how to reduce current to power LEDs

Question

I am replacing a microscope lamp with a motorcycle LED headlight. The light is 20W, 9-85V DC; the driver built in and a complete black box to me. I power it with a 65W power supply (18.5V/3.5A). For as long as I have tested it (short), this seems to work well.

Now the original housing had a potentiometer to adjust the brightness of the lamp. Naively, I soldered that in serie with the headlight. For as long as it lasted it worked functionally well to reduce the light, but of course, within a minute, smoke, smell, and the potmeter even emitted red light (it had to endure 20W).

If I understand it well now, I am to reduce the output power fed to the light. Similar questions here suggest a FET is the way to go. What type of FET should I select here (and why?). I sketch below how I understand it now:

^{simulate this circuit – Schematic created using CircuitLab}

Is this the idea? It doesn't need to be very precise as it is only a light dimmer.

Answer 1

LEDs need to be driven with a constant current source to be able to regulate the brightness without flicker (automotive LEDs are PWM'd fully on/fully off, and it surprises me that there doesn't seem to be any regulation about flicker, I find it annoying). Since any linear circuit is going to dissipate the 'unwanted' power from the supply, you'll need either

1 - a large variable resistor/rheostat. These things are fairly pricey as well as getting hot.

2. a linear regulator circuit. There's many ways to drive a transistor or FET as a constant current source or sink, here's one from a TI app note - ratings of components aren't selected for this particular application. The output device will again be dissipating a fair amount of heat, so it'd need to be heatsinked well.

3. There are buck regulators that can operate in constant current mode, and at frequencies high enough that the any flicker that there is from ripple in the current is not noticeable on even photography. Here's a possible arrangement from here This approach has the advantage that the efficiency is high, so there'll be little heat to dissipate.

One thought - is there any existing regulator included with the LED you are using? That voltage range, aside from sounding odd for an automotive product, wouldn't be achievable without some regulator.

Answer 2

I'd recommend a low-side N-channel MOSFET with a PWM control line to control brightness. Basic schematic concept:

^{simulate this circuit – Schematic created using CircuitLab}

When the PWM signal is high, the NFET conducts, current flows, and the light turns on. When the PWM signal is low, the NFET stops conducting, current stops flowing, the the light turns off.

By varying the duty cycle (percentage of time the PWM signal is high), you can control the brightness. A simple microcontroller like an Arduino can provide the PWM signal. You'll want to look for an NFET that uses logic level signaling so the Arduino's GPIO pins can fully turn the NFET on and off.

Answer 3

Could you not use a potentiometer in series with the LED to give you the desired effect? Obviously with the Highest Resistance being close to max current of the LED. This would allow you brightness control.

If you have the datasheet for the LED to find the forward voltage & current, you can use this link LED Series Resistor Calc to find the largest value of resistance for your potentiometer.

Sorry this doesn't answer your questions but seems a simple solution to your problem. Drop me a message if you need a better explanation.