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# What Kind of Diode to Use with ADC Inputs

1. How to reduce voltage without reducing the current using zener diode?

For something like that, I think you should rather use a 5V voltage regulator

2. Why does a power diode have a much larger p-n junction area than the regular signal diode?

A power diode has a much larger p-n junction area because all solids are ultimately limited to a maximum current density. Beyond this maximum they will heat up so much that they melt. Power diodes are designed to handle lots of power, P = IV, which means that for a fixed potential, V, the current must be large. Current density is current per unit area. If you want a particular maximum current, all of the cross sectional areas of the diode must have a minimum area so that the maximum current divided by the area never exceeds the maximum current density.In most solids, this number is between one tenth of a million and one million amps per square centimeter, 10^6A/cm^2. It sounds like a big number but most junctions are quite small. A typical diode for small signal applications will have an area about one micrometer square, mu m^2, which is 10^-8 cm^2. Such a diode can only handle 0.01 Amps or 10 milliAmps (10^6 * 10^-8). To handle 1 Amp the diode area must be 100 times larger 10x10 mu m^2 . Power diodes need to handle tens of amps even for household power. Some household diodes rated at 6 amps have surge current capacity of 400 amps. That makes the area of such diodes at least 200 x 200 mu m^2. This is huge for integrated circuit technology.Why does a power diode have a much larger p-n junction area than the regular signal diode?

3. What diode should I use that doesn't drop power?