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LED Dazzler: A Driver Circuit For Really Bright LEDs

Caution: these LEDs are so bright they will burn your eyeballs!

by Nicholas Vinen

Specifications

Input voltage 12-30V

Output current 0-3A

Input current Up to 3A

LED power 1-10W each

Number of LEDs 1-3 (12V supply), 1-6 (24V supply)

Efficiency Up to 94.5% (see graphs)

Drop-out voltage 0.5V

Features Dimming, standby switch

Line regulation <4% output variation over 12-30V

Temperature regulation <3% output variation under typical conditions

Low battery cut-out settings 12V, 24V or disabled

Low battery cut-out, 12V setting 11.5V (negative-going), 12.0V (positive-going)

Low battery cut-out, 24V setting 23.0V (negative-going), 24.0V (positive-going)

Low battery cut-out current <1.5mA for 12V, <2mA for 24V

We’re not joking about the warning. Even a brief glance at these white LEDs while they are operating at full power will leave spots before your eyes for quite a while afterwards. They are blinding and they do hurt your eyes. We definitely do not recommend looking at them for even the briefest glance. You wouldn’t look at a laser – well don’t look at these either!

And don’t be fooled into thinking that the light output is in a narrow beam; the built-in optics do an excellent job of distributing it over a wide area, with a viewing angle of 130°. So even if you are well off-axis, they are painfully bright.

Until now, you might have thought that a 50W halogen lamp was pretty bright but these LEDs are much brighter (at 900 lumens) and they use a fraction of the power – just 10W. It doesn’t take a mathematical genius to realise that this means big energy savings. At the time of publication, these are the brightest LEDs you can get (as far as we know).

They are made by Seoul Semiconductor in Korea and they go by the utterly prosaic description of type W724C0-D1. Their rated brightness is 900 lumens, with a colour temperature of 6300K and a colour rendering index (CRI) of 70. Careful examination shows that they consist of four LED dies connected in parallel under a plastic lens which does a good job of focussing the light.

However, this is all academic if you have no way of driving them. LEDs are quite difficult to drive correctly, especially when they need 2.8A at 3.6V. They require an efficient current source, otherwise the high efficiency of the LEDs can be spoiled by wasteful driving circuitry.

This project will drive up to six of these dazzling LEDs (depending on supply voltage) and it also provides dimming. The efficiency of the circuit is up to 94.5% (see Figs.2-4). The operating supply voltage range is from 12-30V.

This design will power virtually any high-brightness LED (1W, 3W, 5W etc) from a low-voltage DC supply, including both white and coloured types. It incorporates a low battery cut-out for 12V or 24V batteries to prevent over-discharge, a standby switch and an integrated fuse.

Click for larger image
Click for larger image
Fig.1: the typical buck step-down regulator configuration (top) compared to the inverted configuration used in this circuit. In each case, the current flow is indicated during the two phases as I1 and I2.

The challenge

Driving high-power LEDs is tricky. If driven just below their nominal forward voltage, little current will flow and not much light will be produced. Conversely, if driven just above their nominal forward voltage, they can overheat and burn out.

The traditional approach is to use current-limiting resistors and a voltage source such as a 12V battery. This works but it wastes power in the current-limiting resistors and also has the disadvantage that the brightness of the LEDs varies quite markedly with relatively small changes in the supply voltage. As a result, it is much better to drive these high power LEDs from a regulated current source.

This new design is a switchmode step-down regulator that uses a single high-current Mosfet. Each 10W LED requires 2.8A at 3.6V and so with a 12V supply, you can drive three 10W LEDs in series. Or with a 24V supply, you can drive up to six LEDs.

Unlike some other LED driver circuits, this one needs no adjustment to suit different LED types, except to change one resistor to set the amount of current they require. Hence, this driver circuit is suitable for driving virtually any high-power LED, including those from Cree and Luxeon.

Where to get the LEDs

These 10W LEDs are available from RMS parts (www.rmsparts.com.au) as Item Code W724C0-D1. At the time of writing they cost $26 each plus GST (less for bulk purchases).

Also available from RMS Parts (but not listed on their website) are the small aluminium PC boards which are used to mount them. These have Item Code STAR-P7 and are available at additional cost (contact RMS Parts for more details).

Heatsinking

The biggest problem with high-power LEDs is heat. Without an adequate heatsink they get stinking hot and can be destroyed in a very short time. Even if you don’t run the LEDs hot enough to melt them, if they are operated at a high junction temperature, they will have a short life. So an adequate heatsink is very important.

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