I have here an interesting piece of hardware, a CD74HC125E that someone gave me. This chip (can I call this an IC ?) is able to set its outputs to high voltage, low voltage or high impedance.

Do I need to put a resistance in series with the chip if I set an output to either high or low voltage, or is there any current-limiting function in it?

If not, how much current can it take before it burns? I could not figure out where to look in the datasheet.

The datasheet can be found at http://pdf1.alldatasheet.com/datasheet-pdf/view/27005/TI/CD74HC125E.html

  • \$\begingroup\$ Please add a link to the datasheet. \$\endgroup\$
    – jippie
    Apr 30 '13 at 20:18
  • \$\begingroup\$ Link added to the question :) \$\endgroup\$ Apr 30 '13 at 20:29

Check the datasheet for Absolute Maximum Ratings.

enter image description here

For normal use you want to look at:

DC Output Source or Sink Current per Output Pin, IO

For VO > -0.5V or VO < VCC+ 0.5V

± 25mA

Keep well below that value, say 20mA max, then you are safe. You'll have to calculate a series resistor for that as the device will be fried if you exceed the maximums.

  • The other currents mentioned are important when the input or output is pulled higher or lower than its own power supply rails, which in most cases is a situation you want to avoid. Look closely at the conditions on every second line eg.: For VO < -0.5V or VO > VCC + 0.5V
  • \$\begingroup\$ So if I'm correct, this means that I have to put resistors on Vin (or ground) and on each output? Would this be enough to not fry the chip, or do I need more ? \$\endgroup\$ Apr 30 '13 at 20:48
  • \$\begingroup\$ Vin and GND do not need resistors. Only the outputs and it depends on what you want to connect. Consider adding a circuit diagram to your question of what you want to build. \$\endgroup\$
    – jippie
    Apr 30 '13 at 20:49
  • \$\begingroup\$ In my case, I use only one output. It allows me to choose between high, low or the value given by a ldr later in the circuit. This is getting complex, so I guess I'll just post yet another question. Thanks for your answer ! \$\endgroup\$ Apr 30 '13 at 20:52
  • \$\begingroup\$ One last question on this one: I read "DC Vcc or Ground Current +-70mA". I can't make this work with you saying that no resistance is needed on Vcc. Any explanation on this? \$\endgroup\$ Apr 30 '13 at 21:12
  • \$\begingroup\$ Nevermind, I've had my answer: since Vcc or Ground is redirected to outputs, they don't need a resistance. However, I have to limit the total current for all 4 outputs to 70mA. \$\endgroup\$ Apr 30 '13 at 21:51

Look in the Absolute Maximum Ratings section for information on the MAX current that the part can safely source or sink. In another part of the data sheet you should find the Recommended Operating Conditions where the output low voltage and output high voltage levels are specified at particular sink and source current levels. These will normally also be the recommended operational limits.

Next time if you posted the link to the actual data sheet for your particular part we could help you get specific numbers.


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