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Comment by KaiserPro | Hacker News Reader

I suspect its not the PSU, its the PWM for the dimming.

compared to actually lowering the voltage/current of a LED/light/other its far simpler and cheaper just to turn it off and on again at varying duty cycles/ frequency.

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Good point. But there's no reason for the PWM to be in the 100 Hz range either. Even the crappest possible microcontroller using timer interrupts could handle 1000 Hz. Hardware PWM peripherals (that can switch at 500 KHz) have been trivially cheap additions to microcontrollers for decades.

It's likely not for dimming but for reducing power usage without reducing apparent brightness. See my comment sibling to yours.

May not be for dimming. PWM control is used also to reduce the current (and therefore power, and therefore heat) required to achieve a particular apparent brightness. This utilizes the persistence of vision effect.

My guess is that, above a certain frequency, persistence of vision no longer applies, and the light is in fact seen simply as being dimmer (which is undesirable in this case). Hence ~100 Hz.

Another technical consideration would be to have grid-based scanning of the LEDs. That is you have some sort of row output, one row at a time, and the columns are multiple parallel outputs. This inherently pulses the LEDs as the rows aren't all lit simultaneously. The persistence-of-vision effect is a great side effect and can be capitalized on. The grid is best for limiting the number of wires to access a substantial number of LEDs (like 16 or so) in a group. I don't think a grid for the turn signals of a car would be a great idea, at least not at the level of the individual turn signal diodes. Perhaps within a single corner's turn signal assembly.

Correct me if I'm wrong, but isn't it just the de facto way of dimming LEDs? Since as opposed to incandescent bulbs, they require certain voltage/current to be lit, if you pass them less, they simply won't emit light.

They require a certain voltage (to a first approximation). Their light output is (again, to a first approximation) directly proportional to the applied current.

PWM control is just a cheap way of reducing the current.

You can feed it DC and vary the current, for example by filtering the pwm.

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