DIODES


The main thrust of Wanna Tinker 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 just sits there imitating a paper weight rather than performing electrical magic for you.  Accordingly, this tutorial is devoted to explaining the fundamentals of diodes.

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.  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 care to wade through a lot of theoretical discussions about the physics of why things work the way they do. I just want the quickest rout to finding where I made a wiring error, or whatever caused my circuit to fail.

Look inside your commercially built rig and you will see dozens of diodes.  Check out the insides 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 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?).

DiodeDiodes can do an amazing variety of electronic tasks, including switching, frequency doubling, voltage regulation, temperature sensing, acting as a fixed or variable capacitor, and more.  One of the simplest and most effective product detector circuits is the diode ring mixer. 

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 in Figure 1 (A). 

As you might suspect, a reverse biased diode will block current flow, as shown in Figure 1 (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.  Figure 2 shows a typical application for a Zener diode as a voltage regulator.  Notice the slight difference in the symbol for the Zener diode in Figure 2 as opposed to the symbol for the “ordinary” silicon diode shown in Figure 1.

Given that D1 in Figure 2 is a 9.1 volt Zener (a commonly used value) and a supply voltage that varies from about 10 to Figure 2about 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

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.

Figure 3Now, let’s stake a look at another type of diode you will be using, the varactor diode, sometimes called a “variable capacitance diode” or “voltage variable capacitor”.  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 Figure 3 make up a parallel resonant circuit; 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 or TV were built since about 1980, chances are the “tuning” is done with one or more varactor diodes, not directly, but as part of a voltage controlled oscillator (VCO) in the phase locked loop (PLL) circuits.

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.  Figure 4 shows the voltage to capacitance relationship for an MV209 varactor. 

Notice that maximum capacitance occurs near zero volts and minimum capacitance occurs near 30 volts for this particular varactor, a type MV209. 

Figure 5As 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, as shown in Figure 5.  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 small!

A typical VCO circuit using a varactor is shown in Figure 6.  Figure 6

No, Figure 6 is NOT a module for you to build, but you will see a similar circuit in the 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.  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).  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 the varactor 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 all 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 in Figure 6 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 

If you apply forward bias to a Zener or varactor diode, they will act much like an “ordinary” diode.  Conversely, it you apply reverse bias to an ordinary silicon diode, it will act as a varactor.  Yes, you can use ordinary silicon diodes as a varactor or voltage regulator, but that subject is beyond the scope of this tutorial, not because it is particularly complex, but because it is not useful right now.  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. 

One Response to DIODES

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