I learned something new about the 555.

Thread Starter

Ian0

Joined Aug 7, 2020
10,008
Forty-two years since I first used a 555, and today I learned something I didn't previously know.
I think we all know that:
  1. 555s always produce clean waveforms, because of the two comparators and the SR latch, and a large amount of hysteresis.
  2. The output pulse length is the either 1.098RC or the length of the trigger pulse, whichever is longer.
but I got this waveform:
DS0010.pngorange is the trigger pulse
blue is the waveform on the capacitor.

The problem is that when the pulse length is controlled by the trigger pulse (i.e. trigger is longer than output pulse) then the output waveform is controlled by just ONE comparator, and so there is no hysteresis, and it jitters just like an LM393 (to which it is closely related) on slow moving waveforms.
When the trigger pulse ends, DISCHARGE should short out the timing capacitor and discharge it, but it only gets halfway before the jitter resets the comparator and it starts charging up again.
The pin 3 waveform is TWO pulses with a very brief interval between them. So small you wouldn't necessarily notice it on the 'scope, but long enough to upset anything that is counting pulses!
The moral of the story is that the "feature" of an output pulse that is as long as the input pulse is a "feature" that is well worth avoiding.

P.S. The real-time clock on my scope is truly hopeless.
 

Alec_t

Joined Sep 17, 2013
14,370
That anomaly is something to be aware of when running a sim then. The standard 555 model doesn't show it in LTspice.
 

AnalogKid

Joined Aug 1, 2013
11,128
If a 555 were designed from scratch today as a clean room project, the result would have a whole lot more than 35 components. Hans' design was a masterpiece within the context of the constraints he was working under at the time, but it has serious personality quirks.

Note that the 555 was not patented, and there are well-documented differences in how chips from different manufacturers behave.

ak
 

Thread Starter

Ian0

Joined Aug 7, 2020
10,008
If a 555 were designed from scratch today as a clean room project, the result would have a whole lot more than 35 components. Hans' design was a masterpiece within the context of the constraints he was working under at the time, but it has serious personality quirks.

Note that the 555 was not patented, and there are well-documented differences in how chips from different manufacturers behave.

ak
Is it related to the LM393/LM358 at all? The internal circuit diagrams of the TRIG comparator looked like a LM393.
 

AnalogKid

Joined Aug 1, 2013
11,128
No. Hans and Bob/Bob were competitors - Signetics versus National Semi. Note that the 555 internal schematic is the true schematic, while the 324/339/358/393 schematics are just concept drawings. Their main benefit (and goal - ?) was to show off that OMG input stage and semi-OMG output stage.

ak
 
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