Small Desk Lamp – A desk lamp is a useful tool for both working with electronics or simply reading on your desk. LEDs save a lot of energy and together they provide more than acceptable lighting. For this project, a 12V DC power supply is used. Instead of adding a series resistor for each LED, the most efficient way to power them from 12V DC is to connect them in series in groups of three. This means that you need 10V DC for each set. But how to get 10V from a 12V source? One way to do this is by pulse width modulation (PWM). A current of pulses is sent, too fast for the eye to see, and the spaces between these pulses are varied to limit the average current.
If a potentiometer is added, it can act as a dimmer. Recently I started wondering if I could downsize and upgrade the original small desk lamp. So I took a fresh look at those traditional, through-hole, single-component LED “indicators” (as they are properly known). The 5mm ones are now a lot more powerful than the 1cm type I used in the past. Some of them, known as “5-chip,” have 5 light-emitting elements squeezed into one 5mm package, sucking down 100mA of forwarding current at around 3.3V DC. They’re still rated at 100,000mcd, but 6 years ago, the ones I used were rated for only 20° of beam spread.
Today’s 5-chip LEDs claim a spread of 60°. I opted for a 12V DC power supply, to make the small desk lamp function in motor homes, where LEDs are ideal to conserve power. For use with 115V AC, you need an adapter that delivers 12V DC at 1 amp. Instead of adding a series resistor with each LED, the most efficient way to power them from 12V DC is by series-wiring them in threes. This means you need 10V DC for each set. How to get 10V from 12V power? Pulse-width modulation is the way to go.