Today we're going to look at 4 1/2 diode tricks that you can use in your designs.
The diode is a fundamental component you have to understand if you want to grow your electronics
prowess.
So today we're going to take a quick look at what a diode is, then look at 4 1/2 uses
for a diode in your designs... but we're not going to use half a diode...
Anyways.
Diodes are generally the first non-linear component one learns about.
Resistors, capacitors, and inductors are all linear devices, meaning that if you were to
plot the input voltage vs. the output voltage, you'd get a plot straight up and to the right.
Nonlinear devices don't follow Ohm's law , and for circuit analysis you cannot replace them
with a Thevenin equivalent.
Diodes are also passive devices, which means they don't need power to function.
Finally, they are two port devices, meaning there's just one input, the anode, and one
output, the cathode . When operating normally, an ideal diode only conducts current in one
direction.
They're conceptually quite simple, but are extremely useful because of their V-I curve,
which looks like this.
The x-axis is voltage, and the Y-axis tells us how much current can flow through the device
when exposed to that level of voltage.
At moderate positive voltages, a diode essentially acts as a short, but with a small voltage
drop called the forward voltage drop.
You can see the forward voltage drop on the VI curve - around .6-.7 V, where the current
starts to ramp up.
That .6/.7V drop is standard for silicon diodes, but for other types of diodes and materials
this forward voltage drop may vary.
You can measure this forward voltage drop? for a specific diode using a multimeter with
a diode testing capability.
You can see that this diode has a forward voltage of roughly .62V.
The forward voltage is also sometimes called the cut-in voltage or the on voltage.
But what you need to remember is that, when exposed to a moderate voltage say 5V A diode
will pass through 5V minus the forward voltage.
So 4.3V for a standard silicon diode.
There are some methods for compensating for this drop, but that's beyond the scope of
today's video.
When exposed to a negative voltage, there will be a nano-amp reverse current (or current
from the cathode to the anode) that you can generally take to be 0A in most situations.
That is, until you get to the other big swing on the VI curve - known as the breakdown voltage.
If your diode is exposed to this level of reverse bias, you blew it.
Often literally.
Diodes essentially can't hold up to that level of negative voltage stress and the device
literally breaks down - allowing negative current flow.
Long story short, you can essentially think of a diode as a one-way conductor.
So, enough preface, let's look at a few different ways you can use diodes for your circuits.
Use #1, rectifiers This has been covered ad nauseum by other
channels, so I'm going to cover this quickly.
The gist is that you can let through the positive voltages from an AC source while blocking
or inverting the negative voltages.
You can put together a halfwave rectifier with a single diode
A full bridge rectifier with four diodes.
That's right, it's not a dumb single diode rectifier, it's a full bridge.
It's worth noting that you have to use a differential probe to measure a full bridge rectifier with
a scope to avoid grounding half your rectifier.
Bring a capacitor into the mix, and you can do a couple things.
1st, you can smooth out a rectifier (although it's generally better to use an active voltage
rectifier chip) 2nd, you can build a voltage multiplier, which
is essentially a bunch of half wave rectifiers in series.
This is used for things that need high voltage but not a lot of current, like electric bug
zappers and particle accelerators which are basically the same thing, right?
Rectifiers are also the backbone of original AM radios.
Use #2 - diode logic gates.
You can build simple logic gates using diodes.
You can create an n-input OR gate or AND gate using 1 diode per input.
For more practical scenarios, you can do some less-standard logic.
Take, for example, a battery backup system.
Using two diodes, you can pass either the intended voltage input (if present), or the
battery source.
In this situation, the diode is reverse biased under normal conditions, so the battery stands
dormant.
When the source voltage is removed, the diode becomes forward biased and provides power
to the device.
Use #3 - diode clamps You can use diodes to limit the range of a
signal.
You can essentially pre-clip the signal by connecting a diode to a reference voltage
or a voltage divider.
If we start with a standard triangle wave, the signal looks normal on the scope.
But, when we add the diode + resistor clamp attached to a positive reference voltage,
the signal starts to clamp.
If I adjust the reference voltage, we can see the clamping levels change.
If you want to get better clamping you can add a bypass capacitor, or you could turn
to an active clamper circuit using an opamp or transistor circuit, but the simple diode
provides an easy and cheap clamp.
They are often used with sensitive inputs, like CMOS logic circuits, to prevent damage
from static discharge.
Clamps can also provide DC restoration when used with a capacitor.
For example, if a signal has been capacitive coupled, you could setup a diode and capacitor
like this to restore it's positive DC component.
Channel 1 is the input, which you can see is centered around ground, and channel 2 is
the output, which you can see is now bottoming out around ground.
Use #3.5 - Diode limiters I'm only counting this as 1/2 of a use, because
it technically falls under clamping, but is different enough to warrant an extra 0.5
Limiters, sometimes known as clippers, are a special type of clamp that limit the output
to plus or minus the forward voltage with a setup like this, which can be used to help
protect high gain amplifiers or other sensitive circuitry from saturating.
Watch what happens if we sweep the input amplitude between 0 and 5 V, you can see that the signal
doesn't break +/- .6 V, which is the forward voltage of the diodes.
Hack #4 - Flyback Diodes Flyback diodes are the pressure release valves
of the electronics world.
Because inductors resist a change in current you can't just instantaneously turn them off
- they'll fight back against the control circuitry to the point that it can shorten its life
or blow it up all together - this is known as inductive kick.
So, you can add a flyback diode in parallel with the inductor.
As I mentioned in the video about the time constant, this can cause a slower current
decay than you might like, so you could also use an RC snubber if you prefer.
It also won't work if the inductor is driven by an AC source (like a transformer), because
the diode will conduct on half-cycles.
In that situation you should use an RC snubber or something like a metal-oxide varistor.
To learn more, check out the component testing with an oscilloscope app note in the description
that discusses how to test both linear and non-linear devices using an oscilloscope:
Are you using any of these tricks on your designs?
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Thanks for watching!
I'm Daniel Bogdanoff, and I'll see you next time!