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Add one resistor to give bipolar LM555 oscillator a 50:50 duty cycle

CircutScoper

Mar 29, 2022
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"1% to 20%"?
I humbly suggest you take a closer look at that LM555 datasheet linked to above. If you do, you'll notice that there are no specified min/max limits on threshold and trigger voltages. Therefore, when attempting to predict the performance of a 555 application circuit, you're left with exactly two choices:

1. Use the typical values,, or else
2. Make up totally arbitrary atypical assumptions for the values.

I you choose #2, it's only reasonable to expect that you'll specify upfront what your assumptions, being arbitrary, were.
 
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Martaine2005

May 12, 2015
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With all due respect, if you don’t understand what tolerance means, look it up.
The data sheet states max, typ and min for many specs.
Not one data sheet “on the planet” (your words), will give tolerances of third party components. These components WILL affect the real world tests. 1% to 20% can be huge.
I find you very argumentative and see no reason for it. 1.67V to 1.8v could be negligible.

Martin
 

crutschow

May 7, 2021
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therefore 1.667V when V+ = 5V, not ~1.85V as shown by your "accurate" simulation?
that simulation that's neither "perfect" nor "set in stone?
I never used any of those straw-man words, so why did you put them in quotes?
Who are you quoting besides yourself?

I was using an old 555 model that doesn't model the nominal trigger points accurately.
Below is the simulation with a better model which uses the nominal trigger points, and does give nearer to 50% duty-cycle (45.3% shown):

The caveat with simulators is that the simulation is never better than the models, of course.

If you don't like simulators that's your prerogative, (there are others on these forums who also poo-poo them), but I have used a Spice simulator for at least 30 years in doing my analog designs, and it has bailed me out many times from design mistake before I built the circuit (the circuit always needs to be built, of course, to verify the design).
Suppose you want to test a design for the range of component tolerances it can experience to determine the worst-case operation.
That's no practical to do with the real circuit but it's easy to do with a Monte Carlo simulation in Spice.
Spice has also helped me troubleshoot problems in designs that have already been built.

upload_2022-4-6_17-11-5.png
 

CircutScoper

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"The data sheet states max, typ and min for many specs."

But not for the trigger voltage, which, please recall, was the subject of my comments. If you don't understand the meaning and significance to this discussion of the 555 trigger voltage, you might follow your own kind advice to me, and look it up.
 

CircutScoper

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I was using an old 555 model that doesn't model the nominal trigger points accurately.
Below is the simulation with a better model which uses the nominal trigger points, and does give nearer to 50% duty-cycle (45.3% shown):

View attachment 54807

Thanks for (now) using a more accurate model.

Please now, just for fun, apply it with its more accurate trigger points, to the circuit in your first post to this thread, and see what happens to the respective duty cycle predictions for both your and my values for R3.
 

Martaine2005

May 12, 2015
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But not for the trigger voltage
Exactly. That is the lab conditions for testing. The ‘resistors’ are NOT tested under the lab conditions as they have no control of where you buy them.
What point of this don’t you understand? Your calculations didn’t specify a resistor tolerance!.
You are arguing uncontrollably with yourself.

Martin
 

CircutScoper

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Exactly. That is the lab conditions for testing. The ‘resistors’ are NOT tested under the lab conditions as they have no control of where you buy them.
What point of this don’t you understand? Your calculations didn’t specify a resistor tolerance!.
You are arguing uncontrollably with yourself.

Martin

The 555 trigger voltage (as a fixed fraction of the supply voltage when the CV pin is unloaded) is a parameter of the 555, independent of the tolerances any external components.

I figured that's what you didn't understand. I see I was right.
 

CircutScoper

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Ok, you are right.
When would you use a 555 without any external /third party component?.

Martin

Well, don't look now, but I just did. I used it as an example of a component with a parameter (trigger voltage) that's independent of the tolerances of external components. :cool:
 

Martaine2005

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I suppose you had a plastic salad bowl over the crown of your head for your perfect hair cut?.
You are pathetically argumentative over something silly.
Simulations are not real world. Data sheets are for the component itself.
As I said before, 1.67v and 1.8v is a working circuit.
Please don’t troll for an argument.
 

CircutScoper

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Please don’t troll for an argument.

Well, Martin, perhaps you came late to this discussion and so missed some useful context. The original subject of this thread is an article I wrote and was recently published in EDN Design Ideas. Opinions were offered here that seemed relevant to some points in my article, so I replied to them.

What rock you crawled out from under, by contrast, I have no idea.
 

Martaine2005

May 12, 2015
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What rock you crawled out from under, by contrast, I have no idea.
You see, this is my point. You insult a fact without a real world example.
My real world example of you having salad bowl hair cut from your mum is perfect. I can’t prove it either.
You are the man, your calculations are perfect. UNTIL a real life external , real life resistor has a tolerance.
I suppose a silicone diode has a forward voltage of 0.6 volts (exactly)?. Do you want to debate that?.
 

CircutScoper

Mar 29, 2022
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You see, this is my point. You insult a fact without a real world example.
My real world example of you having salad bowl hair cut from your mum is perfect. I can’t prove it either.
You are the man, your calculations are perfect. UNTIL a real life external , real life resistor has a tolerance.
I suppose a silicone diode has a forward voltage of 0.6 volts (exactly)?. Do you want to debate that?.

Sorry, but I have no idea what a "silicone" diode is or what its forward voltage drop might be.

Silicon diodes, by contrast, I'm fairly familiar with, and how their forward voltage drop varies with current density, temperature, etc. I made considerable use of that familiarity here, for example: https://www.edn.com/diodes-derive-logarithms-and-exponentials-to-optically-sense-ac-dc-current/
 

Martaine2005

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Sorry, but I have no idea what a "silicone" diode is or what its forward voltage drop might be.
My golly gosh, you are a piece of work.
Ever heard of predictive text?.
Sometimes we read ‘between’ the lines to establish communication. You obviously can’t and take syllable as gospel. I wish you well in your endeavours. Pls don’t quote me again, I don’t want or wish to reply to an arrogant know it all like you. Just a shame that you argue with yourself!.
 

CircutScoper

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The caveat with simulators is that the simulation is never better than the models, of course.

View attachment 54807

Just to mention: If you look back over all my posts to this thread, I think you'll see that I never even once criticized your chosen (nor any other) simulator. I questioned only the accuracy of the simulation, which we now know was compromised by the model being used.

I wish I'd noticed the atypical trigger voltage issue quicker and so better conserved your valuable time. Sorry 'bout that.
 

crutschow

May 7, 2021
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Just to mention: If you look back over all my posts to this thread, I think you'll see that I never even once criticized your chosen (nor any other) simulator. I questioned only the accuracy of the simulation, which we now know was compromised by the model being used.

I wish I'd noticed the atypical trigger voltage issue quicker and so better conserved your valuable time. Sorry 'bout that.
Yes, I misunderstood your comments about simulators.
I'm used to having the models (and my results) be fairly accurate, so I was lulled into a false sense of security and wasn't sufficiently critical of my results with that old 555 model.
I appreciate you noticing the error.

As far as time goes, I'm retired, so time is not a particular issue.

Below is the simulation of my original circuit using your suggested 11.2kΩ resistor for R3 with the proper 555 model:
It's duty-cycle is 49.7%, which is close enough to 50% for government work. :D

upload_2022-4-7_11-42-24.png
 

CircutScoper

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Below is the simulation of my original circuit using your suggested 11.2kΩ resistor for R3 with the proper 555 model:
It's duty-cycle is 49.7%, which is close enough to 50% for government work. :D

View attachment 54820

Thanks for the new result. Since in my calc's I fecklessly neglected such refined details as the ~5mV saturation voltage of the pin 7 discharge transistor, "close enough for gov't work" is about the best I could have expected, and fully believe your simulation is now more realistic than my crude approximations.
 
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Martaine2005

May 12, 2015
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As we all get on again, I too apologise for my outburst last night.
It’s unlike me .


Martin
 

crutschow

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As we all get on again, I too apologise for my outburst last night.
It’s unlike me .
The problem with written communication is that you don't hear the voice inflections or see the expression of the other person conveying the message, so its intent is sometimes misinterpreted.
 
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