7 ms·

Calculating Current Limiting Resistor Values for LED Circuits

51 points by lightlyused 5y ago

po1nt 5y ago
This is nice article but every engineer would say this is oversimplification for simple usecases like status LEDs on router. LEDs are tricky to make behave predictably as they are non-linear components, sensitive to temperature and no two LEDs are made the same. This really shows when connected in parallel. Also different colors of LED have different efficiency. These issues are more prevalent if you're trying to make high-power LED circuits or some RGB circuit with accurate representation of colors. Always thought LEDs are my friend but they made me lose some hair in last years.
- tyingq 5y ago
  The oversimplification for me is not talking about PWM. Pulsing to control brightness is extremely common, and typically sends more current (smaller resistors).
  - MayeulC 5y ago
    Putting LEDs in parallel isn't a good idea. Using a resistor for each is better: LEDs are different one from another, so the one with a lower voltage drop will have more current flowing trough it. A lot more, and it will heat more. Luckily, heating it up will make its internal resistance higher, so all LEDs will end up with the same current. It will shine brighter though, and fail earlier, especially if it heats a lot. What happens after it fails? If it fails open, which is usually the case IIRC, more current flows trough the other LEDs, making them fail earlier. If it fails with a short, the other LEDs turn off, and the resistor dissipates all the current (with a higher voltage drop, so more current). If you're lucky, it stays at that. If not, you can burn the resistor, and/or the power supply due to increased current. Always use a single resistor for each LED, except in very specific cases (like putting them in series). Don't put diodes in parallel.
    - tyingq 5y ago
      I'm confused what "don't put LEDs in parallel" has to do with PWM. Did you maybe reply to the wrong comment?
      - MayeulC 5y ago
        I'm replying to: > The oversimplification for me is not talking about PWM Omitting PWM isn't very important IMO; showing diodes in parallel like this sounds like a critical omission.
        tyingq 5y ago
        Ok, I'll try and explain. The title of the article is "Calculating Current Limiting Resistor Values...". How you do that calculation is entirely different when you use PWM. Hence, I felt like it was an oversimplification around calculating current limiting resistor values. Also, there are some cases where LEDs have to be in parallel, for practical purposes. Like LED matrix displays, or things meant to be inexpensive and not durable, like holiday lights.
        mindslight 5y ago
        Matrix displays don't drive LEDs in parallel - each leg is a different shared connection that you multiplex over time. Holiday lights generally aren't in parallel, as driving in series makes for more efficiency (higher voltage) and easier wiring. Once strings are long enough they're probably put directly in parallel, but that's just cheapos being cheap and relying on statistics.
        deleted 5y ago
        [deleted]
        tyingq 5y ago
        >Matrix displays don't drive LEDs in parallel Many of them do, especially the outdoor single color ones, where 1 led per pixel isn't bright enough. >Holiday lights generally aren't in parallel Granted, "holiday lights" is a broad category, but there are many that are obviously in parallel. Two 1.5 volt batteries couldn't power many LEDs in a serial string. And many of the AC strings are parallel strings of serial connected LEDS as well. Like: https://midimagic.sgc-hosting.com/chrsleds.htm https://midimagic.sgc-hosting.com/chrsleds.htm
  - foobarian 5y ago
    Using analog brightness control is nice because there is no flicker, and there is no stepping. But, with common potentiometers there is only a very narrow operating band due to how nonlinear the LEDS are, and it tends to be pretty touchy due to construction of the pot. I would love to figure out how to make an arbitrary transfer function shape using a pot as input, and some combination of transistors/op-amps to keep the nice analog behavior.
    - JKCalhoun 5y ago
      Perhaps a feedback loop then with a small photo-resistor situated beneath the LED (looking up between its legs?).
    - sdfasf 5y ago
      Flicker sucks, but LEDs are much more efficient when driven hard. So PWM is much more energy efficient for a given luminosity. That being said, if you're not on battery power, we're talking negligible energy. So, put a current source in there and stop giving me a seizure with your flicker.
    - MayeulC 5y ago
      What if you replace your potentiometer with a DAC plus a low-pass filter (a RC circuit)?
    - tyingq 5y ago
      That would be some sort of constant current power supply.
- dvh 5y ago
  Oversimplification would be putting 220 ohms everywhere.
  - analog31 5y ago
    Guilty. ;-)
- jcims 5y ago
  I got into the ‘stage dads (and sister)’ group for my kids show choir. Our job was to take the directors vision for set design, convert it into something that the kids could build and then supervise them building it. I got lighting because i was handy with electronics, and that sent me down a rabbit hole of the vagaries of current limiting circuits, buck/boost converters, color dependent forward voltage, temperature management, cabling and dmx control. All while staying on a modest budget and maintaining sufficient safety and durability that a bunch of highschoolers can build it and set it up and tear it down in a timed set change at high school gyms and theaters across the country. Very interesting experience, highly recommended and i concur, slapping a resistor on it to limit current is focusing on a small and somewhat straightforward portion of the LED spectrum (har har). That said, it’s still useful for folks that just want to slap a light on something.
- segfaultbuserr 5y ago
  +1. Unless you really know what you're doing, LEDs in parallel is a bad idea. A slight forward voltage or thermal mismatch causes one LED to dominate the current consumption until it fries. Then the cycle repeats for the next LED. When I was an ignorant child I fried $60 UV LEDs, but it was a good lesson... But at least the article mentioned it, > It's important that there are n LEDs (connected in series) in each of the m parallel branches of the circuit and that the LEDs all have the same Vled and Iled. Otherwise, all bets are off.
- mindslight 5y ago
  I'd say this article is overcomplicated. You don't need to drive status indicators at full current. In fact, my eyes would prefer you don't. 1k and done. If you're using LEDs for illumination, then do some math. Also, as a general rule don't put LEDs in parallel unless you really know what you're doing with matching I-V curves.
amelius 5y ago
Digikey has some nice in-depth articles about driving LEDs, e.g.: https://www.digikey.com/en/articles/driving-leds-to-cap-or-not-to-cap https://www.digikey.com/en/articles/driving-leds-to-cap-or-n... https://www.digikey.com/en/articles/optimizing-led-performance https://www.digikey.com/en/articles/optimizing-led-performan... https://www.digikey.com/en/articles/maximizing-led-luminosity-to-drive-down-system-cost https://www.digikey.com/en/articles/maximizing-led-luminosit... https://www.digikey.com/en/articles/driving-leds--choosing-between-analog-and-digital-topologies https://www.digikey.com/en/articles/driving-leds--choosing-b... https://www.digikey.com/en/articles/driving-leds-how-to-choose-the-right-power-supply https://www.digikey.com/en/articles/driving-leds-how-to-choo... https://www.digikey.com/en/articles/ac-leds-gain-in-popularity https://www.digikey.com/en/articles/ac-leds-gain-in-populari... https://www.digikey.com/en/articles/how-to-drive-multicolor-leds https://www.digikey.com/en/articles/how-to-drive-multicolor-...
unwind 5y ago
It seems to be an article-length piece, leading up to a downloadable spreadsheet. That's ... not so accessible. I quickly found [1] at Digi-Key which is a simple online calculator. It also features a handy table of reference voltages for various colors of LED, which I appreciated. Ideally I guess the color should be choosable and used to populate the voltage, but that seems to be Super-Advanced Thinking in this area. The full set of calculators [2] seems quite impressive and useful, too. [1]: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor https://www.digikey.com/en/resources/conversion-calculators/... [2]: https://www.digikey.com/en/resources/online-conversion-calculators https://www.digikey.com/en/resources/online-conversion-calcu...
- JKCalhoun 5y ago
  Plug: wrote an iOS app years ago that provided some simple electronic calculators for these kinds of things: https://apps.apple.com/us/app/perfboard/id465951871 https://apps.apple.com/us/app/perfboard/id465951871
londons_explore 5y ago
It's worth noting that while it isn't within spec, and certainly isn't good engineering practice, in most cases connecting an LED directly to a microcontroller output pin with no resistor works just fine. Most microcontrollers have effectively a fixed output drive current of something between 10 milliamps and 50 milliamps (which depends on the size of the drive mosfet on the output pin, and might be different for pull-high vs pull-low mosfets). Most LED's can handle that just fine. The energy that would have been dissipated in the resistor ends up dissipated in the microcontroller instead, but that normally doesn't lead to issues.
- makoto12 5y ago
  That sounds really dangerous. You're depending on circuitry built to protect the microcontroller, to limit the current. Sounds like a good way to kill the LED, but worse also kill the microcontroller. Also I'm quite skeptical at the number of microcontrollers that have this protective circuitry?
  - qazxcvbnmlp 5y ago
    Many modern MCUs have GPIO that can be configured via software to have an internal resistance in line with the GPIO. This resistance will limit current to within the specs of the MCU just fine.
  - MayeulC 5y ago
    True that: I know very few microcontrolers with overcurrent protection, you usually permanently disable a pin when draining too much current. I would still use a resistor with it, though having the microcontroller supply the current is usually fine. Protip: microcontrollers can usually absorb more current with pull-downs (due to n-mosfets, plus internal power supply routing) than they can supply. So have VCC -> res -> LED -> µC and turn the LED on by writing a 0 to the GPIO. Make sure the µC doesn't overheat, and you can drive a lot of LEDs (or other circuits) like this. Double check the datasheets for the max ratings, of course.
    - tzs 5y ago
linker3000 5y ago
https://www.reddit.com/r/AskElectronics/wiki/design/leds https://www.reddit.com/r/AskElectronics/wiki/design/leds
compumike 5y ago
Even when you apply the formulas in the linked spreadsheet, you can get into trouble because it seems to assume V_led is fixed. For an example of how quickly this can go wrong, see my accepted answer to “Why is my LED circuit drawing 50% more mA than I calculated?” https://electronics.stackexchange.com/questions/572739/why-is-my-led-circuit-drawing-50-more-ma-than-i-calculated https://electronics.stackexchange.com/questions/572739/why-i...
- amelius 5y ago
  Yes, the high sensitivity is because the V-I curve of the PN junction is exponential.
phkahler 5y ago
While we're talking LEDs: https://gist.github.com/phkahler/1ddddb79fc57072c4269fdd6716bfb72 https://gist.github.com/phkahler/1ddddb79fc57072c4269fdd6716... In 6 lines we smoothly blink (fade on and off) an LED on a regular GPIO pin. Uses some fixed-point math and pulse-density modulation.
- qazxcvbnmlp 5y ago
  Chances are, if you understand your embedded system well enough to ‘call the function every 1ms or so’ consistently and reliably the additional challenge of fading the led will be minimal. The hard part about fading leds on an embedded system is usually the timing rather than the math.
  - phkahler 5y ago
    >> The hard part about fading leds on an embedded system is usually the timing rather than the math. I do a lot of power electronics work where we have an interrupt every PWM cycle (in the kHz range). In that case the PWMs are already used for critical things and the timing is already solid, so this really is a drop-in blinker/fader. Of course lots of applications have a spare PWM they can utilize for changing brightness of an LED, but the setup is still probably more than the code here ;-)

51 points by lightlyused 5y ago

This is nice article but every engineer would say this is oversimplification for simple usecases like status LEDs on router. LEDs are tricky to make behave predictably as they are non-linear components, sensitive to temperature and no two LEDs are made the same. This really shows when connected in parallel. Also different colors of LED have different efficiency. These issues are more prevalent if you're trying to make high-power LED circuits or some RGB circuit with accurate representation of colors. Always thought LEDs are my friend but they made me lose some hair in last years.

tyingq 5y ago
The oversimplification for me is not talking about PWM. Pulsing to control brightness is extremely common, and typically sends more current (smaller resistors).
- MayeulC 5y ago
  Putting LEDs in parallel isn't a good idea. Using a resistor for each is better: LEDs are different one from another, so the one with a lower voltage drop will have more current flowing trough it. A lot more, and it will heat more. Luckily, heating it up will make its internal resistance higher, so all LEDs will end up with the same current. It will shine brighter though, and fail earlier, especially if it heats a lot. What happens after it fails? If it fails open, which is usually the case IIRC, more current flows trough the other LEDs, making them fail earlier. If it fails with a short, the other LEDs turn off, and the resistor dissipates all the current (with a higher voltage drop, so more current). If you're lucky, it stays at that. If not, you can burn the resistor, and/or the power supply due to increased current. Always use a single resistor for each LED, except in very specific cases (like putting them in series). Don't put diodes in parallel.
  - tyingq 5y ago
    I'm confused what "don't put LEDs in parallel" has to do with PWM. Did you maybe reply to the wrong comment?
    - MayeulC 5y ago
      I'm replying to: > The oversimplification for me is not talking about PWM Omitting PWM isn't very important IMO; showing diodes in parallel like this sounds like a critical omission.
      - tyingq 5y ago
        Ok, I'll try and explain. The title of the article is "Calculating Current Limiting Resistor Values...". How you do that calculation is entirely different when you use PWM. Hence, I felt like it was an oversimplification around calculating current limiting resistor values. Also, there are some cases where LEDs have to be in parallel, for practical purposes. Like LED matrix displays, or things meant to be inexpensive and not durable, like holiday lights.
        mindslight 5y ago
        Matrix displays don't drive LEDs in parallel - each leg is a different shared connection that you multiplex over time. Holiday lights generally aren't in parallel, as driving in series makes for more efficiency (higher voltage) and easier wiring. Once strings are long enough they're probably put directly in parallel, but that's just cheapos being cheap and relying on statistics.
        deleted 5y ago
        [deleted]
        tyingq 5y ago
        >Matrix displays don't drive LEDs in parallel Many of them do, especially the outdoor single color ones, where 1 led per pixel isn't bright enough. >Holiday lights generally aren't in parallel Granted, "holiday lights" is a broad category, but there are many that are obviously in parallel. Two 1.5 volt batteries couldn't power many LEDs in a serial string. And many of the AC strings are parallel strings of serial connected LEDS as well. Like: https://midimagic.sgc-hosting.com/chrsleds.htm https://midimagic.sgc-hosting.com/chrsleds.htm
- foobarian 5y ago
  Using analog brightness control is nice because there is no flicker, and there is no stepping. But, with common potentiometers there is only a very narrow operating band due to how nonlinear the LEDS are, and it tends to be pretty touchy due to construction of the pot. I would love to figure out how to make an arbitrary transfer function shape using a pot as input, and some combination of transistors/op-amps to keep the nice analog behavior.
  - JKCalhoun 5y ago
    Perhaps a feedback loop then with a small photo-resistor situated beneath the LED (looking up between its legs?).
  - sdfasf 5y ago
    Flicker sucks, but LEDs are much more efficient when driven hard. So PWM is much more energy efficient for a given luminosity. That being said, if you're not on battery power, we're talking negligible energy. So, put a current source in there and stop giving me a seizure with your flicker.
  - MayeulC 5y ago
    What if you replace your potentiometer with a DAC plus a low-pass filter (a RC circuit)?
  - tyingq 5y ago
    That would be some sort of constant current power supply.
dvh 5y ago
Oversimplification would be putting 220 ohms everywhere.
- analog31 5y ago
  Guilty. ;-)
jcims 5y ago
I got into the ‘stage dads (and sister)’ group for my kids show choir. Our job was to take the directors vision for set design, convert it into something that the kids could build and then supervise them building it. I got lighting because i was handy with electronics, and that sent me down a rabbit hole of the vagaries of current limiting circuits, buck/boost converters, color dependent forward voltage, temperature management, cabling and dmx control. All while staying on a modest budget and maintaining sufficient safety and durability that a bunch of highschoolers can build it and set it up and tear it down in a timed set change at high school gyms and theaters across the country. Very interesting experience, highly recommended and i concur, slapping a resistor on it to limit current is focusing on a small and somewhat straightforward portion of the LED spectrum (har har). That said, it’s still useful for folks that just want to slap a light on something.
segfaultbuserr 5y ago
+1. Unless you really know what you're doing, LEDs in parallel is a bad idea. A slight forward voltage or thermal mismatch causes one LED to dominate the current consumption until it fries. Then the cycle repeats for the next LED. When I was an ignorant child I fried $60 UV LEDs, but it was a good lesson... But at least the article mentioned it, > It's important that there are n LEDs (connected in series) in each of the m parallel branches of the circuit and that the LEDs all have the same Vled and Iled. Otherwise, all bets are off.
mindslight 5y ago
I'd say this article is overcomplicated. You don't need to drive status indicators at full current. In fact, my eyes would prefer you don't. 1k and done. If you're using LEDs for illumination, then do some math. Also, as a general rule don't put LEDs in parallel unless you really know what you're doing with matching I-V curves.

amelius 5y ago

Digikey has some nice in-depth articles about driving LEDs, e.g.: https://www.digikey.com/en/articles/driving-leds-to-cap-or-not-to-cap https://www.digikey.com/en/articles/driving-leds-to-cap-or-n... https://www.digikey.com/en/articles/optimizing-led-performance https://www.digikey.com/en/articles/optimizing-led-performan... https://www.digikey.com/en/articles/maximizing-led-luminosity-to-drive-down-system-cost https://www.digikey.com/en/articles/maximizing-led-luminosit... https://www.digikey.com/en/articles/driving-leds--choosing-between-analog-and-digital-topologies https://www.digikey.com/en/articles/driving-leds--choosing-b... https://www.digikey.com/en/articles/driving-leds-how-to-choose-the-right-power-supply https://www.digikey.com/en/articles/driving-leds-how-to-choo... https://www.digikey.com/en/articles/ac-leds-gain-in-popularity https://www.digikey.com/en/articles/ac-leds-gain-in-populari... https://www.digikey.com/en/articles/how-to-drive-multicolor-leds https://www.digikey.com/en/articles/how-to-drive-multicolor-...

unwind 5y ago

It seems to be an article-length piece, leading up to a downloadable spreadsheet. That's ... not so accessible. I quickly found [1] at Digi-Key which is a simple online calculator. It also features a handy table of reference voltages for various colors of LED, which I appreciated. Ideally I guess the color should be choosable and used to populate the voltage, but that seems to be Super-Advanced Thinking in this area. The full set of calculators [2] seems quite impressive and useful, too. [1]: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor https://www.digikey.com/en/resources/conversion-calculators/... [2]: https://www.digikey.com/en/resources/online-conversion-calculators https://www.digikey.com/en/resources/online-conversion-calcu...

JKCalhoun 5y ago
Plug: wrote an iOS app years ago that provided some simple electronic calculators for these kinds of things: https://apps.apple.com/us/app/perfboard/id465951871 https://apps.apple.com/us/app/perfboard/id465951871

londons_explore 5y ago

It's worth noting that while it isn't within spec, and certainly isn't good engineering practice, in most cases connecting an LED directly to a microcontroller output pin with no resistor works just fine. Most microcontrollers have effectively a fixed output drive current of something between 10 milliamps and 50 milliamps (which depends on the size of the drive mosfet on the output pin, and might be different for pull-high vs pull-low mosfets). Most LED's can handle that just fine. The energy that would have been dissipated in the resistor ends up dissipated in the microcontroller instead, but that normally doesn't lead to issues.

makoto12 5y ago
That sounds really dangerous. You're depending on circuitry built to protect the microcontroller, to limit the current. Sounds like a good way to kill the LED, but worse also kill the microcontroller. Also I'm quite skeptical at the number of microcontrollers that have this protective circuitry?
- qazxcvbnmlp 5y ago
  Many modern MCUs have GPIO that can be configured via software to have an internal resistance in line with the GPIO. This resistance will limit current to within the specs of the MCU just fine.
- MayeulC 5y ago
  True that: I know very few microcontrolers with overcurrent protection, you usually permanently disable a pin when draining too much current. I would still use a resistor with it, though having the microcontroller supply the current is usually fine. Protip: microcontrollers can usually absorb more current with pull-downs (due to n-mosfets, plus internal power supply routing) than they can supply. So have VCC -> res -> LED -> µC and turn the LED on by writing a 0 to the GPIO. Make sure the µC doesn't overheat, and you can drive a lot of LEDs (or other circuits) like this. Double check the datasheets for the max ratings, of course.
  - tzs 5y ago

linker3000 5y ago

https://www.reddit.com/r/AskElectronics/wiki/design/leds https://www.reddit.com/r/AskElectronics/wiki/design/leds

compumike 5y ago

Even when you apply the formulas in the linked spreadsheet, you can get into trouble because it seems to assume V_led is fixed. For an example of how quickly this can go wrong, see my accepted answer to “Why is my LED circuit drawing 50% more mA than I calculated?” https://electronics.stackexchange.com/questions/572739/why-is-my-led-circuit-drawing-50-more-ma-than-i-calculated https://electronics.stackexchange.com/questions/572739/why-i...

amelius 5y ago
Yes, the high sensitivity is because the V-I curve of the PN junction is exponential.

phkahler 5y ago

While we're talking LEDs: https://gist.github.com/phkahler/1ddddb79fc57072c4269fdd6716bfb72 https://gist.github.com/phkahler/1ddddb79fc57072c4269fdd6716... In 6 lines we smoothly blink (fade on and off) an LED on a regular GPIO pin. Uses some fixed-point math and pulse-density modulation.

qazxcvbnmlp 5y ago
Chances are, if you understand your embedded system well enough to ‘call the function every 1ms or so’ consistently and reliably the additional challenge of fading the led will be minimal. The hard part about fading leds on an embedded system is usually the timing rather than the math.
- phkahler 5y ago
  >> The hard part about fading leds on an embedded system is usually the timing rather than the math. I do a lot of power electronics work where we have an interrupt every PWM cycle (in the kHz range). In that case the PWMs are already used for critical things and the timing is already solid, so this really is a drop-in blinker/fader. Of course lots of applications have a spare PWM they can utilize for changing brightness of an LED, but the setup is still probably more than the code here ;-)