6 ms·

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 pred

by 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.

by po1nt 5y ago

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.