Running Light: 1 W LED Switched Parallel Resistors

Manually selecting the current through the 1 W amber LED with a switch actually intended for LED flashlights:

1 W LED Running Light - switched parallel R
1 W LED Running Light – switched parallel R

The resistors on the low side of the LED use the MP1584 regulator for current control, with the orange wire feeding the resistor voltage into the error amplifier.

The 15 Ω unswitched resistor sets the LED current at 53 mA = 0.8 V / 15 Ω, with the LED dissipating about 100 mW. The resistor dissipates 43 mW.

Closing the switch puts the two parallel 4.7 Ω resistors in parallel with the 15 Ω resistor to produce 2.0 Ω, which sets the LED current to 390 mA and runs it at 950 mW. Each of the 4.7 Ω resistors dissipates 140 mW.

That much power raises the aluminum body to 50 °C = 120 °F: definitely uncomfortable but probably survivable for the LED inside.

Eyeballometrically, a decimal order of magnitude difference in the LED current produces an obvious brightness difference. My first try ran the LED at 500 mW (a binary order of magnitude less than 1 W) and wasn’t visually different. Given that the LED will run from the Bafang’s headlight output, saving power isn’t all that important.

If this is the first time you’ve encountered parallel resistors, this is why your calculator has a reciprocal button: the total resistance is the reciprocal of the sum of the reciprocals of all the resistances:

1/R = 1/R₁ + 1/R₂ + …

A real engineering calculator does not have a shifted reciprocal function.

4 thoughts on “Running Light: 1 W LED Switched Parallel Resistors

  1. The HP41CX sneaked back in the house when I was preparing for the amateur radio exam(s), and has stayed there. The 15C is the quick-response calculator, with easier to find batteries, but both seem to be willing to go forever. I just wish they had the same key layout for the 4-function bits. Apparently, touchtyping across product generations isn’t important to some calculator designers. (Missed an opportunity to get a HP97 scientific/desk calculator when our department shut down. Could have been nice.)

  2. Not that I think the datasheets are anything but bad fiction, but they seem to suggest a maximum junction temperature of 115C and ambient of 80C… so your piddling 50C ought to roll right off its back with any kind of reasonable thermal conductivity to the die.

    1. It has some kind of thermal glop under the package and the PCB is now epoxied to the shell, so the conductivity is as good as it’ll get. I do, however, have some motivation to ensure Mary doesn’t go ouch when she touches it …

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