555 astable; pin 3 output voltage always equals 5V

The simple astable shown works fine at a supply voltage of 5V, but when the supply is raised to 10 or 15V, there is no output. I've found other cases where a 555 circuit will operate at 15 volt, yet pin 3 only swings 5 V.

This circuit is copied from your sample circuit files: "555 as astable multivibrator".

Thank you for your help.

by lsears
April 26, 2023

It should not behave like that.

In a real situation, the output cannot goes to ground, neither to Vcc. For Vcc = 15V, it is possible that it "may" go not lower than 2.5 volt and not higher than 12.5 volt (see datasheet for the specific "555" that you use to get a more appropriate range).

In this simulator, it seems that its main goal was to be used with logic level components (of 0 volt or 5 volt), which does not match the real-world component.

Note 1: the first pulse is a little bit larger than the typical one, so I suggest that you "run" the simulation from 0 to 2 seconds, given the implied resistor and capacitor that you used, if you wish to see the pulses going on and off, as exemple.

Note 2: you can use a VCVS (Voltage Controlled Voltage Source), tied to the output pin of the 555, to bring back the voltage to 15 volt. Again, that will not be an exact simulation of the real thing, since the real thing cannot reach Vcc.

by vanderghast
May 05, 2023

Thank you very much for taking the time to reply. Let me start by saying that there was a typo in my original question. I meant to say "but when the supply is raised to 10 or 15V, there is NO CHANGE in the output voltage". Anyway, your comment that CircuitLab "expects" that the 555 will be used on 5v logic does explain the behavior. Unfortunately, this is quite a major issue in many, many designs. I have found a similar deficiency in the software in that all op amp models have incorrect, rail-to-rail output swing. It seems that CircuitLab designs models that are somewhere between ideal and convenient. I find this imprecision to be quite unfortunate and frustrating. My university has purchased a seat, but I'm going to rethink this going forward. I truly like many other aspects of this simulation software, and is unfortunate that there is this one, important deficiency. It would help if the accompanying documentation mention this. Thank you again for your help.

by lsears
May 05, 2023

You may get a more exact model of a NE555 by Texas Instruments through their free TINA (limited version, but free (*), and a little bit "old style") on their web site, ti.com. (Don't download it from elsewhere than from the TI site.)

That TINA simulator, an old style one, even if it is graphical, is a little bit harder to use proficiently (if you look for something a little bit complicated), and uses a proprietary model (not LTSpice, which is used by one of their competitors, Analog Device, which has an even older "look and feel"), and cannot be mixed with logical devices, like here. But their models of some INTEGRATED CIRCUITS like the NE555, TL431, LMxxx and Ti-OpAmp (including specialized ones, produced by TI ) is worth mentioning.

(*) The free limited version of TINA is not limited by time, or size, but by what is already available in the user interface. You have to download it to run it. The support is far from what it was.

by vanderghast
May 06, 2023

2 Answers

Answer by lsears

Thank you very much for your comments. Its seems that the perfect simulation package is not available. My students have access to MULTISIM, which has, of course, an incredible choice of components, features, and accurate models. However, it just isn't as quick and easy to set up and use as CircuitLab – logging in to our VPN, more steps to building a circuit, schematic capture is more time consuming, etcetera. The kids just seem to universally prefer CircuitLab -probably because it was designed for education. I just wish the developers had gone a little farther in making the models more realistic and versatile. I’ll probably play with Multisym this summer – I see it has a “simplified” version which, at first glance, is no help at all. Unfortunately, the proprietary packages seem to have their own disadvantages.

+1 vote
by lsears
May 09, 2023

Answer by betterwound

I appreciate you sharing your thoughts. It appears that the ideal simulation tool is not yet available. https://www.circuitlab.com/questions/j7d7xe7w/555-astable-pin-3-output-voltage-always-equals-5v/ idle breakout

+1 vote
by betterwound
May 15, 2023

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