555 control pin__________

[boylesg]

What is the impedance of a 555 control pin?

I have found plenty of info on the impedance of the other input pins but not on this pin.

 

[goldsmith]

What is the impedance of a 555 control pin?

I have found plenty of info on the impedance of the other input pins but not on this pin.

Hi Boylesg
May i ask you why you need to know it's impedance please ?

 

[betwixt]

It varies by manufacturer but will be somewhere between 3.5K and 9.5K.

Brian.

 

[IanP]

What is the impedance of a 555 control pin?
I have found plenty of info on the impedance of the other input pins but not on this pin.

The basic version of 555 has a ladder of 3 x 5k resistors associated with the control pin (5), for simplicity forget about all transistors connected here and there, their influence on the "resistance" of pin (5) is negligible .. see attached drawing ..
So, the value is the top 5k in parallel with the bottom (5k + 5k) ==> 3.3k

:wink:
IanP

 

[boylesg]

Hi Boylesg
May i ask you why you need to know it's impedance please ?

My aim to to create a 'singing arc' with my jacobs ladder circuit. The examples on youtube seem to be fairly low volume so I thought I would try amplifying the audio signal a little before feeding it into the 555 control pin.....see if it makes any difference.

- - - Updated - - -

The basic version of 555 has a ladder of 3 x 5k resistors associated with the control pin (5), for simplicity forget about all transistors connected here and there, their influence on the "resistance" of pin (5) is negligible .. see attached drawing ..
So, the value is the top 5k in parallel with the bottom (5k + 5k) ==> 3.3k

:wink:
IanP

Thanks Ian, you probably saved me a fair amount of trial and error. It didn't occur to me to look at the internal schematic of the 555 in the datasheet. But then I wouldn't have known which resistors etc to ignore anyway.

By the way I assume you can adjust the output impedance of a class AB amplifier stage. If so I think I can guess which resistor it probably is. But I have found sweet f all net resources on how to do the biasing calculations for class AB amplifiers. There is abundant material for bias calculations on class A amplifiers however.