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LM555 comparator headache

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1LM555 comparator headache Empty LM555 comparator headache Thu Mar 30, 2023 9:24 pm


I am fighting against an amp/comp in a 555 emulation, where the output is very instable (50mhz it appears). At least I suspect this is where the problem is.
It's happening only for the low comparator ; so I figured maybe it needs some hysterisis but I must admit it is kind of above my pay grade :-)

If someone has a solution or some ideas (the aim of this small project is to view step by step what is happening inside a 555 - probably someone already did it before).

LM555 comparator headache Lm555-10

Last edited by ci@telemack.net on Fri Mar 31, 2023 10:50 pm; edited 1 time in total

2LM555 comparator headache Empty Re: LM555 comparator headache Fri Mar 31, 2023 10:15 am



To avoid parasitic oscillations in a gated flip-flop you have to check the box "Initial High State" in ONE of them, in properties. You have a couple of examples obout the 555 in the section "Projects made in Simulide", numbers 39, 79 and 251.

Another idea is to insert a non-inverter driver at the base of the transistor, before the resistor, this produces a small delay that prevents parasitic oscillations.

Last edited by Defran on Wed Apr 19, 2023 9:50 am; edited 2 times in total

3LM555 comparator headache Empty Re: LM555 comparator headache Fri Mar 31, 2023 2:28 pm


Aaah - thank you for pointing me in the gate direction. I simply changed the threshold actually - seems to do the trick (note sure why - the duty cycle is very small on this comparator, but it should not produce this phenomenon imho - could it be a bug in 1.0 ?).
I did spy on your project in 79 - bistable - actually - but could not spot what could cause this oscillation on my similar model.

Thanks again, all good now !
(also I corrected the three 5-5-5 resistors values, even if it does not change much of the operation  - and I removed the resistor on vin low comp I put there only for testing ;-)

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