[SOLVED] Astable IC555 in astable mode to get 100Khz frequecy

[milind.a.kulkarni]

Hi to All,

I had tried to make a IC555 chip to generate 100 to 50 Khz frequency I had selected the correspoding resistance RA and RB of 10k POT and 2k POT where I got the capacitor of 0.1nf that is 100PF.....by the computation formula for .....There was no frequecy generation at all....Is there is restriction to use the Capacitors in micro-farad range only ?....... Any help is appericated ....Thanks in advance.....

 

[kak111]

Standart 555 datasheet does not show lower capacitance values than 1nF
**broken link removed**

still there is 100kHz oscillators with 100pF
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I suggest to use CMOS version 555, when frequency is near 100kHz or with small duty cycles

......3 MHz astable frequency capability
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[milind.a.kulkarni]

Hi Kak111 ,

QUASAR project kit is really good information

thanks for that

regards,

milind

 

[jpanhalt]

An obsolete application note (AN170) is quite definite about capacitor size:

Source: AN170
Capacitor size has not proven to be a legitimate design criteria. Values ranging from picofarads to greater than one thousand microfarads have been used successfully.

However, there are cautions about resistor sizes:

In selecting the values for the timing resistors and capacitor, several points should be considered. A minimum value of threshold current is necessary to trip the threshold comparator. This value is 0.25mA.

There are also cautions about those configurations that give less than 50% duty cycle. If you still cannot get it to work, please post your circuit here.
The current NXP Semiconductor datasheet ( **broken link removed**) has most, but not all that was in the earlier AN170.

John

Here is a link to the obsolete AN170: