Capacitors on pin 8 of a 555 timer circuit.

Martin_H

I'm reading a schematic for a 555 based 1 mHz oscillator circuit, and it has two capacitors (10 nF and 10 uF) from pin 8 to ground. I'm guessing that these are decoupling capacitors. But I have a few questions.

Why two capacitors?

How do they pick the values?

Do they protect the rest of the circuit from the oscillator, or the oscillator from the rest of the circuit?

lanternfish

The 555 is one current HOG. Every time the output changes state it draws a huge current pulse from the supply. Therefore it is necessary to have more than one decoupling cap' usually a large, slow electrolytic or faster tantalum for the bulk of the charge in parallel with a small ceramic cap. The two in parallel have a synergistic effect with the smaller capacitor (with usually very low series inductance) supplying a small current very fast. The larger cap (with higher inductance) takes longer to get into action. The reverse occurs when the current draw of the load stops.

EDIT: The datasheets recommend a minimum 1uF electro and 0.1uF ceramic at low (output?) frequencies.

erco

My favorite 555/556 website: http://www.kpsec.freeuk.com/555timer.htm

Beau Schwabe

The video link below is a good refresher course as to parasitic effects that cause a capacitor to be self resonant, as well as what can cause an inductor to be self resonant. With a higher frequency, a capacitor becomes less capacitive and more inductive. With an inductor, a higher frequency will cause it to be more capacitive and less inductive. By placing different valued capacitors in parallel to 'decouple' the noise on a power line, you create an overlap that allows the circuit to function over a greater frequency range.

http://www.youtube.com/watch?v=vi24SpKYYoQ

Mark_T

lanternfish wrote: »

The 555 is one current HOG. Every time the output changes state it draws a huge current pulse from the supply.

Indeed - and everytime I see the 555 mentioned I feel compelled to mention there are CMOS variants of the 555 that are much better behaved - the 7555 for instance. The original 555 can pull 0.5A spikes on switching IIRC, hence the need for thorough decoupling to prevent excessive supply noise.

Martin_H

Thanks for all the information. I shall read through it.

It makes sense that two capacitors in parallel would approximate a better capacitor based upon their different properties. In another thread I asked about the ability to substitute capacitor types of similar capacitance values and a similar set of issue came.