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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:

schematic

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.

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  • \$\begingroup\$ As shown, the MOSFET will be 100% on - there is effectively no current through the gate of the MOSFET, which means the potentiometer (regardless of its setting) will just pull the gate down to 0V. That means you've got 18.5V across the MOSFET gate, turning it on 100% (side note: double check the abs max gate voltage of your MOSFET, it's often +/-15V or so). \$\endgroup\$
    – Selvek
    Nov 1, 2018 at 15:58
  • \$\begingroup\$ The key is to understand that MOSFETs are voltage-controlled devices, and BJTs are current-controlled devices. \$\endgroup\$
    – Daniel
    Nov 1, 2018 at 16:07
  • \$\begingroup\$ "Driver built in" ? You might want to investigate this. If you add an external brightness control, you could have two control systems fighting each other (one internal to the driver, and the other -yours- external to the driver). \$\endgroup\$
    – glen_geek
    Nov 1, 2018 at 16:20
  • \$\begingroup\$ @daniel: am I mistaken to think that in this case the control is voltage and the output current? \$\endgroup\$
    – user508402
    Nov 1, 2018 at 16:49
  • \$\begingroup\$ Altering the current flowing into a MOSFET changes how quickly it transitions, not how much it conducts. I would research MOSFETs further on your own to gain a better understanding of how they operate. \$\endgroup\$
    – Daniel
    Nov 1, 2018 at 17:23

3 Answers 3

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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.

enter image description here

  1. 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.

enter image description here

  1. 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.

enter image description here

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

schematic

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.

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  • \$\begingroup\$ I can see how this would work, however, I didn't tell the light is not only to see with my eyes, but also the lightsource for photography. I am not sure the camera would handle the flicker well at shorter shutter times. Additionally, using arduino would complicate the project, that would be simple to build, if not to understand (for me). I think analog is the way to go here. \$\endgroup\$
    – user508402
    Nov 1, 2018 at 16:46
  • \$\begingroup\$ @user508402 you can add a couple passive filter elements and use a relatively high frequency PWM and you can basically have DC. Also, the controller built into the lamp might have enough filtering to make it work. \$\endgroup\$ Nov 1, 2018 at 22:34
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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.

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  • \$\begingroup\$ Potentiometers have a relatively low maximum wiper current. They are not designed for current-limiting, typically. \$\endgroup\$
    – Daniel
    Nov 1, 2018 at 17:25

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