4 ms·

The PWM function is built-in into the electronic switches that drive the light outputs, so you can programmatically control the brightness, it also accepts diff

The PWM function is built-in into the electronic switches that drive the light outputs, so you can programmatically control the brightness, it also accepts different switching frequencies so it’s just a matter of increasing the frequency for the “strobe effect” to be unnoticeable. The advantage of these switches over analog are several, including output diagnostic and lower power dissipation.

Zak 2y ago
Keeping in mind that it's on a moving vehicle, and road users often pan their eyes around rapidly, the frequency would need to be higher than for many other situations. Or they could use a variable linear regulator with no flickering at all.
- ssl-3 2y ago
  Is a higher switching frequency problematic, somehow? Is there an upper bound on switching frequency that makes LEDs difficult or unnecessarily inefficient or something? There's nothing I'm aware of relating to the electronics that drive them that would place such a low bound on frequency that the resulting pulses would be visibly distinct. MOSFETs are generally good for > tens-of-KHz frequencies, and so are the hardware PWM channels in every-day MCUs.
  - Zak 2y ago
    The effect described in the StackExchange answer to increase perceived brightness at a lower power level seems to rely on a low frequency and short duty cycle. It's not necessary to use that effect given that LEDs rated for higher power levels are available, and the amount of power is insignificant at the scale of a car, but it does allow the use of cheaper LEDs.
- brokenmachine 2y ago
  Linear regulators waste a lot of power and generate a lot of heat, so that's why they don't use them. Switching regulators can be used without having PWM on the LEDs though. LED flashlights use constant current sources, I don't know if that's infeasible or not for some reason on high-power car headlights. Personally I wouldn't be surprised if it's just to save the 47c. The car industry is competitive.
  - Zak 2y ago
    A series of web searches found one of the driver boards often used in LED tail lights: the Valeo b003809. Some images of it show an identifiable TLE4242 linear driver IC. I was starting to suspect that would be the case after reading the explanation about slow PWM and a short duty cycle resulting in greater perceived brightness from cheap LEDs that aren't rated for much power. Direct drive from the car's electrical system wouldn't result in the constant brightness seen in LED tail lights because the voltage is lower at idle, so there has to be some kind of regulation. There would be little reason to use a more efficient, more expensive switched-mode power supply, though an aftermarket replacement for the Valeo board does use one. Even an inefficient LED design will be far more efficient than an incandescent bulb, and the power requirement of any taillight is trivial relative to a modern car's electrical system. Linear makes the most sense for the application. Linear regulators aren't rare in LED flashlights nor is direct drive with PWM on a FET for low modes. Neither is good in flashlights, but that only matters to the manufacturer if their customers know the difference. Most customers don't; lights marketed to discerning customers use a switched-mode power supply.
    - neuralRiot 2y ago
      The problem could be the LED driver itself rather than the vehicle driver output. Most LED lights use some kind of PWM or multplexing, but people only notice the bad or rather the slow) ones. It’s easy to find out, just take a few videos at different frame rates an you’ll see.
- amluto 2y ago
  The term you’re looking for is “constant current driver”, not “linear regulator”.
  - Zak 2y ago
    There are two ways to implement a constant current LED driver; a linear regulator is one of them. The other is a switched-mode power supply. A linear regulator is the cheaper of the two.

Zak 2y ago

Keeping in mind that it's on a moving vehicle, and road users often pan their eyes around rapidly, the frequency would need to be higher than for many other situations. Or they could use a variable linear regulator with no flickering at all.

ssl-3 2y ago
Is a higher switching frequency problematic, somehow? Is there an upper bound on switching frequency that makes LEDs difficult or unnecessarily inefficient or something? There's nothing I'm aware of relating to the electronics that drive them that would place such a low bound on frequency that the resulting pulses would be visibly distinct. MOSFETs are generally good for > tens-of-KHz frequencies, and so are the hardware PWM channels in every-day MCUs.
- Zak 2y ago
  The effect described in the StackExchange answer to increase perceived brightness at a lower power level seems to rely on a low frequency and short duty cycle. It's not necessary to use that effect given that LEDs rated for higher power levels are available, and the amount of power is insignificant at the scale of a car, but it does allow the use of cheaper LEDs.
brokenmachine 2y ago
Linear regulators waste a lot of power and generate a lot of heat, so that's why they don't use them. Switching regulators can be used without having PWM on the LEDs though. LED flashlights use constant current sources, I don't know if that's infeasible or not for some reason on high-power car headlights. Personally I wouldn't be surprised if it's just to save the 47c. The car industry is competitive.
- Zak 2y ago
  A series of web searches found one of the driver boards often used in LED tail lights: the Valeo b003809. Some images of it show an identifiable TLE4242 linear driver IC. I was starting to suspect that would be the case after reading the explanation about slow PWM and a short duty cycle resulting in greater perceived brightness from cheap LEDs that aren't rated for much power. Direct drive from the car's electrical system wouldn't result in the constant brightness seen in LED tail lights because the voltage is lower at idle, so there has to be some kind of regulation. There would be little reason to use a more efficient, more expensive switched-mode power supply, though an aftermarket replacement for the Valeo board does use one. Even an inefficient LED design will be far more efficient than an incandescent bulb, and the power requirement of any taillight is trivial relative to a modern car's electrical system. Linear makes the most sense for the application. Linear regulators aren't rare in LED flashlights nor is direct drive with PWM on a FET for low modes. Neither is good in flashlights, but that only matters to the manufacturer if their customers know the difference. Most customers don't; lights marketed to discerning customers use a switched-mode power supply.
  - neuralRiot 2y ago
    The problem could be the LED driver itself rather than the vehicle driver output. Most LED lights use some kind of PWM or multplexing, but people only notice the bad or rather the slow) ones. It’s easy to find out, just take a few videos at different frame rates an you’ll see.
amluto 2y ago
The term you’re looking for is “constant current driver”, not “linear regulator”.
- Zak 2y ago
  There are two ways to implement a constant current LED driver; a linear regulator is one of them. The other is a switched-mode power supply. A linear regulator is the cheaper of the two.