# What is this for

Discussion in 'Electronic Basics' started by Wong, Nov 19, 2003.

1. ### WongGuest

Hi,
I am learning electronics and I found something like this:

Vcc
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/ \ Diode
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signal_in --------|----------------------- output
|
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/ \ Diode
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|
|
-----
---
-

What is the purpose of the circuit ? As what I can understand, both
diodes would not conduct if the signal_in range from 0 to 5v, but once
the voltage of signal_in below 0v or more than 5v, either one will be
conducted, am I right ? So what's purpose of the circuit ?

2. ### Michael A. TerrellGuest

It is a clamping, or limiter circuit to prevent damage to an input by
out of range voltages.
--
I say, the boy is so stupid that he tried to make a back up copy of his
hard drive on the Xerox machine!

Michael A. Terrell
Central Florida

3. ### Robert MonsenGuest

Its intended to clip the input. If the input gets higher than Vcc+0.6, the
top diode conducts, and protects whatever output is feeding. If the input
goes less than -0.6V, the bottom one conducts, and again protects the next
device.

Its often used to protect sensitive transistors from abuse. For example, a
PIC microcontroller has this circuit on its inputs to protect from abuse.

Regards,
Bob Monsen

4. ### WongGuest

If either one is conducting, are we going to say that it is a shorted
circuit ? Since there is no current limiting resistor.

5. ### BaphometGuest

Although not specified, I guess it can be assumed that the current is
limited by the source impedance.

6. ### Jim LargeGuest

That would depend on what the input is connected to.
Either the clamping diodes are going to conduct as
much current as it takes to keep the voltage from
going more than 0.6V outside the rails, or the Magic
Smoke is going to come pouring out.

You may wonder why the designer would give you such
an easy way to let the smoke out of a circuit. Well,
if the circuit is a CMOS logic gate, and the input
voltage goes as much as 1V above Vcc or 1V below
ground, the smoke is going to come out anyway, and it
may even damage circuit board traces and power
supplies in the process. Search Google for a fun
phenomenon called SCR Latchup, and you will see why
most CMOS ICs has those diodes on every pin.

-- Jim L.

7. ### Robert MonsenGuest

Its not intended to handle current forever, simply to clip transient spikes.
Thus, the diodes will survive the abuse, because they are not getting
whacked too often.

If you connect the input to a voltage source outside the range {-0.6,
Vcc+0.6} you'll fry the diodes.

Regards,
Bob Monsen