7 segment display Charlieplexing with pic

i am Charlieplexing 6 1 inch 7 segment display with pic16f886, the segments are driven using uln2803 and each individual digits are driven using 547 and 557 transistor. i am using common anode display.

i am using 3.3 ms timer interrupt to display the data. every thing works fine expect the brightness
i am using 100 ohms resistor @5v for each segment . the 100 ohms is in series to the uln2803 ic.


if i measure the current in multi meter i am getting only 4ma. this is due to fast switching of segments in timer intrupt, if i reduce the resistor to 47 ohms the brightness is ok and current in multimeter is around 9ma but actual theoretical current level is 40ma that exceeds the data sheet limit of 25ma per segment. but nothing is happen tested for 2 hours. my doubt is whether this is safe to operate in Charlieplexing mode.

That sounds more like multiplexing then Charlieplexing. Charlieplexing is a 'nickname' for a method of driving more LEDs by reversing the polarity across them, multiplexing keeps the same polarity but divides the time allocated to each digit.

The answer is you need to check the data sheet for the LED. It should tell you the maximum continuous current for each segment and the peak allowed current, including how long the peak current can be tolerated. As long as you stay within the peak current figure and the time limit you should be OK.

Brian.

A complete display datasheet has specifications for continuous and peak current, the latter preferably with duty cycle and pulse width. 40 mA peak current in 4:1 or 6:1 mux should be fine for most displays.