decoupling capacitors

i guess you are talkin abt bypass cap. bypass cap provide a low impedence path for noise to ground so that you hv a clean Vcc. the impedence of a cap is calculated as Xc = 1/(2*PI*f*C) which f for frequency and C for capacitance. for DC, whose f = 0Hz, cap is basically a open circuit. for high frequency noise, it's a short circuit.

if you are talkin abt decoupling cap in RF applications. the reason is pretty simple. to prevent RF energy goes from one stage to another.

hope that helps.

Yeah ta, its possibly the RF side included so you can use circuit as a building block, but also as its DC pulse generator maybe its for clean Vcc?

I have seen .0047uf used and 47uf in another was wondering why that was?

In a timer app using 555 where you are generating DC pulses which is sort of AC what size is used? What does it depend on? power source is battery, so that would be clean anyway wouldnt it?, not like a noisy DC adaptor?

Many circuits will still benefit from having small bypass capacitors connected across the 9 volt supply lines. The point of these capacitors is to have some charge storage very close to any load that can change its current draw in a very short amount of time. Without the cap right up against such a load, the current changes pass through all the wiring all the way back to the battery, bouncing the supply rail voltage around with the voltage that wiring creates do to its inductance. Remember the formula that relates inductor voltage to the rate of change of current, V=L*(di/dt). This bounce couples all the sub circuits together and can cause all sorts of trouble. So you decouple this path by putting enough supply bypass capacitance at each changing load. If a load is really a bad actor, you may want to add additional inductance in series with its supply line (with even more capacitance after the inductor), to let one side bounce without dragging the rest of the supply rail with it.

Much logic is capable of drawing significantly different current in sub nanosecond times, and many opamps can, too. And when the average currents get larger (motors, relay coils, switching power supplies) the rate of change can be significant, even when the transition time is fairly long.

that makes sense!

Thanks!, thats explained a lot, leaves/creates a few questions

so how do you calculate capacitor size, is it possible to put too large a capacitor as the decoupler.

What size of capacitor do you need before you can feel a discharge?.

Building a Hulda Clarke "zapper" and that says .0047uF for decouple, If you use a larger one, is that entire charge delivered for each pulse

I guess I need to read more about capacitor charge times and discharge rates.

looking at another circuit seems using 47 times the circuits other capacitance (ratio maintained across 2 similar but not same circuits)

(snip)

Excellent! If I somehow explained everything in one paragraph, there would be no reason to go on living. ;-)

The formula that relates current to the rate of change of capacitor voltage is a lot like the one that relates inductor voltage to the rate of change of current through it. I=C*(dv/dt) I is in amperes, C in farads, and dv/dt in volts per second. This tells you that you can pass an ampere through a 1 farad capacitor and the voltage will change 1 volt per second. The trick is to apply this to short and variable spikes of current and calculate how the voltage will change. Lots of people skip the calculus and find out the needed capacitance by experiment (look at the supply voltage with a scope and keep increasing the capacitance till the bounce gets small enough).

That is more dependent on the voltage stored across the capacitor, than the size. As long as the voltage is less than about 50 volts, dry skin will limit the current to less than a painful amount. Tough your tongue across one, and the voltage has to be a lot lower to make the experience not "shocking". The size of the capacitor just varies how long the shock will go on, before the voltage has run down too low to feel.

Decoupling caps do not normally discharge during circuit operation. They just sit there across the supply lines, holding a nearly constant voltage. I would have to see a schematic to be sure what they are describing is really a decoupling cap, and not some other circuit function.

They are very versatile components with lots of applications.

You went right over my head with that.

This is schematic I am looking at, description of circuit at

Also gives 1/4v positive offset to pulses

I just built a 555 timer test circuit from

So I am now more familiar with the formula to control the frequency.

Yeah, Cool! Thanks for your help!

(snip)

The only capacitor performing a decoupling function in this schematic is C1. And it is not bypassing the battery lines, but 2/3rds of that voltage divided down by an internal resistor divider in the 555. The CV label on the pin refers to the use of this fraction of the supply as a control voltage (that is compared to the voltage on pin 6, to decide when to reset the output flip flop). See:

Here, the decoupling action is greatly improved by the resistors between the CV pin and the supply rails.

Thats a useful reference ta, do you mean the test circuit on this page where 2 resistors and 2 leds cross and join the output pulse line

On the zapschematic.gif, if I put a 4.7 uf capacitor (or what size would you recommend) across the power terminals would that help the circuit at all?

WOW, wish I had continued to study electronics 20 years ago when I started... got into software design/prog instead... Never too late I guess!

(snip)

No. I am talking about the 3 equal resistors inside the 555 that divide the supply voltage by 1/3 and 2/3, to act as reference voltages for the trigger and threshold comparators.

The 555 draws a very brief spike of current, each time the output changes state. Any load on the 555 output has to be considered, also. Your 4.7uF sounds like a fine guess as a bypass capacitor, unless you find that the circuit is showing mysterious malfunctions.

Not till you are dead.

? I've still a lot to learn!

the load is going to be resistance of my body holding two copper tubes positive to earth... difficult to calculate but could use multimeter?

Long time yet I hope...

The 555 tutorial I pointed you toward shows what is inside a 555. It is essentially a flip flop with a high current output, a reset input, a second output that can only pull down to the negative supply rail or turn off (to discharge the timing capacitor, in some configurations, and two comparators. One sets the flip flop, and one resets it.

A comparator is just a high gain amplifier that switches its output when the voltage on one of its inputs passes that in the other. In other words, it compares two voltages and its output tells you which is more positive. A very common quad comparator is the LM339. Good for lots of projects.

One comparator has one input connected to the internal divider that sits at 1/3rd of the supply and the other has one input connected to

2/3rds of the supply. When the voltage on the other input of the first comparator goes lower than 1/3rd of the supply, the flip flop is set (output goes high). When the input to the second comparator goes higher than 2/3rds of the supply voltage the flip flop is reset (output goes low).

i think that capacitor is to reduce ripple on the voltage. Other reason that i know is to provide some energy if there are energy demand that come very fast