led matrix power consumption

Suppose that I have an LED matrix, each of its led has a forward current of 20mA.
If I want to turn on all LEDS, by turning them on and off one by one quickly, will this consume only the power equal to turning on just one LED (20mA)?

Basically, yes, if you only have on LED on at a time then only 20mA will be drawn as a whole by the matrix.

If you are trying to display, say, a character on the matrix, it won't appear as bright as having the LED's on continuously.

i think, although the LED is turned on alternately one by one quickly, it will drain more than 20mA current, because timimg led on much longer than the led off, so be led in on it together

So would a SR, each pin can sink 30mA (total 70mA ) be enough to sink the current from the cathodes?
If I would like to increase the brightness, is there any other efficient way to drive the LEDs, avoid turning them all at the same time? perhaps increasing the LED current will do?

Yes, you can increase the LED current to compensate for the dimming. Check the maximum peak LED current (and time it is allowed), maximum average current, and the maximum current through your drivers, then calculate the series resistors to give the maximum current without exceeding any of these.

The average LED forward current is simply given by the proportion of time it is on, multiplied by the driving current. For instance, if an LED is on for 50ms every second, and is driven with 200mA, then its average current is (50/1000) x 0.2 = 10mA.

Remember, don't exceed the peak or average currents, or drive the peak current for longer than the datasheet allows. Especially when testing - don't allow the peak drive current too long!

So let say to turn on a 10 by 10 LED matrix, if each LED is on for 1 ms, the average ON period for each LED will be 1/100 duty cycle. Assume the 1st LED will be on right after the last LED is off, this is equal to 1% of continuous 20mA brightness...

But then if I wanted to make the intensity adjustable by implementing PWM, each LED having 50% PWM will only have 0.5% of 20mA brightness...maybe I should drive 10 LED in series at a time around the peak current

Can you please explain more about the average current part? Say if I intend to drive each LED with 30mA for 1ms, the current through each cannot exceed 0.001 x 0.03 = 30micro-amps?

Hi, I am looking forward to any reply

That is very suspicious math!

Basically, the LED current must NEVER exceed the manufacturers limit, no matter for how long.

The perceived brightness is proportional to the average current.

The average is taken over time, at least the duration of one full on and off period. It is the current you have decided multiplied by the ratio of on to off time. For example, if you set the current to 10mA and it was on for 25% of the time, off for 75%, the average current would be 2.5mA (25% of the total).

If you want to make the LED brighter, you can increase the current but it must stay within manufacturers limit.

To reduce the brightness, you can either leave a longer 'off' period before next LED is powered so the average is lower or, you can 'chop' the on periods into smaller on/off periods using PWM or, you can use analog circuits to reduce the LED current.

You should also consider the time taken for human perception of light and persistence of vision. You want the matrix to appear as though all the intended LEDs are on at the same time. This imposes other limits, if you 'scan' through the matrix too quickly the LED drivers will not have time to turn the current on and off, this will result in the display losing brightness and may show smearing if you animate the LED shape. If you scan too slowly, you start to see flicker and moving shapes may appear to have steps in them, for example horizontal scrolling text may look like italics. The 1mS you mention is probably bordering on too short, you might like to consider 2 - 10mS as being more appropriate but you need to experiment to find the optimum speed for your application.

Brian.