Using Resistors with LEDs

Boy, that escalated quickly... I only wanted an answer to something that seemed counter intuitive.

I constructed a test sketch that turns 3 LEDs on in off in every possible combination, using a separate 200 ohm resistor for each cathode (I have lots of those!) LED brightness never changed.

I then tried using a single 1k Ohm resistor and 2 200 ohm resistors in series tied commonly to each cathode. Each turned on fine, but I could not turn on all 3 at the same time. Too much current, I guess.

I'm going to spring for some 1k resistors (I know, my maker shopping list is quite long right now...), but I find it strange that it worked with the ArduinoISP sketch, but not this test sketch.

A note to Grumpy_Mike: I'd greatly appreciate it if you wouldn't try to "haze" me. I am not trying to "mislead beginners" and I don't deserve for anybody to "come down on me like a pile of bricks". Simple questions deserve simple answers, and I don't merit insult, even if perhaps my question is ridiculous.

If you are using 200 ohms in series with one LED you must use 200/3 ohms in series with three LEDs if you are connecting the LEDs in parallel and expect to keep the light intensity the same. You must use the equivalent of three 200 ohm resistors in parallel since you are using three LEDs in parallel. If I understand you correctly you are attempting to use a higher value resistor when you connect the LEDs in parallel. This is the opposite of what is required.

If you put different colors in parallel, only the lowest voltage LED will turn on. And hog all the current.

polymorph: If you put different colors in parallel, only the lowest voltage LED will turn on. And hog all the current.

As I said in reply #5

It seemed to bear repeating.

Indeed it does. :)

legonick22: LED brightness never changed.

Our eyes do not perceive brightness linearly.

See http://www.telescope-optics.net/eye_intensity_response.htm

From that:

In its basic form (Weber law), this implies that eye response to object luminance, as brightness discrimination, is not proportional to its actual (physical) intensity level; rather, that it changes with the intensity level, remaining nearly constant relative to it. This, in turn, under assumption that the relative value of just noticeable difference in brightness sensation is a unit of the sensation change, means that the perceived object brightness changes with the logarithm of object's actual brightness.

So for one thing, your perception changes as the log of the actual brightness, and secondly, depending on how you conducted the experiment, they may have "seemed" same if there was a pause between different tests.

Just as an experiment, I hooked up a LED connected to a 5V supply via a resistor substitution gadget. I could barely tell the difference in brightness between 100 ohm and 1k although the current must have varied by 10 times.

Certainly, small changes were imperceptible, even 820 ohms to 220 ohms.

So the "it seems to work" doesn't really hold up here. Now if the LEDs are all the same, you might get away with sharing a 1k resistor, but honestly, it is best to do it properly.

A while ago I made a clock ( here ) where I lazily shared a current limiting resistor for all of the segments of a 7-segment display. That should work, huh? The trouble was, the more segments that were on, the dimmer it got. So for example, an 8 (all segments) was noticeably dimmer than a 1 (two segments).

So even for a hobby gadget, the results are just annoyingly noticeable when you finish the project, if you cut corners.

You can even see this effect in this photo of it:

charliesixpack:
My hobbies are to please only me.

Is this one of yours then?

hobby.jpg

People should stop blindly copying old circuits and look around at what's available today.

This comment sort of bugged me as I would beg to differ. 5 minutes of hunting on Instructables will show you many more of today's fools giving bad advice as compared to people who learned how to be good techs when a "super sale" on LED's meant you got 5 for $1.00. As Grumpy_Mike said, this is about physics and that doesn't really change. Good electronics technician bench top practices have not drastically changed either.

Sure, some LED's are brighter, like the ones who's datasheet says "1000MCD @ 10mA with 2.1V forward voltage" but that is... wait for it... "datasheet and device specific". Many LED's operate at what I would call normal brightness versus the trendy "blind a gnat at 200 feet" brightness but it comes down to 1) buying the right part and 2) "Oh My Gosh" reading whatever data is available about the LED. It might even have a datasheet if you selected and bought it from a reputable vendor.

So here is my point...

Different color LEDS are made varying the semiconductor content in the die. This different "content" can mean that LED's require different voltages and current to operate most efficiently (as Grumpy_mike mentioned). It is these parameters that guide the designer/hacker/parts-abuser in determining the correct current limiting resistor for a specific device and using just any just any old resistor part lying around (or none at all) will only get you so far.

pwillard: This comment sort of bugged me as I would beg to differ.

PWillard, I'm puzzled because I see little difference in what you're saying, from what I said. Or at least, they may address different subjects.

But... a data sheet can't tell you the correct value of a limiting resistor. It can only tell you the maximum allowable and typical current values. My point was that the minimum is a user choice.

By "look around", I didn't mean on forums. :)

There's not much to be said for the OP's laziness. He would have a hard time defending his approach on the grounds he can't afford the additional resistors. To even make a statement on the forum like:

since I only have a few 1k resistors.

really leaves one shaking their head. What's the problem, you can't afford more or you don't know how to find them ?

Nevermind that 1 k is the wrong value. We can address that when he learns to wire his circuit correctly.

raschemmel: Nevermind that 1 k is the wrong value. We can address that when he learns to wire his circuit correctly.

Is it wrong?

At least you can't do much damage with 1k.

Is it wrong?

Let me check:

Let Vcc = 5V Let LED Vf=2.3V @ 20 mA 5V - 2.3V = 2.7V RCL = 2.7 V/0.020 V = 135 ohms

1 k != 135 ohms

Note: The OP's using RGB leds so his results (forward voltages, all different for R, G & B) may vary.

Yes, it's wrong.

raschemmel: 1 k != 135 ohms

No, but with modern LEDs 2.5 mA will be plenty visible but will not burn your eyes. And it certainly won't burn anything else. :grinning:

A LED can't burn your eyes or they wouldn't sell them.

Look at the value of LED current limiting resistors RN2A, RN2B and RN2C on the UNO. 1.0K.

;)

raschemmel: Nevermind that 1 k is the wrong value. We can address that when he learns to wire his circuit correctly.

As I mentioned earlier when I tested with different resistances, I could hardly tell the difference visually between 1k and 100 ohms. So 1k would "work" and use less current.

100 ohms. So 1k would "work" and use less current.

can't argue with that

raschemmel:
A LED can’t burn your eyes or they wouldn’t sell them.

I don’t think he meant it literally. Super-bright LEDs (particularly the blue ones, in my experience) really are quite bright unless there’s very little current–a milliamp or so–running through them. They’re bright enough to be painful and cause at least temporary blindness if you look directly into the LED.