Hi everyone,
I'm planning on making a digital 4 digit, 7 segment wall clock. As light sources I wanted to use these led chip bars from banggood (1W):
I was thinking of using some kind of darlington array transistor (ULN2003A?) for the 7 segments and 4 transistors for the common anodes of the digits.
Does anyone have experience in this area?
Thanks alot!
These COBs seem to be made with many LEDs in parallel.
Low voltage/high current means you probably need a transistor for each segment.
Even a ULN2003A can't switch five segments (5*330mA) at the same time.
Would be easier to use high voltage (12volt)/low current strips.
Then you can use a TPIC6B595 shift register per digit, and cascade them all from three Arduino pins.
Leo..
Thanks for your answer!
What if I multiplex through each segment and digit from start to end? Then the current flowing through one pin of the ULN won’t exceed the 300mA right? I world just need a higher current translator for the common anode?
I think you need to figure-out how you're going to power/control ONE LED first and I wouldn't recommend matrix wiring!
LEDs are "current driven" devices and high power LEDs (1W and up) are normally powered by a switchmode constant current power supply. (Not easy to build an usually not cheap.) The resistance of an LED (or any diode) is non-linear and it changes drastically with slight changes in voltage. With constant current the voltage "magically falls into place" and you get the correct power.
With "regular little LEDs" we use a series resistor with about half of the voltage dropped across the resistor. For example, we can have a 5V supply with about 2V across the LED and 3V across the resistor. We then use Ohm's Law to calculated the required resistance for the desired current. (In a series circuit the voltage divides and the same current flows through both devices.)
Power is calculated as Voltage x Current, so in the above example the resistor is dissipating more power than the LED and the circuit is inefficient. With regular LEDs (maybe 50mW) that's not a big deal. But, when the resistor has to dissipate 1W or more you need a power resistor, you're wasting energy/heat, and it's rarely done in "real products".