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LED Dimmer Control Schematic

Current Controlled LED Dimmer

The occasion for this project was retrofitting an illuminated wall picture with LED backlighting. As soon as I applied LED power, the brightness blinded me — much brighter than the original T20 fluorescent tube, so I knew immediately that a dimmer control was required.

Schematic of the LED Brightness Control Circuit

LED Dimmer Control Schematic

LEDs strip

LED Strip ebay Screen Shot

How do the Chinese make any money at only $4.60 per reel (free shipping)—and the quality is good! (Note that there has been a modest price increase since…)

The LEDs come on a 5M reel with a total of 300 devices (100 series strings of 3LEDs per string). Measured current was 1A @ 12V for an input power of 12W. Since my wall wart power supply was rated for 0.8A, I pared it down to 240 devices (every 3 devices, there is a cut point indicated).

Assembly was simple. I was having a hard time determining how I was going to glue down the LED strips, but finally realized that it already had an adhesive backing.

The circuit

I opted for a simple adjustable current regulator. While adjustable voltage works, the limits of voltage are not clearly defined and there can be a lot of dead band on the adjustment. The current regulator, on the other hand, has good range of adjustment, low dropout voltage and minimal dead band.

The power device is a composite PNP /NPN pair that acts as a high gain PNP transistor. This could also be done with a PNP Darlington, but the dropout (saturation) voltage would be a little higher. This function could also be done with an NPN /PNP composite pair, but I wanted the LEDs to be grounded to the negative bus for possible future enhancements.

It is essentially an emitter follower with a 0.6Ω emitter (shunt) resistor. The voltage across this resistor is determined by the base voltage at Q1. The collector (LED load) current is essentially the same as the emitter current. Since the voltage at the base of Q1 is adjustable, the load current is also adjustable. Schottky diode D1 provides a fixed 0.3V drop in order to reduce potentiometer dead band. A standard diode could prevent Q1 from turning off completely—this is important because the potentiometer is also employed as an On /Off switch. No, it does not completely remove power from the circuit, but the losses in the Off mode are far less than the magnetic and winding losses of the continuously operating wall wart.

C1 is a bypass capacitor that helps prevent oscillation—yes, a circuit like this can oscillate—I connected a scope to it to prove to myself that it was stable.

C2 filters out the AC wall wart ripple so that the load current is pure DC. I wanted pure, flicker-free light. Many LED circuits pulse the current—I wanted to avoid this.

Scaling for other currents & input voltage

Regulating the load current also regulates the output voltage, so the raw input voltage may be higher than necessary and need not be exactly matched to the load. However, additional voltage causes additional power dissipation in power transistor Q2. In my case, only a small heatsink was required and it ran comfortably warm under worst conditions. Specific circuit voltages are indicated on the schematic. For other current ratings, only R3 & 4 need be scaled (drops 0.5V @ full load)(may be combined into one resistor). R2 must be scaled for the raw input voltage at full load.

Transistor selection

I had the 2N3906 (PNP TO-92) and D44H8 (NPN TO-220) devices on hand, so that is what I used. Transistor selection is not critical.

Photo Gallery

For the future

Light alternate rows in pseudo random fashion to make it shimmer (if possible)—original picture rotated a wavy filter to make it shimmer.

4 Comments

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  • Sam Star

    Wouldn’t it have been better to use a PWM power supply rather than wasting the extra energy in the transistor?
    It could be a simple design with a 555 and a MOSFET.

  • KROKKENOSTER

    Handy circuit as it stands as a experimenter’s power supply. I had so much grief with I’C’s that I got fed up and built my psu with discreet components Had a lot of grief with SN 72723 Thhey were the flavour of the year at that time they were “Fond” of just dying without any reason. I had this problem in “Wire guidance” and other electronic controls Everywhere I replaced them with discrete circuits and my troubles were over.

    • krokkenoster

      Keith Hi The manuals of the 78xx and the Lm 338Ksteel and the smaller ones LM 317 is the power rating is said to be “internally controlled” and what does this mean? I found in a old “Television” of ca 1992 magazine that these beasties can only handle 15 (FIFTEEN) WATTS AND THE METAL TO3 CAN ONLY about 50WATTS or there about and this make them not very popular as this info is only nowhere really to be found except if you have the entire manual and look at the back pages where this is discussed

    • Jim Keith

      Agree–the LM723 is perhaps the first power supply IC ever–it was introduced over 40years ago. While I have never had issues with the LM723, I experienced reliability problems with the early LM78xx series devices.

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