Hi all In toys, there's a lot of cost cutting. I just saw a small LED bargraph PCB based on a 74HC164 shift register, and there is no power connection to the chip. Pin 14 is not even soldered or touching the PCB. So the chip is getting its power through the in, clock and clear pins. Fine.
I don't really see a problem with this. Makes me wonder though if the clamp diodes can handle being continuously used. The datasheet says they can handle 20mA. I don't know if that's each diode on each input or some sort of sum through the ground pin.
Are these diodes special in some way? Can they be conducting all the time? I don't see why not.
The only thing I can see wrong is that with all 8 LEDs on it seems to draw ~60mA. That's pretty much the maximum if all three inputs are supplying current equally. I guess this is where it gets ugly, if you can't garantee the current through a particular pin, you'll exceed a diode somewhere, and if it blows open the other ones will fail too.