Diodes Tutorial: How to Test Diodes?

A diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.). It has unidirectional conductivity, that is, when the forward voltage is applied to the anode and cathode of the diode, the diode conducts. When a reverse voltage is applied to the anode and cathode, the diode is turned off. Therefore, the turn-on and turn-off of the diode are equivalent to the on and off of the switch.

The diode is one of the earliest semiconductor devices, and it is widely used, especially in various electronic circuits. Diodes and resistors, capacitors, inductors, and other components are reasonably connected to form circuits with different functions, which can realize various functions such as AC rectification, detection of modulated signals, limiting, clamping, and voltage regulation. Whether in the common radio circuit or in other household appliances or industrial control circuits, you can always find the diode.

Catalog

I Diode structure

The diode is made up of a PN junction plus corresponding electrode leads and packages. Using different doping processes, the P-type semiconductor and the N-type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate by diffusion, and a space charge region called a PN junction is formed at their interface.

The electrode drawn from the P area is called the anode, and the electrode drawn from the N area is called the cathode. Because of the unidirectional conductivity of the PN junction, the direction of the current when the diode is turned on is from the anode to the cathode through the inside of the tube.

The circuit symbol of the diode is shown in the figure. The diode has two electrodes. The electrode drawn from the P area is the positive electrode, also called the anode; the electrode drawn from the N area is the negative electrode, also called the cathode. The direction of the triangle arrow indicates the direction of the forward current, and the symbol of the diode is indicated by VD.

Diode circuit symbol

II Diode recognition

Crystal diodes are also called semiconductor diodes, or diodes for short, which are semiconductor devices with a PN junction. There are many types of diodes with different shapes and sizes. The more common ones are glass-shell diodes, plastic-encapsulated diodes, metal-shell diodes, high-power bolt-shaped metal-shell diodes, miniature diodes, and chip diodes. Functionally, it can be divided into a detector diode, rectifier diode, switch diode, voltage regulator diode, etc.

types of diodes

III Diode characteristics

1. The main parameters of the crystal diode are: (1) The maximum rectified current IFMFM refers to the maximum average current allowed to pass forward through the PN junction (Figure a). The actual working current in use should be less than IFM, otherwise, the diode will be damaged. (2) The maximum reverse voltage URM refers to the maximum voltage applied reversely across the diode without causing the breakdown of the PN junction (Figure b). Diodes with URM greater than 2 times the actual operating voltage should be selected during use. (3) The maximum operating frequency fM of the detection or high-frequency rectifier diode should be at least twice the actual operating frequency of the circuit. (4) The stable voltage UZ value of the Zener diode should meet the circuit requirements.

Simple diode circuit

2. The two pins of the crystal diode have positive and negative poles. In the circuit symbol, the bottom of the triangle is positive, and the end of the short bar is negative. In reality, some printed circuit symbols on the diode to mark the polarity; some printed a color circle as the negative mark on the negative end of the diode; some diodes have different shapes at both ends, the flathead is the positive pole, and the round head is the negative pole. Pay attention to identification during use.

anode and cathode of diodes

3. Crystal diodes have unidirectional conduction characteristics, allowing the only current to flow from the positive electrode to the negative electrode, but not allowing current to flow from the negative electrode to the positive electrode.

Unidirectional conductivity characteristics of diodes

4. Germanium diodes and silicon diodes have different forward tube voltage drops during forwarding conduction. The figure shows the volt-ampere characteristic curve of the germanium diode. When the applied forward voltage is greater than the voltage drop of the forward tube, the germanium diode is turned on. The forward voltage drop of the germanium diode is about 0.3V.

the volt-ampere characteristic curve of the germanium diode

5. The figure shows the volt-ampere characteristic curve of the silicon diode. When the applied forward voltage is greater than 0.7V, the silicon diode is turned on. In addition, at the same temperature, the reverse leakage current of silicon diodes is much smaller than that of germanium diodes. It can be seen from the above volt-ampere characteristic curve that the voltage and current of the diode have a nonlinear relationship, so the crystal diode is a nonlinear semiconductor device.

the volt-ampere characteristic curve of the silicon diode

IV How to test diodes?

1. Low power crystal diode

(1) Identify positive and negative electrodes

1) Observe the symbol mark on the shell. The diode symbol is usually marked on the outer shell of the diode, with one end with a triangular arrow as the anode and the other end as the cathode.

2) Observe the color point on the shell. In the case of the point-contact diode, it is usually marked with a polar color point (white or red). Generally, the end marked with a colored dot is the positive electrode. Other diodes are marked with a color ring, and the end with the color ring is negative.

3) Based on the one-time measurement with lower resistance, the end connected to the black test pen is positive, and the end connected to the red test pen is negative.

4) Observe the diode housing, with a silver band at one end as the negative pole.

(2) Detecting the highest reverse breakdown voltage. For alternating current, because of constant changes, the highest reverse working voltage is the peak AC voltage that the diode withstands.

2. Bidirectional trigger diode

Put the multimeter in the corresponding DC voltage block, and the test voltage is provided by the megohmmeter.

During the test, shake the megohmmeter to measure the VBR value in the same way. Finally, compare VBO and VBR. The smaller the difference between the absolute values of the two, the better the symmetry of the bidirectional trigger diode under test.

3. Transient voltage suppression diode

Use a multimeter to measure the quality of the diode. For unidirectional TVs, according to the method of measuring ordinary diodes, the positive and negative resistances can be measured. Generally, the forward resistance is about 4kΩ, and the reverse resistance is infinite. For the bidirectional pole transient voltage suppression diode, the resistance value between the two pins measured by any red and black test leads should be infinite, otherwise, it means that the tube is poor or damaged.

4. High-frequency varistor diode

The difference between the high-frequency varistor diodes and ordinary diodes is that their color codes are different. The color code of ordinary diodes is generally black, and the color code of high-frequency varistor diodes is lighter. The polarity rule is similar to that of ordinary diodes, that is, the end with a green ring is negative and the end without a green ring is positive.

5. Varactor diode

Switch the red and black test leads of the multimeter to measure the varactor diode. The resistance between the two pins of the varactor diode should be infinite. If during the measurement, it is found that the pointer of the multimeter slightly swings to the right or the resistance value is zero, it means that the tested varactor diode has a leakage fault or has broken down.

6. Monochromatic LED

Attach an energy-saving 1.5V dry battery outside the multimeter, and set the multimeter to R × 10 or R × 100. This connection is equivalent to giving the multimeter a voltage of 1.5V in series and increasing the detection voltage to 3V (the turn-on voltage of the light-emitting diode is 2V). When testing, use a multimeter to alternately touch the two pins of the LED. If the tube performance is good, it must be able to emit light normally at this time. At this time, the black test pen is connected to the positive electrode and the red test pen is connected to the negative electrode.

7. Infrared LED

(1) Identify the positive and negative electrodes of the infrared LED. Infrared light-emitting diodes have two pins, usually, the long pin is positive and the short pin is negative. Because the infrared light-emitting diode is transparent, the electrodes in the case are clearly visible. The wider and larger one of the internal electrodes is the negative electrode, and the narrower and smaller one is the positive electrode.  

(2) First, measure the forward and reverse resistance of the red LEDs. Normally, the forward resistance should be around 30k, and the reverse resistance should be above 500k so that the tube can be used normally.

8. Infrared receiving diode

Identify the polarity of the pins

1) Identify from the appearance. The appearance of common infrared receiving diodes is black. When identifying the pin, facing the light-receiving window, the left is positive and the right negative, respectively. In addition, there is a small chamfered plane on the top of the tube body of the infrared receiving diode. Usually, the pin with one end of this chamfered plane is the negative electrode, and the other end is the positive electrode.

2) First, use a multimeter to distinguish the positive and negative electrodes of the ordinary diode for inspection, that is, exchange the red and black test leads to measure the resistance between the two pins of the tube twice. Normally, the resistance value should be one large and one small. Based on the one with the lower resistance, the pinning step connected to the red test pen is negative, and the pin connected to the black test pen is positive.

(2) To test the performance of an infrared receiving diode. Use a multimeter to electrically measure the forward and reverse resistance of the infrared receiving diode. According to the value of the forward and reverse resistance, you can initially determine the quality of the infrared receiving diode.  

9. Laser diode

According to the method of testing the forward and reverse resistance of the ordinary diode, the pin arrangement order of the laser diode can be determined. However, the forward voltage drop of the laser diode is larger than that of the ordinary diode, so when detecting the forward resistance, the multimeter pointer deflects slightly to the right.

V Diode Functions

1. One of the main functions of crystal diodes is detection. The figure shows the superheterodyne radio detection circuit. The amplitude-modulated wave output from the second intermediate amplifier is applied to the cathode of the diode VD. Its negative half cycle passes through the diode, while the positive half cycle is cut off, and the high-frequency components are filtered by the RC filter. The output is the audio signal modulated on the carrier wave. This process is called detection.

Diode detector

2. Another function of diodes is rectification. The figure shows the rectified power supply circuit. Due to the unidirectional conduction of the diode, when the diode VD is turned on during the positive half cycle of the AC voltage, it has an output. When the diode VD is turned off, there is no output during the negative half cycle of the AC voltage. The pulsating voltage rectified by the diode VD is DC voltage after RC filtering.

Diode rectifier

3. A full-bridge rectifier is usually referred to as a full-bridge. It is a combined device of rectifier diodes. It has shapes of rectangular, circular, flat, square, etc., and has a variety of voltage, current, and power specifications.

Full bridge

4. The text symbol of the full-bridge rectifier is "UR". The full-bridge rectifier stack contains four rectifier diodes, which are connected according to certain rules. As shown in the figure on the right, it has two AC input terminals (~) and DC positive (+) and negative (-) pole output terminals.

Inside the full-bridge rectifier

5. The full-bridge rectifier stack is mainly used for bridge-type full-wave rectifier circuits. When the AC voltage U is positive for half a cycle, the current I is forming a loop through VD2 and the load R and VD3, and the voltage UR on the load is positive and negative. When U is negative for half a cycle, the current I is negative through VD4, R, and VD1 to form a loop. The voltage UR on the load is still positive and negative, achieving full-wave rectification. Using a full-bridge rectifier stack can simplify the structure of the rectifier circuit.

Bridge type full wave rectifier circuit

6. The figure below shows the volt-ampere characteristic curve of the Zener diode. It can be seen that the Zener diode works after the reverse breakdown of the PN junction, and its terminal voltage remains basically unchanged within a certain range. As long as the reverse current does not exceed its maximum operating current IZM, the Zener diode will not be damaged.

Zener diode volt-ampere curve

7. Voltage stabilizing diodes with a voltage stabilization value below 15V can be measured with a multimeter "R × 10k" block (containing a 15V high-voltage battery). When reading, the left end of the scale line is 15V, and the right end is 0. The original 50V scale of the multimeter can be used for reading, and substituted into the following formula to obtain: the voltage regulation value is (50-X) / 50 · 15V, where X is the number on the 50V block scale line.

Multimeter test Zener diode

8. The function of the voltage stabilizing diode is voltage stabilization. The figure shows a parallel voltage stabilizing circuit. The voltage on the voltage stabilizing diode VD is the output voltage.

a parallel voltage stabilizing circuit

9. The three-pin Zener tube is a Zener diode with temperature compensation, and its casing contains two back-to-back Zener diodes connected in series; its shape is the same as that of the crystal triode, with 3 pins: pin①and pin② are the negative poles of the two voltage stabilizing diodes respectively. Because they are symmetrical, they can be interchanged at will. When in use, one is connected to the positive pole of the power supply and the other is grounded; Three-pin voltage regulators are mainly used in precision voltage regulator circuits that require high-temperature stability.

Three-pin Zener diode

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