Our new LED torch uses the Luxeon STAR/O 1W white LED which comes with its own collimating lens assembly.

We previewed the Luxeon 1W and the truly awesome 5W version in the May 2003 issue and this torch is the first of a series of drive circuits for the 1W version.

In the last 12 months LED torches have finally arrived. This white LED torch provides a similar light output to its incandescent bulb counterpart yet uses far less current from the battery. It gives a beautifully soft light beam which maintains a constant colour and similar brightness over the whole battery life. And the LED should never need replacing.

Compared with a typical conventional torch, this LED torch has a much wider and more evenly distributed beam. Torch bulbs typically have a very small bright spot with weak diffuse light surrounding it.

The white LED torch provides a beautifully even distribution of light which can light up a fence gate (or whatever) at more than 15m. At this distance the beam is about 5m in diameter.

Apart from its sheer light output, this LED torch provides produces a natural white light instead of the yellowish light from torch bulbs.

And it continues to produce this constant white light regardless of the battery condition, until they are virtually flat.

High efficiency

These new white LEDs are much more efficient than torch bulbs. The Eveready KPR102 Krypton light bulb used in the torch we are using, is rated to deliver 16 lumens of light output, when drawing 0.7A from a 2.4V battery; equivalent to 1.68W.

In effect, the KPR102 bulb produces 9.52 lumens/watt. By comparison, the Luxeon 1W white STAR/O LED is rated at 18 lumens/watt - almost twice as efficient!

Consider also that this LED torch will continue to operate when the cells are down to less than 1V (0.5V each).

This is long after a conventional torch would have expired. The LED torch also gives you plenty of notice. We estimate that typical alkaline D cells will last for several days before they give up.

The Luxeon 1W LED assembly includes a lens which focuses the light into a narrow beam. Heat produced by the LED is dissipated onto a 25mm square aluminium PC board which is an integral part of the LED package.

Note that this is all the heatsinking required as the maximum heat developed would be less than 1W and the heatsink size is sufficient to maintain the temperature only a few degrees above ambient. In practice, the heatsink runs slightly warm to the touch.

Drive requirements

The Luxeon 1W LED requires about 3.4V in order to produce its rated output. If we are using a 2-cell torch, this means we need to step up the voltage with a DC-DC converter which should be as efficient as possible. After all, we do not want to use an efficient light source and then waste power in the converter.

In practice, our DC-DC converter has an efficiency of well over 80% over the likely operating battery voltage range of 3V down to 2V. Below 2V the batteries are essentially exhausted but compared to conventional torches, battery life will be considerably extended.

The complete Luxeon LED torch circuit is shown in Fig.1. It uses a number of semiconductor devices specially manufactured by Zetex to achieve high efficiency in a DC-DC converter. Heart of the circuit is IC1, a DC-DC converter which can operate from a supply voltage between 0.93V and 3.5V. It includes current sensing and voltage sensing inputs.

In operation, IC1 switches base current to a low saturation transistor, Q1 which turns on to build up current build through a 22μH inductor, L1.

This current is monitored by the emitter resistor R1 and when it reaches 0.53A, transistor Q1 is switched off and the current flowing in the inductor is diverted to the LED via diode D1. This switching runs at around 60kHz, depending on the battery voltage. The resulting current pulses are filtered by the 220μF capacitor to provide DC to the LED.

Losses in this conversion are mainly in the inductor, the switching transistor Q1, current sense resistor R1 and the diode D1. Efficiency will be high if we can minimise these losses.

Since the inductor current is limited to 0.53A (peak) while it is rated at 3A, it will not saturate and will therefore have minimal heating losses. At the same time, transistor Q1 is a low saturation device. Its collector emitter voltage is a maximum of 45mV at 1A which means that there will be little power loss in this device.

R1, the current sensing resistor has a value of only 33mΩ (33 milliohms) so the maximum voltage drop when the inductor current reaches 0.53A is a mere 17.5mV.

Power dissipation in this resistor is so low that even with a constant 0.53A through it, the power would be less than 10mW. In practice, it will be less than 5mW.

Losses in diode D1 are kept to a minimum because it is a Schottky type with a rated 385mV forward voltage at 1A. Further efficiencies in the conversion are due to the very low quiescent current drain of IC1 at less than 300μA, and the way Q1 is driven.

Transistor Q2 is used to boost the current drive to the base of Q1. IC1 senses the voltage across the 3.3Ω resistor at Q2's emitter and limits current flow to around 7.5mA into Q1's base.

Q2 therefore operates as a current source providing the base current to Q1. When the Vdrive output of IC1 at pin 8 goes to ground, the base drive to Q1 is off and so the transistor switches off, allowing L1 to deliver its power to the load via diode D1.

The output power delivered to the 1W LED is related to the peak current in L1, the switching frequency and the difference between the input voltage and the voltage across the LED.

The power is regulated using the sense resistor R1 to detect peak current and by sensing the voltage across the LED.

VR1 and the 22kΩ resistor divide the LED voltage down and feed it to the FB (feedback) input, pin 6 where it is compared to an internal voltage reference which is around 730mV (nominal).

Heavy switching currents drawn from the battery and delivered to the load are smoothed out using low impedance capacitors.

Note that good efficiency of the conversion is also dependent on the low effective series resistance (ESR) of the decoupling capacitors. We have specified two 220μF 10V ZL series capacitors from Rubycon. These have an ESR of 130mΩ at 100kHz.

You could improve efficiency slightly by using the ZA ultra-low impedance 220μF 10V Rubycon capacitors with 44mΩ impedance instead. However, these cost around ten times more than the ZL series!

Construction

The 1W LED torch is installed into an Eveready WP250 water-proof torch which uses two D cells. We have designed a PC board (coded 11211031) measuring 33mm in diameter to mount the DC-DC converter components. Note that all components mount on the copper track side of the PC board, opposite to what you would normally do.

At the time of writing, none of the kitset suppliers had decided to make a kit available for this project. However, the parts can be obtained from the suppliers mentioned below.

You can obtain the 1W LED from Alternative Technology Association, PO Box 2001, Lygon St North, East Brunswick, Vic 3057. Phone (03) 9388 9311; Fax (03) 9388 9322; website www.ata.org.au. Parts listed with a Farnell catalog number can be obtained from Farnell. Phone 1300 361 005; Fax 1300 361 225; website www.farnellinone.com.

The PC board can be obtained from RCS Radio Pty Ltd, 41 Arlewis Street, Chester Hill, NSW 2162. Phone (02) 9738 0330; Fax (02) 9738 0334; website www.cia.com.au/rcsradio.

Begin construction by checking the PC board carefully. The board should be circular as shown and may need to be cut and filed to shape first. Check for any possible shorts or undrilled holes. The PC board only has five holes, four for the mounting screws and one for the PC stake. The mounting holes can be drilled out to 2.5mm in diameter or you can file the hole in from the edge of the PC board to form an elongated slot.

The three main semiconductor devices are small surface mount types which should be soldered in first. The orientation for these is shown in the overlay diagram of Fig.2, with the labelling oriented as shown.

To solder these in, you will need a fine tipped soldering iron and a magnifying glass. Place one of these parts in position and solder one outside pin first. Check that it is oriented correctly and that the remaining IC pins lines up with the tracks on the PC board. When correctly lined up, solder the remaining pins. Now solder in the other semiconductor devices in a similar manner.

Next, solder in the 33mΩ resistor and the other surface mount resistors. Note that the 3.3Ω and 22kΩ resistors can be standard 0.25Ω resistors instead of surface mount types and provision has been made to install these with an extra circular pad allocated and spaced for the extra resistor length. All components must be installed on the copper side of the PC board, except for the +3V supply PC stake.

Trimpot VR1 is mounted by bending the leads as shown in Fig.4, so that they contact the PC pads allocated for this component and soldering in place. The remaining components are installed by soldering the leads to the copper pads.

Keep components below 12mm above the PC board. The capacitors and inductor need to be bent over as shown in the photographs.

Cut out a 32mm diameter disk of tinplate from a tin can lid and place this on the back of the main PC board. Fig.5 shows the details. Drill a hole where the PC stake fits through and solder this tinplate disk in place. Cut the PC stake flush against the tinplate. Also drill and file out the four mounting holes.

As mentioned, the LED torch is built into a standard Eveready WP250 water-proof torch. The reflector needs to be removed from the lens cap so that the 1W LED can be installed.

To remove the reflector, scrape around the inside of the lens cap where the reflector sits, to remove the plastic that has been heat welded to the reflector. We used a flat screwdriver and scraped away till the reflector came loose.

The 1W LED assembly will require a small amount of filing at each corner base so that it will sit comfortably within the reflector and no more than 5mm above the reflector lip. This is to prevent the LED assembly making contact with the inside of the torch lens. Fig.6 shows how the Luxeon LED is installed and connected.

Note that if you install the LED in a different torch, you may need to drill four holes in the reflector so that each corner of the LED assembly can sit inside the hole.

The PC board is installed at the rear of the torch reflector assembly using a 240VAC bayonet lamp holder skirt. This is cut down to 16mm in height from the screw thread end and glued to the plastic flange at the rear of the torch reflector using super glue.

The PC board is placed over the rear of the bayonet lamp holder and the four holes are drilled 2mm in diameter for the securing screws.

Note that you will need to scrape away a little of the bayonet holder for the solder connections to sit into allowing the PC board to sit flat against the rear of the holder.

Also mark the orientation of the PC board onto the bayonet lamp holder so that it will be installed with the same orientation each time. We used a red marking pen to show the correct orientation. Fig.7 shows these details.

Setting up

Start the lens assembly by feeding the LED leads through what was the lamp hole in the reflector. The cathode (black) wire needs to be soldered to the threaded section, as shown above. Next goes the bayonet lampholder skirt which we removed earlier and cut down to 16mm deep. The lampholder (and of course the lamp itself) are not used - that's the whole point in making this very efficient LED conversion! Finally the assembled PC board is secured into position. This already has the tinplate disc soldered to it, with the whole assembly ready for mounting inside the torch body. The torch switch will still work and battery position will be the same.

Wire the circuit up as shown but with a 0.1Ω, 5W resistor in series with the LED.

Set VR1 fully anti-clockwise and connect a multimeter across the 0.1Ω resistor set to read DC millivolts. Using a piece of wire, connect the two D cells to the torch (take care to get the correct polarity) and adjust VR1 for a reading of 35mV. Then remove the 0.1Ω resistor and finish wiring.

Attach the PC board to the bayonet lamp holder skirt with the M2 screws. Assemble the torch together, making sure the batteries are placed in with the positive side up.

Parts List - 1W Star LED Torch 1 Eveready 2 D-cell WP250 waterproof torch (KMart) 1 Luxeon 1W white STAR/O LED (LXHL-NW98) (Alternative Technology Association) 1 PC board coded 11211031, 33mm diameter (RCS Radio Pty Ltd) 1 Ringgrip mains bayonet lamp holder skirt (LH19/RB-WE) (KMart) 1 32mm diameter tinplate disk (or brass) 1 22μH 3A axial choke 7mm diameter x 26mm long (Epcos B82111-E-C22) (Farnell 608-671) 1 PC stake 4 M2 x 6mm screws 1 50mm length of red hookup wire 1 50mm length of black hookup wire Semiconductors 1 ZXSC100N8 Zetex DC-DC Converter SO8 package (IC1) (Farnell 384-7962) 1 ZXT13N20DE6 Zetex low Vcesat NPN Transistor (Q1) (Farnell 334-6870) 1 BC559 transistor (Q2) 1 ZHCS2000 Zetex Schottky diode (D1) (Farnell 411 5843) Capacitors 2 220μF 10V Rubycon ZL series Ultra Low Impedance electrolytic (Farnell 768-080) 1 1nF ceramic capacitor (code 102 or 1n0) Resistors 1 33 milliohm 1W surface mount resistor (R033) (Welwyn LR series 2010 case) (Farnell 361-0238) 1 22kΩ 0.063W surface mount 0603 case resistor coded 223(Farnell 911-392) 1 3.3Ω 0.063W surface mount 0603 case resistor coded 3R3 (Farnell 357-1130) 1 100kΩ miniature horizontal trimpot coded 105 (VR1) 1 0.1Ω 5W resistor (for setting up); coded 0R1

Notes & Errata Some constructors have had difficulty obtaining the ZXT13N20DE6 transistor (Q1) for this project. A suitable alternative is the ZXT13N50DE6, which has a higher Vceo rating but is otherwise identical. The ZXT13N50DE6 is currently available from Farnell, Cat. 334-6882.