Step-up Converter for White LEDs

Published:2011/8/22 1:43:00 Author:Li xiao na From:SeekIC

By Dirk Gehrke

White LEDs are increasingly used for lighting applications in battery-powered equipment. To cope with the effects of a fluctuating supply voltage, a constant-current source is a must.

There are now countless applications where while LEDs (light emitting diodes} are used as (background! light sources. As examples we could mention ICD or keyboard backlighting. Alternatively, white LEDs have advantages in bicycle lights and energy-friendly pocket flashlights.

The circuit shown hare allows two variants to be built:

-up to (our series-connected white LEDs powered from two series-connected NiMH, NiCd or alkaline cells;

-up lo six series-connected white LEDs powered from three series-connected NiMH, NiCd or alkaline cells, or from a single lithium-Ion cell.

For the first option it is sufficient to have on input voltage range of 1.8 V to 6.0 V, for the second, a range of 2.5 V to 6.5 V. The TPS61042 from Texas Instruments is a PWM (pulse width modulation) step-up converter configured us a constant-current source. This setup allows a constant current How lo be set up through a number of series-connected white LEDs. The value of resistor R1 between the FB and RS pins of the chip and ground fixes the LED current al 19 mA. An on-chip OVP circuit protects against damage from overvoltage. The circuit deactivates the switching transistor at an output voltage greater than 30 volts. A further useful function is the ability to switch the chip on and off via the CRTL pin. The switch-mode converter is activated when the CTRL pin is taken to the input voltage level When CTRL is pulled to ground potential, the TPS6I042 goes into ’sleep’ mode, reducing its current consumption to o mere 0.1 uA.

DIY Through-Plating

For a double-sided printed circuit board to be produced successful^ with minimal tools, it is first necessary to have perfectly aligned films of the solder and component sides of the board. Another important aspect is the through-plating of those holes in the board that establish a contact between tracks o* pads at the component and solder side. A common error mode by beginners is IC sockets being nearly aligned and soldered at the solder side and then discovering that it is impossible to reach the pads al the component side that also need soldering. Without trough-plated holes, component leads with a pad contact at both sides of the board need to be soldered at the component as well as the solder side!

A method often applied in the Elektor labs when stuffing prototypes of boards that will be moss-produced ’through-plated’ is to run o very thin wire (copper litz) along with the component lead and soldering it onto a copper track. The wire is easily obtained by pulling it from a multi-strand cable. In this way, contact is established between the board sides without having a real through-plated hole available. There are, however, alternative ways. Using the ’Copperset’ system from Multicore it is possible lo produce o perfect electrical contact with a professional look in no time al all.

Holes ’hat need to be through-plated are first filled with lubes (bail bars) snapped off from a larger bar. The tubes contain o solder core (I). Using the mini anvil and the centre punch tools |?) the tube is fixed in the hole (3} and then soldered (4). Applying the usual desoldering methods, the lube is then opened (5) allowing the component lead to be inserted and soldered (6). If the hole only serves as a ’via’ then step* (5) and (6) ore not required, the Multicore Copperset kit contain the special tools and three bars of 500 tubes with diameters of 0.8 mm, 1.0 mm and 1.2 mm. The kit is not cheap (Farnell) but fortunately the bait bar lubes are also sold separately and what’s more they are easy to handle with just common-or-garden loots. The system is suitable for boards with a thickness of up to 1.6 mm. QKHZ2 D

Simple Mains Failure Alarm
Jacob Gestman Geradts

A ’Mains Failure Alarm’ was published on page 70 of the July/August 2004 issue of Eleklor Electronics. Although that circuit is full of Ingenuity, it may be a bit over the top given the purpose, which boils down to making a noise when the mains voltage disappears. The author thought the same could be achieved with considerably fewer parts.

If you compare the two schematics, you will agree that although a lot of parts have been omitted, a back-up energy supply remains a central and indispensable part. In the version proposed here, an array of NiCd cells is used. The author salvaged them from on old cordless drill that was binned because of a duff battery pack. As in many other cases, just two cells turned out to be beyond hope, the others working just fine!

With mains voltage normally present on the power outlet, the cells are charged with a small current to keep them lopped up. The charge current is supplied via the mains adapter, Dl and R l. The latter is dimensioned such that a trickle charge current of a just a few mA is obtained.

When a power cut occurs, the transistor in the circuit will conduct and DC buzzer Bz l will produce some noise. Once the circuit has done its job — alerting you that the mains voltage has vanished — you or your housemates will soon want la silence the buzzer. This is done by pressing SI, which takes the transistor’s base to ground.

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