- [Instructor] There are three primary characteristics
that I consider when choosing which diode
to use in my circuit.
Forward voltage, which is the voltage at which
the diode turns on and allows forward current
to flow through it,
the breakdown voltage,
which is the negative voltage
at which the diode fails to stop current
from flowing backwards through it,
and the maximum forward current,
which is the amount of forward current
the diode can handle before it will overheat and burn up.
To show you how to buy in and interpret
these diode characteristics,
I'll look at the data sheet for a common 1N4148
small signal diode.
The 1N4148 diode is designed for general-purpose usage.
It has a moderate forward voltage drop,
but it's not designed to handle much forward current.
To find the data sheet for that diode,
I'll go to my search engine
and type in the part number 1N4148
and the word datasheet.
I see that there are several datasheets available
for the 1N4148 diode from different manufacturers.
For this video, I'll look at the datasheet from Vishay.
The top part of the first page lists
some general information about this diode,
what it's typically used for
and the types of packages it comes in.
Below that, is a table labeled absolute maximum ratings.
Like all other electronic components,
diodes have physical limitations.
And these are the values you need to pay attention to
to avoid destroying your components.
The most important of these to worry about
is the maximum forward continuous current.
Abbreviated as IF,
which describes the average amount of current
the diode can safely conduct in the forward biased mode.
Ideally, a diode will be able to conduct
an infinite amount of current
when it's forward biased,
but in reality as the amount of forward current
through this diode increases,
the diode produces more heat,
which it will need to dissipate.
So this maximum forward continuous current rating
is based on the thermal limitation of how much heat
the diode can handle.
If I try to maintain a forward current of
more than 300 milliamps through this diode for too long,
that will cause it to overheat and it'll burn up.
However, this diode can handle more than 300 milliamps
if it's only for a short period of time.
The peak forward surge current is the maximum amount
of current the diode can handle during a surge.
I like to relate the maximum continuous current
and surge current ratings to how long I can hold my hand
over a hot stove.
If I pass my hand over the stove quickly,
that represents a surge of current.
Even if the stove is really hot,
it's not going to burn me because of the short exposure time.
However, if I hold my hand over the stove,
representing a continuous current,
even if the stove is only moderately hot,
it will eventually burn my hand due to the long exposure.
According to the datasheet,
this diode can survive up to two amps of current
for a surge lasting up to one microsecond.
If I ever exceed that two-amp peak
forward surge current even momentarily,
then there's a pretty good chance my diode will overheat.
Moving on down the page
to the electrical characteristics section,
there are two main parameters to pay attention to,
the forward voltage and the breakdown voltage.
The forward voltage is the amount of positive voltage
between the anode and cathode
that will cause the diode to turn on,
allowing forward current to easily flow through it.
For silicon diodes the forward voltage is typically
somewhere between 0.6 to one volt.
The breakdown voltage is the amount of reverse voltage
the diode can withstand before it fails
to stop reverse current from flowing backwards.
And for silicon diodes,
the breakdown voltage is typically anywhere
from 50 to a hundred volts or even more.
For this particular 1N4148 diode from Vishay,
the forward voltage is listed as having
a maximum value of one volt,
meaning that one of these diodes
should never require more than one volt
to become forward biased.
A little bit lower in the table,
the breakdown voltage for this diode is listed
as being a minimum of 100 volts.
So, I can expect that one of these diodes
will withstand at least 100 volts
in the reverse biased direction
before it allows reverse current to flow freely.
These minimum and maximum breakdown
in forward voltage values in the datasheet
are extremes that you can expect from this part.
But, like all electronic components,
that will vary from part to part due to variations
in the specific design and manufacturing process.
When I'm building a circuit with one of these diodes,
sometimes I'd like to know the actual forward voltage
that I can expect the diode to turn on at,
rather than just the worst case maximum
that's listed in the datasheet.
And I can find that out using my digital multimeter.
To do that, I'll turn the knob on my DMM
to the diode test mode,
which is indicated by the diode symbol.
This is the same DMM setting that you can use
to check for electrical continuity
to find short circuits.
So, if I touch the probe tips together,
the DMM will beep at me to let me know
that there's a short circuit between them.
To measure the diodes forward voltage,
I'll touch the black probe to the cathode terminal,
which is the side of the diode
with the little black stripe,
and I'll connect the red probe to the anode terminal.
I can see that the forward voltage of this diode
is actually closer to 0.6 volts
than the one-volt maximum that was listed in the datasheet.
It's a good thing because I generally want my diode
to switch into the forward biased on mode
at the lowest positive voltage it can,
so that the diode acts as close
to a real short circuit as possible to forward current.