555 oscillator queer behavour

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In a 555 the capacitor charge currents and comparator levels are ratiometric to Vcc. This gives you the same output frequency for any stable Vcc. However, if the value of Vcc changes significantly _within_ a period then a frequency deviation will result. I assume that you could also see significant injection locking with a 555.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I have a question concerning the function of the 555 oscillator in astable mode. In particular the external elements i have connected to the 555 are those depicted at the datasheets of all the 555 ics (LM555, NE555 etc). The 2 resistors are above

1K Ohm (Ra=1K & Rb=6.8K), the C capacitor's value is greater than 0,0005uF (C=0,001uF) whereas a 0,1uF capacitor is connected between the "Control Voltage" pin and the ground. The above connection with the aforementioned values for the external elements adjust the 555 so that it theoretically produces a frequency of 98KHz, more or less. The problems lies in the fact that the experimental frequency, produced by the ic, is lower than the theoretical by around 30KHz. The discrepancy of theoretical and experimental value appears also in other "valid-practical" values of Ra,Rb & C with different deviations. I have also tried the LinCMOS version of 555 (TLC555) with the same results. Another odd phenomenon, equally unwanted with previous one, is the shifting of the produced frequency whenever the value of power supply of the 555 is changed, always ofcourse within the allowed limits (4.5->16Volts). The datasheets of the various 555 mention clearly that the HIGH and LOW times, and hence the frequency, are independent of the power supply.

Any help is wanted

panagiotis

Very odd. If you hadn't explicitly said above that you'd got the same problem even after changing the capacitor, I'd have guessed that was the culprit. Simulating, I get F(Vcc=15V) = 95.7kHz; F(Vcc=5V) =

90.6kHz. Were your caps of different types too? Maybe a bad batch..,

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Terry Pinnell
Hobbyist, West Sussex, UK

This was a *simulation* (using CircuitMaker's Spice facilities), not a real life breadboard test. And, using different models of the 555, I found other slight variations. But in any case, this was only 5%, not the 30% you found.

'Culprit' means 'guilty party', IOW, 'the cause of the problem' in this case.

Did you double-check your circuit wiring? (Can you publish it?) Did you try the frequency measurements with an alternative instrument or method? Was there an unusual load connected? Did you view output on a 'scope?

-- Terry Pinnell Hobbyist, West Sussex, UK ====================

Hi

Thanks for your answer. You say that you measured the frequency for Vcc=5V & Vcc=15V and you found it different by 5KHz? So the same "problem" (if it is one) appears to you to. About the caps, they were indeed of various types. Also what is "culprit"?

panagiotis

Try using this 555 tool