On 1/18/12 9:29 PM, Thomas K. wrote:
I have been trying to get this circuit to work for a few days and it
is driving me nuts!
I'm trying to make an array of tricolor LEDs (RBG with common
cathode). The goal is to use 5v logic to turn on MPSA92 and light up
the array.I have 4 LEDs with three anode resistos (one for each
color). I'm working with just the red now, as the others will be
clones of the same circuit.
When I connect the emitter to to +13v, base to ground or +5 through
any resistor and collector to the array, it lights up. Only when I
disconnected the resistor (open), does the array turn off.
I have tried base resistors from 200 ohm to 200k ohm, and the same
deal. When I use +5v on the emitter, however, the circuit works as it
should. I feel like I am missing a fundamental property of PNP
transistors. Any suggestions?
Connect the emitter to the same voltage as the base circuit, and it will
work. The base-emitter junction is a forward-biased diode, so any more
than 0.3 volts will cause the transistor to turn on.
A better design would use common-anode RGB LEDs and an NPN transistor
pulling each cathode of the LED to ground with a 0 to 5V signal on the
base through a series resistor of >10K value.
--
David Forbes, Tucson AZ
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