PaulRB:
Hi,The actual instantaneous current will be a bit less than 20mA because the transistors or other drivers on the columns and the rows will "drop" some of the 5V, so the voltage accross the LED + resistor could for example be as much as 1.5V less than the full 5V. This is is still true even if the Arduino outputs themselves are driving the matrix directly.
That is the case in my current project. 2x5 (rows/columns) LED matrix.
Less voltage means less current, of course, but you could adjust the resistors to a lower value to compensate (as long as there is still enough voltage to drive your LEDs: blue and white LEDs can need up to 3.2V to run at all).
I'm using blue ones.
If an LED is rated at 20mA maximum continuous current, it may be able to take 80 - 100 mA for short periods. You need to check the data sheet for the LED to find out how much and how long. Usually I have found its OK to use double the max continuous current, if the LEDs will be on for only one eighth of the time.
That's the issue when you order bunch of LEDs on eBay. The 5mm blue ones I've got didn't come with that information (short period current).
There is a danger to over-driving LEDs. If the multiplexing relies on your sketch running correctly (rather than using an external chip like max7219 to perform the multiplexing), then if your sketch stops for some reason, the LEDs that were switched on at the time will then receive more than their max current for a long time and could be damaged.
I'm fairly sure that can never happen - I'm testing my code for couple of months already. But, yeah, it relies on sketch running correctly and that is useful information.
So, I shouldn't expect any issues connecting that same 2 row/5 column matrix on Arduino using 100 Ohm resistors for rows?