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74LS147N 10:4 priority encoder not working... Answered

Hi,
   This is a follow up of this question- https://www.instructables.com/answers/Logic-circuit-to-generate-and-convert-3-bits-paral/
I set all the 10 inputs of the encoder high and was expecting to see all the 4 outputs give a high(4 LEDs were connected to the outputs to see this happen).But that wasn't the case and just 2 of the LEDs glowed.I tried different configurations on the inputs but all the time it was either these two LEDs that glowed or none .Can this ic power 4 LEDs directly from its outputs? Could I have wrecked the ic?
Something a bit off the topic- What should I keep in mind while mixing TTL and CMOS logic gates?
I powered the ic with 5 volts.

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Downunder35m

Best Answer 4 years ago

It is a logic IC and simply checking the datasheet shows it can provide 8mA on the output - this is enough for a transistor to switch the LED but not to switch the LED directly.
You might have fried your IC.

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Adarsh_tronixDownunder35m

Answer 4 years ago

Thought so! I am no expert in electronics and I found all those specifications on the datasheet very confusing.Gotta be more careful next time..thanx!

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steveastrouk

4 years ago

Oops. You can't drive leds directly!

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Adarsh_tronixsteveastrouk

Answer 4 years ago

But say I connect the outputs to the inputs of another gate,I wouldn't need transistors then,would I? Especially if the second ic is CMOS,I infer that it won't draw much current.

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steveastroukAdarsh_tronix

Answer 4 years ago

No problem !
The inputs of logic draw tiny currents.
The outputs can sink and source a "reasonable" current.

Look hard at all the small writing in a datasheet. Its there for a reason.....

Imagine I have a current sink/source capability of 1mA, and an input current of 10uA. I have what is called a "fanout" of 1000/10 or 100. So ONE output could drive up to 100 inputs.

If you want to test with LEDs, use the high efficiency red leds, which can run on 1mA, or make up a little tester with a ULN2803 driving your LEDs.

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Adarsh_tronixsteveastrouk

Answer 4 years ago

Thank you! I will give greater importance to the data sheets.

Have heard the term 'fanout ' before..I think in PCB desiger softwares.Thank you for explaining what it means.

Yes,I will get a Darlington array as I don't want to burn out another ic.