# PCB Heaven

## General Category => Digital discussion => Topic started by: cheerio on May 08, 2013, 11:44:06 AM

Title: 555 weird behaviour
Post by: cheerio on May 08, 2013, 11:44:06 AM
i build an astable 555circuit using the NE555P from texas i. (http://www.ti.com/lit/ds/slfs022h/slfs022h.pdf (http://www.ti.com/lit/ds/slfs022h/slfs022h.pdf))
RA = RB = 10k
C = 330pF

i use the circuit in figure 12 on page 10 of the datasheet.
i calculate the output frequency like this:
1/ (ln(2)*(RA+2*RB) * C)

so i expect 1/ (ln(2) * 30000 * (330*10^-12)) -> 145.7267718kHz
what i actually get on my scope is ~113.6kHz

how come? i tried it on the breadboard and on a pcb as well. same behaviour.
Title: Re: 555 weird behaviour
Post by: kam on May 08, 2013, 14:45:03 PM
Just by chance, i calculated the tolerance of the 555 timer (+-2.25%), a typical ceramic capacitor (20%) and the resistors (5%) at the worst case. Doing so, the output frequency would be 113 KHz

Since you've gone into this research, how about getting some precision parts and post us the results?
Title: Re: 555 weird behaviour
Post by: cheerio on May 09, 2013, 02:29:47 AM
interesting calculation. the resistors are 1% tolerance. The cap states 330pF and is measured 330pF (not a good meter though)
used 5% parts before. exactly the same result. It looks like it is not the precision but i will double check this.
Title: Re: 555 weird behaviour
Post by: kam on May 09, 2013, 16:23:22 PM
Frankly speaking, 555 timers are not the most precise elements in the Cosmos. They are supposed to have 2.25% precision (as mentioned in the datasheet) but let me doubt about this. I can't remember if i ever had a situation where the theoretical and the practical values matched.

One question - What do you use to measure the frequency?
Title: Re: 555 weird behaviour
Post by: cheerio on May 10, 2013, 20:40:29 PM
i use my oscilloskope Rigol DS1052