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Does slow PWM save money? I first noticed slow PWM in factory tail lights on Cadillacs somewhere around the turn of the century. I pointed it out to my dad on
by ssl-3 2y ago
Does slow PWM save money?
I first noticed slow PWM in factory tail lights on Cadillacs somewhere around the turn of the century. I pointed it out to my dad one night when we were out for a walk, and told him how to see it on a car that was disappearing down the road.
It took a few seconds for him to understand and to observe it himself, and then he said "I can accept that you're annoyed by that, but maybe they just build them that way because they think it looks cool."
Ever since that night, I have always assumed that slow PWM was used as a deliberate visual effect: After all, General Motors isn't really trying to squeeze a few pennies out of a new Cadillac [that's what Chevrolet is for!].
(I also think it's a dick move on the manufacturer's part, whether it is to save forty-seven cents, or if it is to look cool, or even if it is both.)
- Zak 2y agoThis prompted me to go look for a better answer, and I found a good candidate: pulsed current at a relative low frequency (~60hZ) allows higher perceived brightness from smaller/cheaper LEDs. From my perspective, it is undesirable to increase the brightness of car taillights relative to what was typical with incandescent bulbs or to make them flicker. https://electronics.stackexchange.com/questions/164078/why-is-pwm-used-to-control-leds-even-when-theyre-only-used-on-or-off https://electronics.stackexchange.com/questions/164078/why-i...
- neuralRiot 2y agoThe 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 agoKeeping 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 agoIs 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 agoThe 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 agoLinear 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 agoA 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.
- alamortsubite 2y agoThose early Cadillac LEDs were the worst, though I've seen some new models from other manufacturers whose taillights are almost as bad.