As usual, Alex at Tinkerlog does an excellent job of clearly describing some electronics arcana, in this case, exactly why one needs to use current limiting resistors on LEDs (and when you can do without them).
If you apply a specific voltage to a resistor, you can compute the resulting current with:
I = V / R Example: I = 5 Volt / 100 Ohm = 50 mA
Obviously that does not work with LEDs because they don’t behave like a linear resistor. If you look at the graph above, you can rise the voltage from 0 Volt to 1.6 Volt without resulting in noticeable current. Apply a bit more voltage and there is current and the LED lights up. We have reached the Forward Voltage which is needed to open the pn-gate. Forward Voltage (VF) for a typical red LED is 1.7 to 2.2 Volt. Now small changes in the voltage produce large effects on the resulting forward current (IF). Datasheets normally state at least the absolute maximum ratings for IF, eg. 25 mA. If you apply a voltage that results in a larger current, the LED may be destroyed.
20 thoughts on “When to use a resistor with an LED (and why)”
Comments are closed.
ADVERTISEMENT
Join Make: Community Today
That quote is VERY poorly worded and would only serve to further confuse someone who’s already having trouble understanding it. It’s trying to squeeze the LED into some weird special case device, saying its like a resistor, only it has these weird properties at certain voltages.
He’s trying to explain the LED as a strange voltage device when the reality is it’s a very basic, straightforward current device. Rather than mangling the wrong model to try and fit the device, why not give a basic explaination of the correct model?
Every single statement of that quoted paragraph is garbage, starting with the fact that you can’t apply a very specific voltage to an LED. Depending on what your power supply can handle, either you load down the supply until its voltage drops to Vf or current becomes so high that using E=IR, the voltage drop across the low resistance of the wires drops the remaining voltage and you still have Vf across the LED. You simply do not apply voltages to LEDs, it is impossible in physics and if you think you are, you are totally misunderstanding what the circuit is doing and how it works.
> It’s trying to squeeze the LED into some weird special case device, saying its like a resistor, only it has these weird properties at certain voltages.
I wrote: Obviously that does not work with LEDs because they don’t behave like a linear resistor.
And you always do apply voltage to something, not current. You have to apply a voltage to a p-n junction to let the current (the elctrons) flow.
Not only may the LED get destroyed, but it may also shatter and expel the pieces very quickly in every direction. If you are as lucky as I was, it will hit the bridge of your nose instead of your eye.
Wow… :| Thanks for the advice…
Augh, this article is terrible. Unless you are using a current-limited source, the answer is “Always”… always use a current-limiting resistor or you are asking for trouble (anywhere from minor trouble: your LED burns out; to bigger trouble: your LED pops and catches fire). The method of using PWM to limit current in this situation is _completely_ incorrect. PWM applies full voltage for a short period of time, and no voltage for a short period of time, to achive “on average” a particular voltage or current, but the important part of PWM is for a short period of time, it is just the same as if you had connected the power to the LED directly, which can cause damage to electrical devices in nanoseconds, not just milliseconds or seconds. Now, it may be the case that you can get away with something like this for quite some time, possibly for a long long time without issue (most microcontrollers have a limited source/sink current, which is effectively a current-limiting resistor built in, but not as good because you could fry your microcontroller if you are unlucky); but the way the author is explaining that this is OK to do is completely wrong.
“And you always do apply voltage to something, not current” — wow, just wow…please, please, study electrical engineering at least a little bit before writing any moore articles that will only confuse people.
To say it doesn’t behave like a linear resistor implies it somehow fits in the general mould of a resistor just a bit different.
And as “J” says, the *only* answer to the question of when to use a resistor with an LED is ALWAYS.
Anything that doesn’t follow a linear V/I curve is technically a non linear device. Any rectifier or transistor is a non linear device. Even a neon lamp, tube, florescent tube, and even lightning and incandescent light bulbs are non linear. The thing to remember about any electronic device is to not go past its power rating. LED’s have an easy to know when you are getting outside of their power rating. Most LEDs have a certain forward voltage drop. This forward drop varies between devices, so it’s not a good idea to run them from a constant voltage source. The current through any batch of LEDs will remain fairly constant for a given power level. The PWM for LEDs for current control is often used when resistors would make it too expensive. Most LEDs have a ability to handle a lot of current for a short period in time. If you average the on/off time the average power rating of the LED is within specs. If you want to have fun blowing up LEDs with a lot of current, try freezing them. The cold allows the LED to dissipate more power than it’s supposed to.
To add to the others: What should that I/V diagram for the resistor do there? If you use a graph to design for a LED you would draw the LED’s I/V curve and a load line, not the resistor I/V curve as given in the diagram.
In synthesis you would select the operating point of the LED. Together with the supply voltage you then have two points to draw the load line. From the load line you would determine the required resistor.
In circuit analysis you would determine the load line from the resistor used in the circuit and from the supply voltage. And you would find the LED’s operating point from drawing the load line.
You are all ZERO help to a beginer. You just argue and use PWM…you didn’t tell us what that stands for. Came to this site to find out what SIZE resistor to use for an LED. 470, 560, 12k ????? How do you figure it out. Voltage drop, series, parallel???
Amen! What a cluster f*%k excuse for helpful info here!