Can someone explain this

142

03:59:57

A fairly bog standard 555 astable, the only quirk being that the timing capacitor goes to pin 3 through a single resistor rather than 7. The simulator shows a square wave output at about 500Hz. Calculating the expected output frequency (1/(2 ln 2 RC) ) yields an output frequency of just about 700 Hz, while actually building the thing (I measured the "47kΩ" resistor at 46.6kΩ, in fact) yields a nice square wave output at just about 1kHz.  So what am I missing?

published 8 years ago

bakupcpu

8 years ago

The chip in the simulator has some voltage drop at the output compare to real life component. It's not 100% accurate but it's always a good way to test circuit then adjust.

hurz

8 years ago

This is, cuz the rise time is not identical to the fall time. But in your formular you just use 2 times the time of ln(2)*RC actually this is only valid for the fall time from 6V to 3V. From 3V to 6V I calculated 1.0986 as factor. Together with 0.693 for fall its then 1.792! So the formular for frequency for this topology is f=1/(1.792 * RC) why is the rise time so different to fall time? Itd because the ouput of 555 does not reach Vcc its always 1.5V below, here only 7.5V so to reach 6V takes much longer. About 58% slower in rise then in falling. Hope this helps. Ps, with this formular I got 539Hz which fits almost perfect to the simulation.

tonyinselby

8 years ago

Makes perfect sense. Thanks, hurtz.

hurz

8 years ago

You are welcome tonito.

jason9

8 years ago

But how is it 1kHz? Even if there isn't a voltage drop, the simulation gives 2/3 of 1kHz.

hurz

8 years ago

Bad setup in real. Tolerances of caps can be quite high. Who kowns.

hurz

8 years ago

BTW, a cmos version of 555 with rail2rail output will have 698Hz cuz the factor will be 2x ln(2) symmetric rise and fall time. While everycircuit 555 is more a BJT model

tonyinselby

8 years ago

I suspect the 1kHz figure is due to an ill oscilloscope. In fact I'm not too bothered about the exact frequency (it's to PWM some LEDs) - just wondering about the apparent discrepancy between calculation and simulator. Again, thanks.

hurz

8 years ago

So with this simulation, you can calibrate your scope.

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