pvelentz
Joined: 03 Dec 2004
Posts: 1
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 11 Dec 2004 1:14 555 oscillator's queer behavour |
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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.
I attach a pdf file with the schematic for the standard astable mode.
Any help is wanted
panagiotis
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