# Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs



## fyrstormer (Jun 30, 2018)

*Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

In my RC hobby, I make frequent use of old-fashioned 5mm and 3mm thru-hole LEDs running at low power, because they're only being used for illuminating the vehicle so I can see where it's going at night, not for lighting up the path in front of it. The power supply is regulated by the RC electronics, generally to a fixed voltage of 6V, so I can get away with using simple resistors as current-limiters for the LEDs so they don't burn out. The formula of

Rrequired​ = (Vsupply​ - Vforward​) / Itarget​

is easy enough, but I'd like to factor-in the LEDs' Rinternal​ as well, so I can subtract it from Rrequired​.

I know ideal LEDs have no internal resistance, but I also know real LEDs are not ideal LEDs, and at the low amperage levels I'm working with (40mA for white LEDs, 10mA for yellow/orange/red LEDs), the LED's internal resistance becomes a significant factor.

If I were _really_ motivated, I could rig up some electronics to measure the LEDs' actual internal resistance, but I'm not that motivated. What I'd like instead is some "close enough" estimates of the typical internal resistance for red (1.6Vf), orange (2.0Vf), yellow (2.2Vf), and white (3.2Vf) LEDs, so I can plug them into my resistor-calculating spreadsheet. Can anyone help me with this?


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## DIWdiver (Jun 30, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

The internal "resistance" of a diode isn't a fixed number. It varies quite a bit with current. Fortunately, it's generally small enough that if you have a volt or so on your external resistor, you can pretty safely ignore the internal resistance of the LED, as it will be quite small compared to your external resistor.

In fact, if you know or measure Vf at the current you are planning to run them, the internal resistance is already factored into that.


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## fyrstormer (Jul 1, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

The resistors I'm working with are as low as 56 ohms, and the current is as low as 10 milliamps. The internal resistance of the LED is a significant factor in those circumstances. Furthermore, I can't calculate what the current flow would be _without_ an external resistor unless I know what the LED's internal resistance is. For my purposes I don't need an exact value, it just needs to be in the ballpark.


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## DIWdiver (Jul 1, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

The 'resistance' of the LED is nothing but the slope of the V-I curve at the point of interest. It's quite easy to measure.

Using a supply at least several volts higher than Vf, test with two resistors of close, but not equal value (or one large one and one small one to add in series). Make two measurements of V and I near the current you want to use. Then R = dV/dI or R = (V1-V2)/(I1-I2).


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## Lynx_Arc (Jul 1, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

There are resistor LED calculators online that will do the job for you. 
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor
This one says a 1.6v led at 10ma with a 6v supply needs about 440 ohms resistor.


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## fyrstormer (Aug 25, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*



Lynx_Arc said:


> There are resistor LED calculators online that will do the job for you.
> https://www.digikey.com/en/resource...ors/conversion-calculator-led-series-resistor
> This one says a 1.6v led at 10ma with a 6v supply needs about 440 ohms resistor.


I'm not looking for a calculator that will help me determine how much external resistance to add -- I already know how to do that. I'm looking for information about how much internal resistance the LEDs have. They aren't superconductors, so they must have non-zero internal resistance. Once I know their internal resistance, I can subtract that from the recommended external resistor to figure out how much external resistance I should _actually_ add to the circuit.



DIWdiver said:


> The 'resistance' of the LED is nothing but the slope of the V-I curve at the point of interest. It's quite easy to measure.


The 'resistance' of _any _electrical component is "nothing but" the slope of the V/I curve at the point of interest. Not sure why you decided to use scare-quotes around the word "resistance", as if I'm using the term incorrectly, when what you just described is in fact the mathematical definition of the thing I'm asking about.



DIWdiver said:


> Using a supply at least several volts higher than Vf, test with two resistors of close, but not equal value (or one large one and one small one to add in series). Make two measurements of V and I near the current you want to use. Then R = dV/dI or R = (V1-V2)/(I1-I2).


Yes, I could do that, but I don't have an adjustable power supply and I'm not going to spend hundreds of dollars to get one when all I want is some information about LEDs that cost a few cents apiece. All I want is a basic estimate of the internal resistance of generic thru-lead LEDs of various colors. This information must exist somewhere.

Don't tell me the internal resistance is zero, or effectively zero, because that is nonsense. If that were true, then I could connect a white LED with a 3.2Vf to a 3.6V LiFe battery and the battery would explode from being short-circuited -- even 0.4V above the LED's Vf rating would be enough to push every amp the battery can muster through the LED if it actually had zero internal resistance.. I know from prior experience that doesn't happen -- in fact, the LED doesn't even light up particularly brightly -- so the LED's internal resistance must be fairly substantial.

I understand that LEDs are not ideal resistors and therefore do not exhibit the exact same resistance at all supply voltages, all amperage throughputs, and all temperatures. That's fine. I'm just looking for a rough estimate in the range of 10-50mA at a supply voltage of 6V.


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## DIWdiver (Aug 26, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*

What I've been trying (apparently ineffectively) to explain is that there is not a single answer to your question. The resistance varies quite considerably with how hard you drive the LED. And since you are trying to get a number to determine how hard to drive the LED, you are asking for something that doesn't exist. It will differ from one model LED to another, from one color to another, and from one drive current to another.

Driving from 6V, you are far better off estimating Vf of each LED you want to drive from the data sheet, then calculating the resistor you need to use by (6-Vf)/I. Trying to calculate (or worse, to know without calculating) _and make use of_ the resistance of an LED is a fool's errand.

I am an electronics engineer. Over the last forty+ years I have used LEDs for indication, isolation, data transmission, displays, backlights, and general illumination in hundreds of projects. I have NEVER had a need to know the resistance of an LED. Because it depends so much on the conditions under which it is measured, it's simply not a useful general purpose piece of information.


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## Lynx_Arc (Aug 26, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*



fyrstormer said:


> I understand that LEDs are not ideal resistors and therefore do not exhibit the exact same resistance at all supply voltages, all amperage throughputs, and all temperatures. That's fine. I'm just looking for a rough estimate in the range of 10-50mA at a supply voltage of 6V.


The problem is that LEDs exhibit higher efficiency at lower current drive levels and when they reach Vf or more the efficiency drops and 10-50ma is closer to the entire range of an LED than rather an small range where you could get more of a rough estimate. My advice is to use a linear regulator instead of a resistor and use it thus eliminating any guesswork involved.


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## MeMeMe (Aug 27, 2018)

*Re: Looking for some "close enough" internal resistance values for 5mm thru-hole LEDs*



fyrstormer said:


> If I were _really_ motivated, I could rig up some electronics to measure the LEDs' actual internal resistance, but I'm not that motivated. What I'd like instead is some "close enough" estimates of the typical internal resistance for red (1.6Vf), orange (2.0Vf), yellow (2.2Vf), and white (3.2Vf) LEDs, so I can plug them into my resistor-calculating spreadsheet. Can anyone help me with this?



Frequent lurker, not a poster, but found the responses to this thread frustrating .. so .. Well you will have to be a bit motivated as I can tell you how to do it without making any measurements, but I will leave the actual work to you.

Every LED data sheet has a Vf vs. I curve. Pick some representative parts similar to what you are going to use, look up a few Vf/I pairs on the graph, say at 10mA, 30mA, and 60mA, and calculate the resistance. As the Vf is quite different between colors and technology, you will likely need to do this for a RED (AlInGaAp technology), Yellow, Blue (same as white), and Green, but careful on the type of green, as it can be yellowy green (AlInGaAp) or a more pure green (InGaAn).

You will need to pick a LED rated for a similar current to what you use, i.e. a 20mA unit, or your numbers will be way off.

While the resistance will vary a lot between different LED types, contrary to the posts above, it will be relatively consistent between 10 and 60mA for a 20mA LED.

Again, contrary to the posts above, the internal resistance of an LED is useful and necessary in many cases, i.e. estimating current ripple, estimating current mismatch in parallel strings (or series-parallel), as a parameter in LED power supply control loops, etc.


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