It seems that I fried my 74LS181N ALU, though I'm operating on 4.95v and have a 1k resistor (plus LED) on all output pins. I was sure it could handle those 5mA, but the datasheet suggests an output current of max 0.1 mA.

Im I misreading the datasheet, is it realistic that the ALUs limit is 0.1 mA?

For those who care, for testing I have the pins Vcc and input A0 tied to 5v, the output pins 14-17 floating, and all other on ground.

EDIT: Oh yeah, the chip gets hot.

  • 2
    \$\begingroup\$ I think you're looking at the input high current rather than the output current. Note that the chip likes pulling current into itself much more than it likes driving out -- if your resistor-LED combos are going to ground instead of coming from VCC the chip wouldn't be able to drive them hard. \$\endgroup\$ – TimWescott Aug 29 '19 at 22:55
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    \$\begingroup\$ Bipolar TTL (74xx, 74LS, ALS, etc) can pull the output down much better than it can pull an output high. If you want to use LEDs to show the output state, you should connect them (and a current limiting resistor) between the output pin and the positive supply. CMOS parts (anything with a "C" in the middle) have equally strong pull-up and pull-down outputs, so LEDs can go to either Vcc or Ground with them. \$\endgroup\$ – Peter Bennett Aug 29 '19 at 23:53
  • \$\begingroup\$ 74181 has MANY gate-equivalent internal operations \$\endgroup\$ – analogsystemsrf Aug 30 '19 at 14:45

The outputs are limited to 400uA (sourcing) if you need it to have valid logic levels in order to drive TTL-level inputs with guaranteed noise margins.

You can load it quite a bit harder if you don't care about connecting it to inputs and are only driving an LED. But as @Tim says, it's better to sink current with the output and have the LED "on" when the output is low- 8mA with guaranteed logic levels and noise immunity. If you have a red LED, you're going to have no more than about 3.2mA for the LED (less for other visible colors) so it can drive a few inputs as well.

See, for example, this datasheet:

enter image description here

You did not damage your chip by connecting an LED from outputs through 1K to ground, if the chip is indeed damaged something else did it.


The old fashioned 74LSxxxx IC gets hot because it is an old TTL IC that uses a lot of power supply current. A modern Cmos iC uses almost no power supply current.

If your power supply voltage is 5V and your LEDs are 3V then the 1k resistor has 2V across it if the input to the resistor is perfect at 5V and the current is 2mA, not 5mA.

But the output voltage of a LS TTL IC never goes anywhere near +5V and its maximum output current is less than 0.5mA.


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