Question by Riz: why are there so many capacitors connected to a PCB power line on the schematic?
and what should be the values of these caps?
Best answer:
Answer by billrussell42
Usually 0.1uF 50 volts or 20 volts. Sometimes 0.01uf is usesd instead.
You want one capacitor very close to each IC and transistor, so that the ICs power supply has as low an impedance as possible as seen by the IC, and to keep pickup of noise and RF to a minimum.
Even a 1/2 inch length of trace has a significant inductance and can pick up RF.
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generally, they’re for filtering the rectified power. Depends on the voltage, what the values should be. uf value should have some high and low values. The high values filter the power, the lower values shunt spikes and noise to ground.
They said right. Which is the main reason.
http://www.toboc.com/forum3/default.aspx?g=posts&t=1362
http://www.toboc.com/forum3/default.aspx?g=posts&t=1360
The basic reason is to reduce the high-frequency impedance of the power plane. The higher the operating frequency and / or power, the more critical this will be.
The reason for so many caps is twofold:
* Each power-using device is a potential noise source, so it needs bypassing nearby to minimize its AC current loop.
* Small-value caps have lower series inductance so perform better at high frequency. Lots of small ones will do a better job at high frequency than one big one.
The small-value / low inductance bypass caps are placed close to the devices that use power. This localizes (bypasses) the AC loop so that the device doesn’t kick back noise to the other devices on the board, as well as providing better response (lower power impedance) locally.
Typical small cap values are 0.1uf, mixed with 0.01 and 1uf – the exact values are selected to create the lowest overall impedance. It gets even more involved when you consider various package types, details of via placement, anti-resonance issues, etc.
These small caps are combined with large-value bulk caps that work best on the lower frequencies. These tend to be placed near the board power entry point.
Even that isn’t enough sometimes. Bypassing is only really effective up to 300-400MHz or so. Beyond this, the board itself becomes a key bypassing element – power/ground plane coupling begins to dominate.
There’s an excellent and very-readable book that goes into this and other topics in high-speed design, called “High Speed Digital Design: A Handbook Of Black Magic” by Dr. Howard Johnson. Linkage below…