CREE XM-L T6 LED Driver Circuit – Specifications and Practical Application

This article explains how to illuminate a Cree XM-L T6 LED using a current controlled driver circuit while the supply input is from a battery, or in case a mains SMPS is intended as the driver unit. The idea was requested by Mr. Jaco.

Technical Specifications

Thanks for the great advice and circuits! Have you had a chance to have a look at a circuit for the LED mentioned by Guruh?

I would like to retrofit my 3 cell Maglite with this Cree LED and upgrade the batteries to Li Polymer. Do you have any advice on the battery voltage I should choose and how would I achieve changing the intensity of the LED to a high, medium and low state with the existing on/of switch?

Typical information on the LED:

1. CREE XM-L T6 Mounted on star board
2. 2.9V-3.5V 3000mA 6500K
3. Maximum Drive Current 3 A
4. Maximum Power 10 W
5. Light Output 1040 lm @ 10 W
6. Forward Voltage 3.1 V

Regards and thanks in advance,
Jaco

The Design

For a battery operated circuit the LED driver could be simply in the form of a current controller stage, because here voltage regulation is not important and can be eliminated.

As per the above request, the Cree XM-L T6 LED driver is required to be operated from a 3.7V/3amp source, with a 3-way switchable dimmer control facility.

The design can be implemented using the following transistorized current control stage. Although it's not one of the most efficient of the designs, the simplicity wins over the slight inefficiency.

Referring to the above diagram, the design is a basic current controlled stage where T2 determines the maximum current limit of T1 by controlling the base potential of T1.

Circuit Operation

When the circuit is switched ON, T1 is triggered via R1 illuminating the LED. The process allows the entire current consumed by the LED to pass through one of the selected resistors (R2, R3, or R4) to ground.

This induces a proportionate amount of voltage across this current sensing resistor, which in turn forms the triggering voltage for the base of T2.

If this sensed voltage exceeds 0.7V, T2 is forced to trigger and ground the base potential of T1, thereby restricting its conduction, and subsequently restricting power to the LED.

The LED is now forced to shut down, however the process as the LED tries to shut off it also begins reducing the voltage across the particular base resistor of T2.

T2 now experiences a loss of triggering voltage and switches OFF, restoring the LED back to its original state via T1, until again the restriction process is initiated and this continues, maintaining a current controlled illumination over the connected LED, which is a Cree XM-L 10 watt lamp in this case.

Here R4 must be selected to allow the LED to illuminate with optimal consumption (max brightness), that is at its rated 3 amp level....R2 and R3 may be selected to offer any other desired lower current operation (lower intensity) to the LED such that by selecting these produces three different intensity levels for the LED.

Parts List

T1 = TIP 41 (on heatsink)

T2 = TIP 31 (on heatsink)

R1 may be calculated by using the following formula:

R1 = (Us - LEDv) x hFe / LED current

= (3.5 - 3.3) x 25 / 3 =  1.66 ohms

Wattage of the resistor = (3.5 - 3.3) x 3 = 0.6 watts or 1 watt

R2, R3, R4 may be calculated as:

Low Intensity = R2 = 0.7/1 = 0.7 ohms, wattage = 0.7 x 1 = 0.7 watts or 1 watt

Medium Intensity R3 = 0.7/2 = 0.35 ohms, wattage = 0.7 x 2 = 1.4 watts

Optimal Intensity = R4 = 0.7/3 = 0.23 ohms, wa ttage = 0.7 x 3 = 2.1 watts

Operating through SMPS

In order to drive the proposed Cree LED from an mains operated SMPS, the following steps may be incorprated in order to implment the required volatge and current controlled operations:

1) Procure a 12V/3amp readymade SMPS.

2) Open it and look for the small optocoupler part on the PCB. This will look like a small 4-pin black IC.

3) Once you have located it, modify its input side by carefully conducting all the instructins as indicated in the following article: https://www.homemade-circuits.com/how-to-make-variable-current-smps/