4 ms·

The oversimplification for me is not talking about PWM. Pulsing to control brightness is extremely common, and typically sends more current (smaller resistors)

by 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
        mindslight 5y ago
        I would assume matrix displays with multiple LEDs for brighter output would still mostly put them in series, because a single LED voltage drop doesn't make for a nice voltage to work with. With parallel strings of LEDs in series, statistics helps match I-V curves. But perhaps some uncommon design constraints make some signs work out the other way. There are always going to be exceptions (like your 2 cell battery example, or "LED throwies"), but we're talking about the common scenarios. In general I think talking about putting LEDs in parallel is just confusing for anyone that's reading an article to know what size resistor to use.
        tyingq 5y ago
        >But perhaps some uncommon design constraints make some signs work out the other way. Yes, they use a lot of ICs that are either 3.3v or 5v as the driver to the LEDs.
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.

by 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
        mindslight 5y ago
        I would assume matrix displays with multiple LEDs for brighter output would still mostly put them in series, because a single LED voltage drop doesn't make for a nice voltage to work with. With parallel strings of LEDs in series, statistics helps match I-V curves. But perhaps some uncommon design constraints make some signs work out the other way. There are always going to be exceptions (like your 2 cell battery example, or "LED throwies"), but we're talking about the common scenarios. In general I think talking about putting LEDs in parallel is just confusing for anyone that's reading an article to know what size resistor to use.
        tyingq 5y ago
        >But perhaps some uncommon design constraints make some signs work out the other way. Yes, they use a lot of ICs that are either 3.3v or 5v as the driver to the LEDs.

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.