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Is it possible to control LED brightness with a linear regulator? I am surprised to hear it - when I was making light effects with LEDs years ago, I understood

Is it possible to control LED brightness with a linear regulator? I am surprised to hear it - when I was making light effects with LEDs years ago, I understood PWM to be the only practical mechanism, due to the strongly non-linear conductivity curve. But maybe it's possible, if you just want to fix your LEDs at a specific brightness which never changes, that you could find exactly the right current level...?

kedikedi 2y ago
You can but it isn’t efficient. You lose (V_in - V_out)*I_led Watts to heat if you use a linear regulator. You might ask why not connect them in series and get the voltage difference as small as possible. But the “forward voltages” of LEDs are highly temperature dependent and car battery voltage is (somewhat) engine rpm dependent (might swing between 12 and 13.8V between no rpm and some rpm. It’s kept flat at 13.8V)
deleted 2y ago
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SAI_Peregrinus 2y ago
Linear regulators can do it easily, if used in a constant-current driver configuration. That constant can be variable, so regulated-current might be a better name. Your classic LM317 linear regulator can do it pretty easily, with a potentiometer instead of a fixed resistor to set the output current. Section 9.3.3[1] of the datasheet describes the circuit, though in practice you might add a couple of capacitors for stability. There are also a number of discrete-transistor circuits. A current mirror with a rheostat is probably the simplest, though not generally ideal performance. [1]https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792#%5B%7B%22num%22%3A391%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C0%2C464.4%2C0%5D https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792...
- marssaxman 2y ago
  Interesting! I have used plenty of LM317s with LEDs, but I never thought of using them to regulate brightness - I would just pick whichever resistor corresponded to the forward current specified in the emitter datasheet. Didn't realize that there was enough room on the curve to use current control as a meaningfully configurable brightness setting.
Zak 2y ago
As a flashlight nerd and reviewer, I know a number of ways to control the brightness of an LED, and linear regulators are a fairly common one. It's important to note that a linear regulator can be designed to target a specific voltage, or a specific current. LEDs should be regulated with current for the reason you mentioned. One that I think would work well in cars is a fixed-current linear regulator like the AMC7135. There's no need to do anything fancy to control it; put a 7135 in the circuit and the LED gets 350mA. Two in parallel and it gets 700mA, and so on. More fancy is a linear regulator with variable current output. Linear regulators are a low-end option for flashlights, with switched-mode power supplies being preferred for their superior efficiency. A car has a lot more energy to work with and more mass with which to sink and radiate heat.

kedikedi 2y ago

You can but it isn’t efficient. You lose (V_in - V_out)*I_led Watts to heat if you use a linear regulator. You might ask why not connect them in series and get the voltage difference as small as possible. But the “forward voltages” of LEDs are highly temperature dependent and car battery voltage is (somewhat) engine rpm dependent (might swing between 12 and 13.8V between no rpm and some rpm. It’s kept flat at 13.8V)

deleted 2y ago

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SAI_Peregrinus 2y ago

Linear regulators can do it easily, if used in a constant-current driver configuration. That constant can be variable, so regulated-current might be a better name. Your classic LM317 linear regulator can do it pretty easily, with a potentiometer instead of a fixed resistor to set the output current. Section 9.3.3[1] of the datasheet describes the circuit, though in practice you might add a couple of capacitors for stability. There are also a number of discrete-transistor circuits. A current mirror with a rheostat is probably the simplest, though not generally ideal performance. [1]https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792#%5B%7B%22num%22%3A391%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C0%2C464.4%2C0%5D https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1734395381792...

marssaxman 2y ago
Interesting! I have used plenty of LM317s with LEDs, but I never thought of using them to regulate brightness - I would just pick whichever resistor corresponded to the forward current specified in the emitter datasheet. Didn't realize that there was enough room on the curve to use current control as a meaningfully configurable brightness setting.

Zak 2y ago

As a flashlight nerd and reviewer, I know a number of ways to control the brightness of an LED, and linear regulators are a fairly common one. It's important to note that a linear regulator can be designed to target a specific voltage, or a specific current. LEDs should be regulated with current for the reason you mentioned. One that I think would work well in cars is a fixed-current linear regulator like the AMC7135. There's no need to do anything fancy to control it; put a 7135 in the circuit and the LED gets 350mA. Two in parallel and it gets 700mA, and so on. More fancy is a linear regulator with variable current output. Linear regulators are a low-end option for flashlights, with switched-mode power supplies being preferred for their superior efficiency. A car has a lot more energy to work with and more mass with which to sink and radiate heat.