NE555: Odd discharge behavior?

J H

Prodigy 10 points

Part Number: NE555

Tool/software:

I’m using simplified 555 astable circuit:

The output is this:

1v/div & 0.5usec/div. What’s that bump? It’s enough to be seen by 74HC gates.

4 months ago

0 Ron Michallick 4 months ago

TI__Guru**** 169866 points

Hello J, welcome to the TI e2e forums.

I would not expect this on a circuit board with a proper bypass capacitor. Is this bump also on pin 1?

Do you have a bypass capacitor between pins 1 and 8? Is this on a circuit board or a 'breadboard'?

0 J H 4 months ago in reply to Ron Michallick

Prodigy 10 points

Hi Ron. I believe I have a bypass, I’ll double check, and also look at pin 1. This is on a breadboard. Here’s a bit more detail; top = pin 3, bottom = pin 7:

I assume there’s a current spike when the comparator flips; is that the likely source?

Thanks ~ J

0 Ron Michallick 4 months ago in reply to J H

TI__Guru**** 169866 points

J H said:
there’s a current spike when the comparator flip

Yes, there is and that is the source.

That's why I suspect bump is also on pin1 (keep scope ground where is has been).

0 J H 4 months ago in reply to Ron Michallick

Prodigy 10 points

Yes it is bypass. Here is pin 1 and pin 7 with no bypass:

Adding 0.1u bypass helps, but not much.

Here they are with 10u bypass:

This seems ok.

So the lesson is comparators might need larger bypass than you might think?

Thank you ~ J

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NE555: Odd discharge behavior?