Four seven segment displays are multiplexed by a 16F648A. Now I need to calculate the required value for the segment resistors.
Supply voltage 5V.
Segment voltage 1,8V.
Segment current 30 mA cont.
Segment peak current 80 mA (1/10 duty cycle, 1mSec pulse width).
Segment array resistors 180 ohm.
When displaying 4×8 supply current 60 mA
When displaying 1×8 supply current 15 mA (cathode removed from the other 3)
Assumptions not sure:
Display duty cycle 25% (4 displays).
Display segments all on at once. ( one instruction in the pic)
Actual segment current 60 mA/7 = 8,6 mA.
How to calculate the segment resistors value for the max. permissible current.?
The 80 mA peak might be to high for the pic. But first I need to find out how to calculate.
Resulting from the answers and comments I have continued as follows.
While multiplexing still active I selected only one digit to be active and connected the oscilloscope over one segment resistor. The outcome of this was that the voltage drop over the resistor is between 1,7 and 1,9 V. So let us take 1,8 V over a resistor of 180 ohm. Resulting in an I/O current of 10 mA. Far within the reach of the 16F648A. I decreased the resistor to 68 ohm and found that the voltage dropped to about 1,2V resulting in an I/O current 17,6 mA. The brightness of the display only increasing very little.
I measured also the duty cycle and it is indeed 25%.