Shift register and Dot matrix led

hello every one

I Would like to know why ceramic and electrolyte capacitors are needed between vcc and gnd for a shift register 74hc595 while controlling dot matrix led segment

thanks

thulasi

D_Hersey over 12 years ago

For the same reason there are water-towers.
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e14 Contributor over 12 years ago in reply to D_Hersey

What are water towers
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jadew over 12 years ago

They're needed to quickly supply current. Those chips will consume current in bursts and when that happens, if you don't have the caps to supply it, your VCC line will get pulled down a bit and that's going to generate a lot of noise on the rail.
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e14 Contributor over 12 years ago in reply to jadew

Hello Razvan

Thanks for reply

why these capacitors are in parallel, what is their individual roll
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jadew over 12 years ago in reply to e14 Contributor

Hey Thulasi, you should do a google on decoupling capacitors.

There's also a wikipedia entry which seems to do a pretty good job at explaining why they're needed: Decoupling capacitor - Wikipedia, the free encyclopedia

Razvan
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e14 Contributor over 12 years ago in reply to jadew

Thanks for solution
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Robert Peter Oakes over 12 years ago

With any digital circuit, noise is generated when the internal gates and other circuits switch from one state to an other, the faster the switching, usually means more noise. Power traces on a board are not perfect (0 Ohms) so therefore there is always a voltage drop along the trace, also there is inductance and capacitance to other nearby traces. decoupling capacitors are designed into the circuits to help manage this and provide easy paths of any noise generated to get back to ground rather than nearby signal traces

Noise is the "Interference" on the power and possibly other lines that if left un-checked can end up interfering with the normal operation of the circuit

larger electrolytic capacitors will provide low frequency decoupling and to a certain extent an additional source for short period increases of current requirements

the smaller ceramic caps (Or other material) are used for high frequency decoupling and will effectively provide a short to ground of this noise.

Why both, well Electrolytic are great for lower frequencies but internal construction tends to add inductance, series resistance and other factors that cause them to be less affective at the higher frequency ranges (capacitors are not perfect even though we would like them to be) so we add small value ceramic caps to provide the high frequency decoupling that the electrolysis can not handle
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