Help me understand direct drive

I have one of those cheap under cabinet/magnetic/velcro mount LED lights. You know the kind, two sets of COB LEDs, high/low/off, 3 AAA cells. I am sure it is direct drive because I can see the light get dimmer as the cells run down. What I am having problems understanding is how the high/low works in a direct drive setup. I know there is a 7135 component that will pass a certain amount of current, so is this a setup where there is always one of those in the circuit, and the high position adds a second 7135, or does it work some other way?

Those almost always use a resistor or 2 for regulation. Different values of resistor will change the current. Unfortunately the light will dim as the battery voltage runs down. If you'd like to know more about this: https://www.evilmadscientist.com/2012/resistors-for-leds/

The 7135 is a linear regulator. This means it acts like an automatic variable resistor that tunes itself to provide a consistent current.

So if I am following the article correctly, there should be two resistors in my light; one of the calculated value for full brightness, and another with a slightly higher value that will drop enough extra voltage that the LED will not operate at normal brightness. And running a risk here that if the second resistor drops too much voltage, the input voltage to the LED will drop below the LED's vf.

One way to regulate direct drive light is pwm.

HKJ said:

One way to regulate direct drive light is pwm.

Click to expand...

But PWM implies the existence of a driver that usually doesn't dim as the batteries drain, doesn't it?
B

Timothybil said:

But PWM implies the existence of a driver that usually doesn't dim as the batteries drain, doesn't it?
B

Click to expand...

No, it requires an electronic switch that can turn the current on/off. For lights with a driver it is often the driver that is switched on/off, this saves an extra switch.

Some lights use a driver only to limit maximum brightness, 7135 is often used that way and it can easily be turned on/off from a microprocessor.