Regulated 24 Watt Broad Spectrum LED Lamp

(C) 2010 G. Forrest Cook

Front of 24W LED array

Above photo: LEDs running at a reduced power level.

Back of 24W LED array

Above photo: Back side of the 24W LED Lamp

24W LED array schematic

Regulated 24 Watt Broad Spectrum LED Lamp


This project involves constructing an energy efficient broad spectrum LED lamp system. The lamp is useful for indoor reflective room lighting. It has a broad color spectrum that more closely approximates the light of the sun when compared to fluorescent bulbs and white-only LEDs. The light output is regulated and the light that is produced does not flicker.

The six differently colored LED stars, made by LedEngin, Inc., are rated at 5 watts (nominal). The LED array and associated current regulator consume 1 amp at 24VDC (24 Watts). Warning: NEVER stare directly at this lamp when it is running at full operating power, it is DANGEROUSLY BRIGHT.

With the LEDs shown, the combined color of the lamp has a pinkish white hue. The 5 Watt ratings of the LEDs are not precise, the white, blue and green LEDs consume about 4W and the lower voltage red, orange and deep red LEDs consume about 3W. The current regulator keeps the LED brightness constant and insures that the LED series string never draws more than 1 amp of current.

The project has also been coined "Bold as LED" in reference to the Jimi Hendrix song "Bold as Love" which has the lyric: "My yellow in this case is not so mellow".


Nominal operating power: 24 Watts (24V DC at 1 Amp)
LED power consumption above regulation point: 18.6 Watts
Maximum operating voltage: 28V DC
Minimum voltage for regulated light: 23V DC
Leds produce light down to 11V

Deep Red LED voltage: 2.55V
Red LED voltage: 2.37V
Amber LED voltage: 2.60V
Green LED voltage: 3.92V
Blue LED voltage: 3.56V
White LED voltage: 3.7V
Voltage across regulator when current becomes regulated: 4.2V


The lamp is wired as a current loop which includes the power supply, the LED series string and the current regulator circuit. The LM317K and 1.2 ohm 5 Watt resistor act as a current regulator that limits the loop current to 1 Amp. During regulation, there will always be 1.2V across the 1.2 ohm resistor. The current regulator insures that the LEDs always run at their maximum brightness, but not so bright that they burn out. A 100uF electrolytic capacitor bypasses the DC power input to the lamp and a 100nF monoblock capacitor bypasses the LM317K input.


The LEDs and current regulator circuit were mounted on a 3" x 8" chunk of 1/8" aluminum stock. The LM317K regulator and LED heat sinks were bolted to the chassis directly, heat sink grease was used on the regulator, the heat sinks and the six LEDs. Connecting the LM317K directly to the aluminum plate makes the plate electrically hot at 1.2V, the plate should not be allowed to come into contact with any live conductors. By using a few more parts, the LM317 can be mounted with an insulator and plastic shoulder washers for electrical isolation from the mounting plate.

The LEDs come mounted on their own small star-shaped aluminum substrates, these were attached to the aluminum plate using two 7/16" 4-40 screws and nuts per LED. A drop of silicone heat sink grease should be applied to the center of each LED star when it is mounted to the plate for heat conduction. It is important to use insulating plastic washers on the top side of the LED stars to prevent electrical contact with head of the screw.

The LED stars were soldered together using short pieces of #20 tinned wire after being mounted on the plate. It is necessary to use a fair amount of heat to solder the contacts, a 200/240W soldering gun did the job. Be very careful not to melt the lenses on the LEDs, they are expensive. The positive and negative leads of the LED series string were connected back to the current reglator circuitry using #20 wire covered with teflon insulation.

The initial mechanical arrangement did not pass the "rule of thumb" test, which says that if a semiconductor is too hot to hold your thumb on, it will not live a long life. Two large aluminum heat sinks were bolted to the back of the aluminum plate and seem to be sufficient to keep the lamp operating at a reasonable temperature. The LED array produces more heat than the LM317K.


Connect this circuit to a 24VDC power supply or other power source such as a solar-charged lead acid battery. Be sure to observe the correct polarity. Look away from the LEDs and apply power. Again, do not stare directly at the LEDs, they are bright enough to harm your vision. A switch-mode power supply rated at 24VDC and above 1 Amp is probably the most energy-efficient way to power this device from line power.


1x LM317K T03 case 1.5A adjustable voltage regulator
1x 1.2 ohm 5W resistor (or 2x 2.4 ohm 2W resistors in parallel)
1x 100uF 35V or higher electrolytic capacitor
1x 100nF 35V or higher monolythic capacitor
1x LedEngin LZ1-10R205 deep red 5W LED
1x LedEngin LZ1-10R105 red 5W LED
1x LedEngin LZ1-10A105 amber 5W LED
1x LedEngin LZ1-10G105 green 5W LED
1x LedEngin LZ1-10B105 blue 5W LED
1x LedEngin LZ1-10CW05 cool white 5W LED
Miscellaneous wire, solder lugs, termination strips and hardware,
Large aluminum mounting plate, heat sinks if necessary.


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