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I searched various datasheets, but I didn't find the information of a max discharge current for the NE555.

The maximum source/sink for the output is mentioned, but not the max sinked by the discharge pin.

I guess it isn't designed to be (ab)used this way, but having an open collector output is very handy sometimes. Specially as it enables the output pin to be used as feedback.

So, what could be the max sink current of the NE555 discharge pin?

Update: Markus mentioned in the comments that it is obvious, and after looking closely at the datasheet, even from ST, it is always about the output pin :

the output structure can source or sink up to 200 mA. ... Iout Output current (sink & source) ±225 mA

But nothing specific about the DISCHARGE pin. The only mention is a low voltage value to some current. Which I guess is for ensuring a pull down strong enough to go beyond the trigger threshold.

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  • \$\begingroup\$ ST's datasheet has a maximum current in the absolute maximum ratings; the fairchild 2002 Datasheet, otherwise pretty bad, literally starts (first sentence!) with a drive capability. The TI datasheet specifies output low voltage up to a specific current, and so on… \$\endgroup\$
    – Marcus Müller
    Commented Nov 27, 2022 at 19:38
  • \$\begingroup\$ Discharge pin sink current is relevant for the standard oscillator design too, at low enough duty. \$\endgroup\$
    – Hearth
    Commented Nov 27, 2022 at 19:51
  • \$\begingroup\$ @MarcusMüller sorry, but I did find the maximum voltage for discharge regarding a discharge sinking current. But no mention of "maximum ratings". Does I out covers both the output and discharge pin? \$\endgroup\$
    – Steve Schnepp
    Commented Nov 27, 2022 at 19:58
  • \$\begingroup\$ Same with the TI one that prominently says "The output circuit is capable of sinking or sourcing current up to 200 mA" but nothing specific about the Discharge pin. \$\endgroup\$
    – Steve Schnepp
    Commented Nov 27, 2022 at 20:01
  • \$\begingroup\$ There's a lot that was never specified (perhaps not even characterized) about the 555. For example, have you ever tried to find out (or work out) what the switching threshold voltage of the reset pin is? \$\endgroup\$
    – Dave Tweed
    Commented Nov 27, 2022 at 21:12

3 Answers 3

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Given that the pulldown transistors for pins 3 and 7 in most 555 die photographs look identical, it's probably safe to assume that their ratings are the same, too.

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I know that this is probably a bit late, but from Philips AN170:

http://www.sophphx.caltech.edu/Physics_5/Data_sheets/555appnote.pdf

from P6:

enter image description here

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The TI Datasheet for the NE555 says the following: "Maximum output sink and discharge sink current is greater for higher VCC and less for lower VCC"

So the cryptic clue, I would assume, is the Discharge Pin sinks the same as the Output Sink.

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