7 ms·

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 l

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.
- MayeulC 5y ago
  I loathe those "too cheap to be designed for minimum durability" products. However, resistors are usually a lot cheaper than LEDs. Each LED comes with its own resistor on cheap LED strips from china, sometimes resistors are even integrated in the LED packaging. As for PWM -- do you have any source on this? I guess you just have to take into account the capacitance of the LED?
  - tyingq 5y ago
    >As for PWM -- do you have any source on this? Well, mostly just "duty cycle" related. If you're pulsing the LED half the time (50% duty cycle), for example, you can pulse with twice the current you would normally use if it weren't pulsed. Then you control brightness by pulsing for a lower percentage of the time.

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.
MayeulC 5y ago
I loathe those "too cheap to be designed for minimum durability" products. However, resistors are usually a lot cheaper than LEDs. Each LED comes with its own resistor on cheap LED strips from china, sometimes resistors are even integrated in the LED packaging. As for PWM -- do you have any source on this? I guess you just have to take into account the capacitance of the LED?
- tyingq 5y ago
  >As for PWM -- do you have any source on this? Well, mostly just "duty cycle" related. If you're pulsing the LED half the time (50% duty cycle), for example, you can pulse with twice the current you would normally use if it weren't pulsed. Then you control brightness by pulsing for a lower percentage of the time.