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If you are using a Schottky diode to protect an input pin from a voltage of greater than +5V, where the diode is connected (in the appropriate direction) between the pin and the +5V Vcc, doesn't that mean that you're dumping current into Vcc when (for instance) +7V is being applied to the pin?

If that is the case, could this result in damaging a battery or anything else? If so, under what circumstances?

(Note: I'm aware that there is usually a resistor in such a circuit as well, but what I'd like to know is why I've seen schematics with these diodes using Vcc as a dumping ground for current).

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    \$\begingroup\$ Welcome to Arduino Stack Exchange! To be perfectly honest, I think this question is better asked at Electronics Stack Exchange, as there is no reference to an Arduino here, and the people at Electronics.SE will be able to give you a comprehensive answer. You may wish to edit the question to include a schematic of the exact setup you are asking about \$\endgroup\$ May 16, 2018 at 6:46
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    \$\begingroup\$ A Schottky diode isn't typically used to protect against overvoltage, but reverse voltage. Are you thinking of a Zener diode? \$\endgroup\$
    – Dampmaskin
    May 16, 2018 at 7:13
  • \$\begingroup\$ Show schematic! \$\endgroup\$
    – winny
    May 16, 2018 at 7:21
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    \$\begingroup\$ As you say, there is usually a series resistor which will drop the voltage, and limit the amount of current you dump onto Vcc. A design without the series resistor is a bad design for exactly the reasons you've stated. \$\endgroup\$
    – Steve G
    May 16, 2018 at 7:36
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    \$\begingroup\$ In some cases fault current through a clamping diode connected from signal pin to Vcc can "pump up" the Vcc rail. This occurs in the fault current is in excess of the Vcc load from all sources. This really can happen in practice and must be designed around. \$\endgroup\$
    – Russell McMahon
    May 16, 2018 at 11:59

2 Answers 2

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where the diode is connected (in the appropriate direction) between the pin and the +5V Vcc, doesn't that mean that you're dumping current into Vcc when (for instance) +7V is being applied to the pin?

Yes, you dump the energy into VCC potential, raising its voltage.

In ESD (electrostatic discharge) situations this is OK, because there is usually at least an 100nF cap present on VCC/GND (and often >=10µF in parallel). ESD discharges are modeled by a small but high voltage capacitor - much smaller than 100nF, so the voltage rise is very small, too.

The other case is a (permanent) connection to a higher voltage. That can burn out the diode, the battery/voltage source and the electronics it was supposed to protect, by raising VCC above damage level.

This is where additional protection like series resistors and TVS diodes come into play.

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A schotky diode actually refers to a “standard diode” ie an in4001 etc employed to short out the low energy very high voltage of a collapsing magnetic field. Much like you’d see across a solenoid coil in (reverse polarity) after de-energisation.

I think what you may be referring to is zener diode which has a reverse breakdown voltage (RBV) this means that 1. They have virtually no resistance in forward bias, but in reverse bias, they will switch from extremely high impedance to virtually no impedance when the voltage reaches the required point (the zener rated voltage) There switching time is also measured in micro seconds, so unless your into computer speed engineering it’s neglible and can be ignored ( Ie there not going to heat up in transition; and they will respond fast enough to protect other ics.

Your correct they are usually used with a resistor; that is so when they go into reverse bias conduction mode, the resultant short (normally to the ground rail) is limited. The voltage between the source (+ve) is now expressed (dropped) entirely across this resistor, protecting both the per sup from overload and the load from overvoltage. This actually happens so fast that the tiny bit of natural impedance inherent in the most basic of these types of circuit arrangements is enough to produce not only a clipped and regulated supply, but also a well regulated one when viewed even with extremely fast scopes.

Negatives of zener regulation? Zener regulation is so simple why isn’t it used all the time? Answer - It probably is but integrated into a voltage regulator. The reason why we don,t often use it raw, is because of the limited power handling capability of the voltage dropping resister. Imagine taking a 110v supply and dropping it to 10v with a zener/diode config’ and expecting 1amp out of it... figured it out yet?

The heat rejection(inefficiency/loss) across the resistor would be equal to VI or 100*1 or 100 Watts!

Zener reg’ is useful if it is driving the gate/base of a power transistor or to protect components, but when it is active the signal will be cancelled...

How to use a zener for protection of dig’ elec’ components. Below describes a classical ‘high or ‘sourcing’ type input. It is assumed that we are talking about very low energy high impedance inputs. Signal > voltage dropping resistor (typical range 1k2 to 6k8 for 5v sig’) > “tee junction of” zener cathode and IC Input. Anode of zener should be tied directly to ground.

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    \$\begingroup\$ I think he really meant shottky, not zener. In a configuration similar to this. \$\endgroup\$
    – dim
    May 16, 2018 at 11:23
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    \$\begingroup\$ The 1n4001 is very definitely not a schottky diode. \$\endgroup\$
    – Hearth
    May 16, 2018 at 11:24
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    \$\begingroup\$ Your answer has some good comment in it BUT you have missed what he is asking about. Schottky diodes have low Vf (forward on voltage). When connected eg from pin to Vcc threy clamp the pin at a Schottky Vf drop above Vcc. This is a standard and useful protection scheme. \$\endgroup\$
    – Russell McMahon
    May 16, 2018 at 11:57
  • \$\begingroup\$ There’s no such thing as a Schottky diode. It’s a term used to describe the way a diode is used.... \$\endgroup\$
    – boonge72
    May 16, 2018 at 14:22
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    \$\begingroup\$ @boonge72, a Schottky diode is a specific construction of diode, not a way it is used. A Schottky diode is a metal-semiconductor junction rather than p-n junction. \$\endgroup\$
    – The Photon
    May 16, 2018 at 15:44

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