" falls into an E6 or E12 series for most definitions but I was just ordering resistors and getting pissed off about the never-ending scrolling required to get to the values I wanted. There are way too many out there - next time I am ordering from a supplier that lets me filter on tolerance or something to limit the number of choices.

 

Obviously the table needs updating.

Those are still called, "standards" but some of them were old when I started in the 1960's. They almost aren't relevant any more.

 

I agree that "standard" falls into an E6 or E12 series for most definitions but I was just ordering resistors and getting pissed off about the never-ending scrolling required to get to the values I wanted. There are way too many out there - next time I am ordering from a supplier that lets me filter on tolerance or something to limit the number of choices.

Wait. You use supplier that does not have filters? Ebay? Amazon is pretty bad too when you search for stuff there.

 

Wait. You use supplier that does not have filters? Ebay? Amazon is pretty bad too when you search for stuff there.

No, not generally. I was helping the local high school robotics club and the administration was so nice to force me to use their choice of suppliers and their Samsung tablet to create a cart full of parts for their approval. Mouser wasn't working so well - I couldn't scroll right to get all of the filters. They wouldn't let me connect my laptop to their network. I felt like I was punished and had to write 500 lines of "I will not offer to help ever again".

 

Attached is a picture of a schematic.
My question is: let's say the current rating for the LED is 20mA and the forward voltage is 2.7V (max.)
Looking at the schematic, there is 5V before the LED and 2.62V after the LED before the resistor. Which number should I go off of, if I'm trying to make sure I don't burn out the LED? Do I need 2.7V before I reach the LED, or is this scenario okay?

View attachment 116343

What matters is the voltage across the LED, not "at" the LED. I assume the voltage indicated is referenced to ground and is the voltage across the resistor. The rest of the voltage is dropped across the LED.

 

What matters is the voltage across the LED.

Not exactly. One has no control of the forward voltage so it doesn't really matter. As I said above, the circuit is not designed to reach a target Vf, it is designed to set (limit) current through the LED. The Vf is used to make that calculation per the method shown by @shteii01 above.

Remember, the values are Typical, not exact. Design for the possible values and error on the side of caution, not excess.

 

Not exactly. One has no control of the forward voltage so it doesn't really matter. As I said above, the circuit is not designed to reach a target Vf, it is designed to set (limit) current through the LED. The Vf is used to make that calculation per the method shown by @shteii01 above.

Remember, the values are Typical, not exact. Design for the possible values and error on the side of caution, not excess.

Re: not exactly

Technically correct, but as we change the current through the LED we change the voltage across it. Buy the point at issue was that it is not the voltage "at" the LED that was important since that does not indicate the voltage "across" the LED in all circumstances.