Diodes

Diodes 

Diodes are very important in electronic circuits. Everyone working in electronics must be familiar with them. Your study of diodes will enable you to predict when they will be on and when they will be off. You will be able to read their characteristic curves and identify their symbols and their terminals. This chapter also introduces several important types of diodes and some of the many applications for them.

Types of Diodes

Rectifier diodes

A rectifier diode is designed specifically for circuits that need to convert alternating current to direct current. The most common rectifier diodes are identified by the model numbers 1N4001 through 1N4007. These diodes can pass currents of up to 1 A, and they have peak inverse voltage (PIV) ratings that range from 50 to 1,000 V.
Here is a list of the peak inverse voltages for each of these common diodes. When choosing one of these diodes for your circuit, pick one that has a PIV that's at least double the voltage you expect it to be exposed to. For most battery-power circuits, the 50 V PIV of the 1N4001 is more than sufficient.

Model Number	Diode Type	Peak Inverse Voltage	Current
1N4001	Rectifier	50 V	1 A
1N4002	Rectifier	100 V	1 A
1N4003	Rectifier	200 V	1 A
1N4004	Rectifier	400 V	1 A
1N4005	Rectifier	600 V	1 A
1N4006	Rectifier	800 V	1 A
1N4007	Rectifier	1,000 V	1 A

Most rectifier diodes have a forward voltage drop of about 0.7 V. Thus, a minimum of 0.7 V is required for current to flow through the diode.

Signal diodes

A signal diode is designed for much smaller current loads than a rectifier diode and can typically handle about 100 mA or 200 mA of current.
The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.
Here are a few other interesting points to ponder about signal diodes:

• They're noticeably smaller than rectifier diodes and are often made of glass. You have to look at them closely to see it, but the cathode end of a signal diode is marked by a small black band.
• They're better than rectifier diodes when dealing with high-frequency signals, so they're often used in circuits that process audio or radio frequency signals. Because of its ability to respond quickly at high frequencies, signal diodes are sometimes called high-speed diodes. They're also sometimes called switching diodes because digital circuits often use them as high-speed switches.
• Some signal diodes are made of germanium rather than silicon. (Germanium the crystal, not to be confused with geranium the flower.) Germanium diodes have a much smaller forward-voltage drop than silicon diodes — as low as 0.15 V. This makes them useful for radio applications, which often deal with very weak signals.

Zener diodes

In a normal diode, the peak inverse voltage is usually pretty high — 50, 100, even 1,000 V. If the reverse voltage across the diode exceeds this number, current floods across the diode in the reverse direction in an avalanche, which usually results in the diode's demise.
Normal diodes aren't designed to withstand a reverse avalanche of current. Zener diodes are. They're specially designed to withstand current that flows when the peak inverse voltage is reached or exceeded.
And more than that, Zener diodes are designed so that as the reverse voltage applied to them exceeds the threshold voltage, current flows more and more in a way that holds the voltage drop across the diode at a fixed level. In other words, Zener diodes can be used to regulate the voltage across a circuit.

In a Zener diode, the peak inverse voltage is called the Zener voltage. This voltage can be quite low — in the range of a few volts — or it can be hundreds of volts.

Zener diodes are often used in circuits where a predictable voltage is required. For example, suppose you have a circuit that will be damaged if you feed it with more than 5 V. In that case, you could place a 5 V Zener diode across the circuit, effectively limiting the circuit to 5 V. If more than 5 V is applied to the circuit, the Zener diode conducts the excess voltage away from the sensitive circuit.
Zener diodes have their own variation of the standard diode schematic symbol.

Transistor Diode Identification

Connect the right leads of a transistor to form a diode.

A diode is similar to a water valve in operation; when open it lets electrical current flow through a wire and when shut it prevents current flow. Transistors, because they contain two diodes, can be configured to operate like a diode. Just tie together the right two transistor leads and you are done. A transistor diode has the same electronic schematic symbol as a transistor. However, the transistor diode schematic will have two of its leads connected together. Be aware that there are two transistor symbols to identify, the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and the Bipolar Junction Transistor (BJT) symbol. Notation on these schematics will also indicate the type of MOSFET and BJT transistor.

Instructions

Required

• Transistor schematics
• Diode schematics
• Electronics textbook
• Electronic circuit schematic

1. 1
   Find a schematic for an electronic circuit. Consider asking an electronics manufacturer for a schematic. Application notes that manufacturers publish often contain schematics. Specifically, ask a transistor manufacturer (the applications or marketing department) for an application note that contains a schematic of a circuit that has transistors connected as diodes. Ask the manufacturer if there is a schematic available that contains both MOSFET and BJT transistors diode connections.
2. 2
   Look for the bipolar and MOSFET transistor schematic symbols in the schematic. Circle each transistor symbol. If you are not familiar with transistor symbols, examine the schematic symbols for MOSFETs and bipolar transistors in elementary electronics textbooks or look in resource 1 and resource 2. Understand that the MOSFET has a drain, gate and channel lead and that the bipolar transistor has a base, emitter and collector lead. Also realise that the MOSFET and the BJT have three leads. Know how to identify each of the leads for these two types of symbols. Know that there are two types of MOSFET symbols, a NMOS, also known as an n-channel MOSFET, and a PMOS, also known as a p-channel MOSFET. Know how to recognise the symbols for each type. Note that their symbols are very similar. A NMOS transistor has a small circle on its gate lead. Also note that there are two types of bipolar transistor symbols, the NPN and PNP. Know how to recognise the symbols for each. Note that their symbols are very similar. A NPN transistor has an arrow on the one lead that points outward. A PNP transistor has an arrow on one lead that points inward toward the base lead. Examine each circled bipolar transistor and check to see if the base and the collector leads are wired together or if the base and emitter leads are wired together. If they are, mark them as bipolar diodes. Examine each circled MOSFET transistor and check to see if the gate and source leads are wired together or if the gate and drain leads are wired together. If they are, mark them as MOSFET diodes. 

   
   
3. Mark the anode and cathode of each MOSFET transistor diode. Consider that a diode has an anode (positive) and a negative (cathode) lead. Identify each MOSFET as either a PMOS or NMOS MOSFET. The anode of the PMOS transistor diode will be the gate lead. Mark that lead with an "A" for anode. The cathode of the PMOS transistor diode will be the lead of the transistor that isn't connected to the gate of the PMOS transistor. Mark that lead with the letter "C" for cathode. The cathode of the NMOS transistor diode will be the gate lead. Mark that lead with a "C" for cathode. The anode of the NMOS transistor diode will be the lead of the transistor that isn't connected to the gate of the NMOS transistor. Mark that lead with the letter "A" for anode. 

   
   
4. Mark the anode and cathode of each bipiolar transistor diode. Identify each bipolar transistor diode as a PNP or NPN transistor diode. For PNP transistor diodes, the cathode of the PNP transistor diode will be the PNP's base lead. Mark that lead with a "C" for cathode. The anode of the PNP transistor diode will be the transistor's lead that isn't connected to the base of the PNP transistor. Mark that with the letter "A" for anode. For NPN transistor diodes, the base lead is the anode. Mark the base lead of a NPN transistor with an "A" for anode. The NPN transistor's lead that isn't connected to the base is the cathode. Mark this lead with the letter "C: for cathode.