Electrical Engineering

William Mariager

00:37

I wonder if someone could help me out with this part: nxp.com/documents/data_sheet/74HC_HCT595.pdf

I need to drive some LEDs and I have that part home at the moment. As far as I can tell, I can draw up to 35mA per Q output. But I'm unsure if I can draw 35mA from all 8 Q outputs at once, for a total of 280mA.

It's a DIP16 package, giving it a 750mW dissipation, which I think might be what I'm looking for.

I've seen a few examples where they drive 8x20mA so I'm pretty sure it's possible. But I'd like to try and also understand how to read it out from the datasheet.

The Photon

01:22

@WilliamMariager In Table 4, I'd be worried about that maximum supply current at 70 mA. But, to be honest, I'm not sure exactly how they define things (if you were to invert the outputs and sink 20-30 mA per Q pin, would that be okay?)

Nick Alexeev

Max supply current I_CC at 70mA. Can't source more than 70mA total.
Max ground current I_GND is also 70mA. Can't sink more than 70mA total either.

These are absolute max characteristics. The IC can get damaged if they are exceeded (not just operate out of spec).

The Photon

@NickAlexeev Oh, good, that makes it more clear. Didn't see the ground current spec. (obviously I didn't look very hard)

Nick Alexeev

01:38

@WilliamMariager By the way, here's a somewhat related question about another HC part by NXP: How much current can this 74HC245 handle on each output pin?

Asmyldof

02:15

Usually up to 120% with HC/HCT will fall in the okay as long as heat is managed properly, up to 150% in "undefined behaviour"/instability. Anything above potential damage

*potential permanent damage that is

But if you're a nostalgic weirdo and have about 50 of each relatively usual type on hand, heck, put it under a glass jam jar, under good ventilation/cooker hood and try it, see what happens. Be aware though that chip to chip the die and bond wire quality will differ slightly, so one test is not proof for all

crasic

@Asmyldof the hat suites you

Asmyldof

Thank you. I actually have one in white

But then I also have a green felt tophat

So....

Going nap times now

If I'm not being grumpy in about 5 hours, wake me ^.^

William Mariager

Thanks @ThePhoton and @NickAlexeev. :) I went with a larger resistor reducing the current to 17mA per LED. I guess I'll have to make do with 4 LEDs to make sure it works correctly.

Asmyldof

02:32

If you don't: Happy New Year

Nick Alexeev

03:52

0

Q: How do I test a USB drive to be sure it is not a “USB killer”

I have recently read of a dangerous USB drive containing a circuit instead of storage, that destroys the hardware of a computer by injecting a high voltage, high current pulse into the port it is plugged into. See this article It should be easy to test an unknown USB drive for this sort of dange...

Phase 1. A disgruntled individual (let's dub him UnaZapper) builds and disseminates a sizable batch of killer USB flash sticks.
Phase 2. IT industry arms itself with USB killer detectors.
Phase 3. Manufacturers of USB sticks respond by switching to clear enclosures (if there are power electronics components, they would be visible). Manufacturers of PCs increase overvoltage protection.

("Honorable" mention: etherkiller and family .)

Shalvenay