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Project Notes

#231 LED Current Regulator

Test a current-regulator style LED dimmer circuit.



I’m looking at some alternatives for dimming a 12V LED strip. This circuit is by Jim Keith at electroschematics. Rather than using PWM, this circuit controls the current and theoretically should provide better flicker-free lighting even when dimmed.

LED Strip Specifications

I’m using this (very) cheap 3014 Led Strip Light. According to the seller specs:

Attribute Specification
Length 5m
LED Density 60 LED/m
Color cold white
View angle 120°
Working Voltage 12V DC
Power consumption 20W, 4W/m, ~67mW/LED
Working Temperature -40 to 70°
Luminous Flux 880-1080 Lumens/Meter
Size L500cm (5M) x W0.5cm x T0.20cm
lifespan >100,000 hours

LED Load and Power Supply

I’m using a 1m length strip for testing. At maximum load, that should be about 4W according to the specs.

Running the LED strip direct from a 12V supply, I actually measure 270mA or 3.24W.

The 12V/1.5A/18W power supply I’m using is quite adequate for this.

Dimmer Circuit

The dimmer circuit is essentially a high-gain PNP (NPN + PNP combo) with a 0.25Ω (1Ω x 4 in parallel) shunt resistor.

A 1kΩ pot controls the PNP current. The 1N5819 diode offsets the pot so it has little dead zone yet can still turn the LEDs “off”. The voltage divider necessarily means there’s at least 1.1mA leakage even when the LEDs are off.

The PNP control and shunt resistor means there’s also ~1.3V knowcked off the supply. So a 12V supply will never drive the LEDs to max brightness. For my purposes, they are bright enough already.

I’m measuring ~150mA on max brightness with this circuit and a 12V supply so 1.8W power consumption.

At that current, I was able to get away with an A42 NPN (rated for 500mA), but higher currents would need something beefier.



The Schematic


Protoboard Layout:




Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.