- [Instructor] One of the quickest way to accidentally
destroy an electronic device
is to connect it to the power supply backwards.
It's an easy mistake to make if the power connector
is not polarized, if you're not paying attention you might
accidentally mix up the red and black power wires,
or maybe you somehow stick batteries
into a device backwards.
Those are all mistakes that I've personally made
at some point in time in the past.
So to protect your electronic circuits
against users like me, it's a good idea to
include protection against reverse current
if it could potentially damage your device.
Fortunately, that's easy to do by connecting a diode
in series with the positive side of a DC power source.
This reverse protection diode insures that the current
will only flow in the forward direction
so the DC power source will only ever apply a positive
voltage to the circuit.
If the power source is connected in the correct orientation
current will flow through the diode to the device
and everything works likes it's supposed to.
But if for some reason that power source
gets connected backwards
or has some sort of malfunction
the diode will become reversed by it
and act like an open circuit.
That keeps current from flowing though
the circuit in the wrong direction.
So the circuit is still protected.
One draw back of using a reverse protection diode
is that you do loose some of the voltage
from the DC power source to the forward voltage
drop across the diode.
For example.
If I was connecting my circuit to a 5 volt power source
and used a reverse protection diode
whose forward voltage was 0.6 volts,
my circuit would only actually get
4.4 volts at it's input voltage.
To minimize that forward voltage loss
many circuit designs use a special type of diode
called a Schottky diode whose unique
semi-conductor composition gives it a much smaller
froward voltage drop than standard silicon diodes.
The forward voltage for a Schottyy diode
is usually somewhere between 0.15 volts and 0.45 volts.
But there reverse breakdown voltage is still large enough
to make them useful for protecting against reverse current.
Schottky diodes are special enough
that they get their own schematic symbol.
Which looks like the standard diode symbol
but with to 90 degree bends
added to each side of the cathode line.
Physically the packages for a Schottky diode looks
very similar to a standard Rectifier diode.
It's possible to tell them apart if you look closely
at the tiny little print on them.
I find that hard to read.
So I usually check the forward voltage of my diodes
using my DMM.
To do that I'll set the selector nob to the diode mode.
When I probe a standard Rectified diode,
I can see that it has a forward voltage of around .6 volts
which is what I would expect from a regular diode.
However when I measure the forward voltage
of a Schottky diode I see that's much smaller
than the regular diodes forward voltage.
Now, you might be wondering why we don't just always use
Schottky diodes in place of regular diodes.
Since they have better characteristics.
And the main reason is cost.
Schottky diodes will usually cost several times more
than their general purpose counter parts.
Now that's not a big deal if you're only buying a handful
of parts for a hobby project,
But when engineers are designing electronic devices
to be mass produced those small differences in cost
really start to add up.
One other thing to consider when selecting a diode
to protect against reverse current,
is the amount of current that the circuit will draw.
All of the current flowing into the protected circuit
if first passing through this diode.
So it's critical that the diode can handle all that current
without burning up.
I also need to be prepared for the unexpected.
If something where to suddenly change in the load circuit
that caused to draw more current than intended,
that could potential melt the diode
causing it to fail as a short circuit.
So my circuits no longer protected.
To be extra careful and defend against that possibility
I can insert a fuse in series with the protection diode.
A fuse is a safety component protects electronic devices
from drawing too much current
and basically consists of a strip of wire.
When the amount of current though the fuse is less
than certain threshold, the fuse simply acts as a wire.
But when the current exceeds the fuses threshold
the fuse wire will melt
causing it to act like an open circuit.
So if something changes in the load
and it tries to draw too much current
the fuse will break and cut off the power.
I will need to replace or reset the fuse
to get the circuit running again.
But at least my circuit will be protected
from whatever was going wrong.