Ever wondered how Tesla combats the problem of LED refresh rates?
Looks like they individually address them and spin them around:
Looks like they individually address them and spin them around:
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Combating LED Headlight/Taillight Strobing
- Thread starter JaredBanyard
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bsd
Member
Has the nice side-effect of keeping the current draw relatively steady as well.
@bsd, To get an LED to "dim", you usually turn it on and off at a specific duty cycle to reduce the amount of light it produces. For example, if you want it dimmed to 50%, you make sure that within a given period of time, it is on 50% of the time and off 50% of the time. The faster the refresh rate, the less noticeable it is. This page has a nice diagram showing the duty cycle:
ATmega168A Pulse Width Modulation PWM - Tutorials
What Tesla has done is instead of just turning the whole thing on or off, they are cycling the LED's so that at any given time, there is always something on.
@bsd, To get an LED to "dim", you usually turn it on and off at a specific duty cycle to reduce the amount of light it produces. For example, if you want it dimmed to 50%, you make sure that within a given period of time, it is on 50% of the time and off 50% of the time. The faster the refresh rate, the less noticeable it is. This page has a nice diagram showing the duty cycle:
ATmega168A Pulse Width Modulation PWM - Tutorials
What Tesla has done is instead of just turning the whole thing on or off, they are cycling the LED's so that at any given time, there is always something on.
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Gizmotoy
Active Member
Neat. Is there a purpose for doing it this way? To the eye, they just look solid on. To look cool in video?
tokuro
Member
Many LED devices (alarm clock etc) does the same. that is to reduce power and heat from LED. It also reduces numbers of components needed to build complex display.
If you have a decent camera, try taking a picture of LED clock at shutter speed of 1/250 or higher. You will see that only portion of LED is "on".
If you have a decent camera, try taking a picture of LED clock at shutter speed of 1/250 or higher. You will see that only portion of LED is "on".
JPP
Active Member
At night, look in your Model S rear camera at some recent production vehicles with front LED lighting (when they are following you), and watch the different light shows--pretty interesting. I guess the frame rate of the rear camera somehow catches the rotation rate of the lights (...maybe like old movies and stagecoach wheels). VFX--we need expertise here...
The effect is called "aliasing". It's exactly the same effect that makes wagon wheels appear to turn backwards in old western movies. The sampling rate of the movie or video is insufficient to keep up with changes in the scene.
Gizmotoy
Active Member
Sure, but I kind of meant "why make the refresh circle around rather than just flash on/off simultaneously like other automakers"
Maybe the OP is wrong? I could see the LEDs being in separate banks that have slightly different refresh rates. What I don't get is why they'd go through the effort to make them refresh circularly ("spin them around").
You can certainly dim LEDs without PWM. Just drop the voltage. PWM is typically used to dim LEDs because it's easy to control from a digital circuit. But if you just need a constant level, the easy thing to do is put a resister in series with the LED to drop the voltage. I suppose they would use PWM for power efficiency. (Though presumably it's possible to make LEDs that achieve the desired brightness at 100%.)
Yes but the resistor would burn a lot of power and get hot. Complete waste of energy and a potential failure mode if it gets too hot. PWM is really the sensible solution. It has very high efficiency because the driver transistor is either all the way on or all the way off, and this means it doesn't waste any power. It is also trivially easy to control the brightness by adjusting the duty cycle, no D/A converter required. Human eyes can't see the flicker. Low cost, efficient, flexible, reliable, effective. That's why everyone does it.
Doug is right. I spent two years running an LED driver design team for a major semiconductor company. Certain applications, such as TV studios and theaters can't tolerate the flicker and must use linear dimming. Super high quality lighting requirements, like museum lighting, also opt for linear dimming. There are also studies that show people's ability to perceive flicker varies widely. LED PWM frequencies tend to be between 300-1000 Hz. Some people can see up to 1000 Hz flicker and others can't perceive 60 Hz.
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I will also point out that the MS DRLs are multiplexing between banks of LEDs as well as PWM dimming (when a turn signal is active). Multiplexing allows one driver to drive several banks of LEDs, further saving cost. White LEDs are so bright these days that they are not always needed to run at full brightness. This allows cost saving tricks such as multiplexing.
Glad you guys enjoyed it. Don't suppose someone could get high speed footage of the effect? I would guess we would need somewhere around 240fps? Maybe more?
They might be using PWM to vary the DRL intensity depending on conditions, maybe making them brighter when it is brighter outside.
Multiplexing like this is clever, it means they will be able to reduce input ripple current, which can allow for smaller conductors, minimal capacitance on the LED driver board, and also reduces electrical interference. If it is done really well, it will look like the module consumes a fixed current.
Multiplexing like this is clever, it means they will be able to reduce input ripple current, which can allow for smaller conductors, minimal capacitance on the LED driver board, and also reduces electrical interference. If it is done really well, it will look like the module consumes a fixed current.
LEDs are more efficient in lumens/watt at low to medium drive levels. They are more efficient in lumens/dollar at high drive levels. So cheap LED bulbs tend to use fewer LEDs, drive them hard and run them hot, which is what shortens their lifetime, and why some cheap LED bulbs will not last 'forever'.
PWM drive and multiplexing lowers average current and die temperature, beneficial to LED lifetime.
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