Archive for October 3rd, 2010
So I added a cheap 72-LED worklight as a box-filler to a recent order. Popped in four AA alkaline cells, clicked the switch, and … huh, that thing looks dim.
Took it apart and what do we find inside? Resistors!
Over on the left lives a pair of 10-Ω resistors that limit the overall LED current. They’re in parallel, so it’s running with a 5-Ω ballast.
Over on the right, each string of 24 parallel LEDs has a separate 10-Ω equalizing resistor.
A quick ammeter check gives the dismal news: total current is 220 mA = 73 mA per string of 24 LEDs = 3 mA per LED.
Well, no wonder why it’s dim!
The ballast resistors drop 0.22 x 5 = 1.1 V, each equalizing resistor is good for 0.073 x 10 = 0.73 V, and (as you’d expect) the LEDs run at about 4 V.
Run it on rechargeables and it’s much worse.
Given the low price, I’d expect these LEDs to fall over dead if I goosed them all to 20 mA… not to mention, 0.02 x 72 = 1.44 A is a pretty stiff load for alkalines and the housing wouldn’t stand up to nearly 9 W of power dissipation.
Ah, well, it’ll come in useful here & there…