Diodes

. . . A Tutorial for inexperienced builders

[[  Most recent update: 13 February 2013.  ]]

The main thrust of WannaTinker is building, testing, and using electronic gear as opposed to theory.

There are times, however, when a little theory can come in handy, such as when the electronic marvel you have just put together simply sits there imitating a paper weight rather than performing electrical magic for you.

Accordingly, this tutorial is devoted to explaining the fundamentals of diodes.

As you can see in the photo showing a few of the dozens of packages for diodes, they come in a variety of shapes and sizes.

On the left side of the photo, there are six (count ’em, 6) tiny diodes attached to a strip of plastic.  These tiny diodes are the surface mount technology (SMT) type, and I seldom use them because they are difficult to work with.

All the diodes pictured here are of the silicon diode type, which are the most common type used.

As you might imagine, the increase in size from left-to-right in the photo represents an increase in the current carrying capacity of the diodes.  The ones on the left are capable of passing only a small fraction of an ampere of current and the large ones on the right can safely pass multiple amperes of current.

I urge you to avail yourself of the many excellent sources of information about semiconductors, such as the ARRL Handbook, magazine articles, and other reference material.

Having said that, I find that reference material sometimes gets a little too theoretical to be of use to me. When I am trying to get a circuit up and running, I really don’t want to wade through a lot of theoretical discussions about the physics of why things work the way they do. I just want the quickest route to finding where I made a wiring error, or whatever caused my circuit to fail.  This tutorial is intended to help you do just that.

Look inside your commercially built electronic gear and you will see dozens of diodes. Check out of just about any electronic device and you will see diodes all over the place! Diodes are one of the most useful electronic components, and one of the oldest in terms of general usage (think “crystal set” radio receivers). Whole books could be (and have been) written about these marvelous little devices. More importantly, diodes will be used in the projects presented on this web site, so it will be useful for you to know a thing or two about them. Besides that, when you know a thing or two about diodes, learning about transistors is a piece of cake (you DO want to know something about transistors, don‘t you?).

Diodes can do an amazing variety of electronic tasks, including switching, frequency doubling, voltage regulation, temperature sensing, acting as a fixed or variable capacitor.   One of the simplest and most effective product detector circuits is the diode ring mixer, and that’s a story for another day.

Diodes are rated by, among other things, the amount of current they can safely pass. A forward biased diode is like a closed switch; it will pass current. “Forward biased” simply means the voltage on the anode is more positive than the cathode, as shown below  at “A”.

bias png

As you might suspect, a reverse biased diode will block current flow, as shown at “B”.

There is a critical reverse bias voltage that must not be exceeded. If this critical voltage is exceeded, the current will begin to flow in an uncontrolled manner, and the diode may turn to smoke.

There are diodes, however, that are designed to safely pass current when the reverse bias reaches a specified voltage. These diodes are called Zener diodes, and they are effective voltage regulators. The Drawing below shows a typical application for a Zener diode as a voltage regulator.

Notice the slight difference in the symbol for the Zener diode, as opposed to the symbol for the “ordinary” silicon diode shown earlier.

Given that the diode is a 9.1 volt Zener (a commonly used value) and a supply voltage that varies from about 10 to about 14 volts, point “A” will be held at exactly 9.1 volts. This assumes, of course, that the load current, IL, is within the design limits of the diode.

The Zener diode will draw varying amounts of current, IZ, in order to hold the voltage constant.

Clever, don’t you think?

You may wonder what happens if the supply voltage drops below the desired 9.1 volts, say about 8 volts. In that case, we’re just out of luck. Zener diodes, clever as they are, cannot create voltage where none exists.

The ARRL Handbook shows simple formulas for calculating the value of R, and determining suitable wattage ratings for Zener diodes. The Handbook does a good job of presenting and explaining these calculations, so I will not duplicate that effort here.

Why are they called “Zener” diodes? – You might want to know.  This type of diode was “discovered” by Clarence M. Zener, an American physicist who did extensive research into semiconductors in general and diodes in particular.

Now, let’s stake a look at a varactor diode, sometimes called a voltage variable capacitor (VVC).

If you are new to electronics, the term “voltage variable capacitor” begs other questions: “What is a variable capacitor?”; “If voltage can be used to vary the value of a capacitor, what other methods can be used?”; “Why would one want to vary the value of a capacitor in the first place?”; etc., etc., etc.

I don’t want to insult knowledgeable readers by being TOO fundamental, but I also don’t want to loose those who are just beginning in this wonderful world of electronics. The main subject in interest at the moment is diodes, not capacitors (variable or otherwise).

Having said that, please allow me to digress, briefly, into resonant circuits. Resonant circuits have two main ingredients: capacitance and inductance. Capacitance is provided by (what else?) capacitors, and inductance is provided by (you guessed it) inductors, which are commonly called “coils”.

When you “tune across the dial” on your broadcast radio receiver, you are varying the capacitance or inductance in a resonant circuit. That’s one reason for wanting to vary the value of a capacitor. In order to r.e.a.l.l.y understand resonant circuits, you need to know about capacitive reactance, inductive reactance, and other stuff that we don’t need to get into during this look into diodes. For now, just remember that any time you see a coil and a capacitor connected together, either series (end-to-end) or parallel (side-by-side) you are looking at a resonant circuit.

The coil, L1 and the capacitor, C1, in the drawing  make a parallel resonant circuit.

D3

The coil, L2, and the capacitor, C2, make up a series resonant circuit. We will take a closer look at resonant circuits elsewhere, but for now, it’s back to diodes.

If your radio was built since about 1980, chances are the “tuning” is done with a  varactor diode, not directly, but as part of a voltage controlled oscillator (VCO) in the phase locked loop (PLL) circuits, all of which is beyond the scope of this tutorial about diodes.

When reverse bias is applied to a silicone diode (positive voltage to the cathode and negative to the anode) it will act as a capacitor. Maximum capacitance will occur near zero volts, and the capacitance will decrease as the voltage is increased, within reason, of course. The drawing below shows the voltage to capacitance relationship for an MV209 varactor.

A similar curve can be plotted for any varactor.

D4

As with Zener characteristics, the varactor characteristics show up in all silicon diodes (and in the junctions of bipolar transistors, but that’s another story). Some diodes are manufactured specifically for the varactor characteristics. The old-style varactors looked like small transistors with the middle leg missing.   Now days, however, virtually all varactor are manufactured for surface mount technology and are so tiny they are very difficult to work with. They look sort of like this: > .. <   Yes, they are very tiny, and are intended for robotic assembly, so I seldom use them in circuits I build, and none are used in the WT40 transceiver because larger discrete components are better suited for DIY projects.

A typical VCO circuit using a varactor is shown here.

D6

No, this is NOT a module for you to build, but you will see a similar circuit in the variable frequency oscillator (VFO) module. The capacitor “C”, the coil “L”, and the varactor diode “V” make up the resonant circuit.

Notice that the circuit is tuned with a potentiometer, “R Tune”, which varies the reverse bias voltage on the varactor.

Notice, also, that the symbol for the varactor diode is different than that for an ordinary diode – there are two lines representing the cathode.

If you are new to radio circuits, this is no big deal, but to those of us who began with mechanical variable capacitors, this is an amazing concept (to be able to tune a circuit without a mechanical variable capacitor).

In the example shown above, the capacitor, C3, couples the tuned circuit to the oscillator transistor (transistor not shown in this drawing). Capacitor C2 prevents the DC voltage from being shorted to ground through the coil, L. The value C2 is quite large, compared to the capacitance of the varactor, V, that is connected directly to the top of the coil, L, as far as the resonant circuit in concerned. The varactor and C2 are in series, so the total capacitance will be a bit less than the capacitance of the varactor.

(See your ARRL Handbook, or other reference, for information about how to calculate values for capacitors in series and parallel.)

Capacitor C4 shorts (bypasses) any radio frequency (RF) energy that gets through the radio frequency choke, RFC, to ground. The reason that the RF signal is bypassed to ground is that you don’t want RF energy running around  through your circuitry because it can cause all kinds of mysterious problems.

For a resonant circuit of about 7 MHz, typical values for the components are shown below.

L – 2.5 uH

C1 – 180 pF

C2 – 0.001 uF

C3 – 150 pF

C4 – 0.047 uF

RFC – 1 mH

R Tune – 10k

V – Maximum capacitance of about 35 pF

Speaking of using diodes in a VFO circuit, as previously mentioned, a VFO circuit similar to the one you will be building for your WT40 Transceiver is shown below.

Diodes in this VFO circuit include:

[] D1 Which is being used as a SWITCH to place C1 into the resonant circuit and provide frequency offset when in transmit (T) mode.

[] D2 & D3 are “ordinary” silicon diodes arranged in a manner that turns then into a varactor to allow the “variable” function in the VFO.

[] D4 is a Zener diode used to regulate the tuning voltage.

[] D5 assists in stabilizing the oscillator circuit.

[] D6 is a Zener diode used to regulate the voltage that powers the oscillator circuit.

[] D7 is a “flux sucker” diode to stabilize the relay, K1 (prevents “chatter” when the relay is de-energized.)

By the way . . .

. . .if you apply forward bias to a varactor diode, it will not act as a varactor, but  will act much like an “ordinary” diode.

Similarly, reverse biasing an “ordinary” diode will cause it to act like a varactor.

When you build the VFO module for the 40 meter transceiver described on this web site, you will find that it does, indeed, use “ordinary” silicon diodes as varactors.

So why bother with varactors at all, you might want to know.  Simply because ordinary diodes offer a limited range of capacitor value, usually from 10 to 25 pF  –  varactors are available with values of over 400 pF.

– END OF DIODE TUTORIAL –

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About w6bky

Retired 29 May 1987. Now do hobbies: blogging, ham radio, gardening, etc.
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