Question, led matrix circuit using decade counter and shift registers

**broken link removed**

I'm planning to try making an LED matrix using this diagram. My question is, will there be an issue if one of the shift registers is all on (11111111) when the sinking transistor is active? The guy has a video of it working, but the LEDs are rated as 3.5v 20mA, and the shift register data sheet says it can dissipate 500mW. 8 LEDs x 20 mA x 3.5v = 560mW > 500mW. Is it just that the small difference doesn't matter? Or is it perhaps that since it is multiplexed they are usually not on for very long?

It's the power dissipated in the shift register that's important not the power dissipated by the external LEDs and resistors. The shift register power is equal to the on current times the output saturation voltage when an output is on (low). This voltage is low if the current is below the maximum current rating of the shift register.

The calculation is: 5v minus 3.5v minus 0.2v = 1.3v x 20mA x 8 = 208mW without taking into account the loss across the dropper resistor.

colin55 said:

The calculation is: 5v minus 3.5v minus 0.2v = 1.3v x 20mA x 8 = 208mW without taking into account the loss across the dropper resistor.

Click to expand...

But that's not the power dissipated in the shift-register driver circuits, which is the op's concern.

It doesn't matter.I have drawn total power from 74HC595 without any problem.Since you are multiplexing the average current will flow through the LED's.

I don't care on the maximum ratings on data sheets.

Gayan Soyza said:

I don't care on the maximum ratings on data sheets.

Click to expand...

Neophytes don't, but any good circuit designer does.