How to distinguish and detect crystal diodes and ordinary diodes

Crystal diodes are abbreviated as diodes, which are made of semiconductor materials like transistors. The so-called semiconductor refers to a class of substances whose electrical properties are between conductors and insulators. Commonly used semiconductor materials are silicon and germanium. We often hear about Silicon Valley in the United States because there were a lot of semiconductor manufacturers there.

Semiconductor materials have two significant characteristics: First, the conductivity is greatly affected by the amount of impurities. For example, if silicon is doped with one part per million of boron, the conductivity can be increased by more than 500,000 times; the second is the conductivity It is greatly affected by external conditions, such as changes in temperature and light, which will significantly change its resistivity. Using these characteristics, semiconductor devices with a wide range of uses, various characteristics, and different functions can be manufactured.

Since most semiconductors are crystals, semiconductor materials are often called crystals. This is how the names of crystal diodes and transistors are derived. There are many types of crystal diodes. Commonly used are ordinary diodes (used for rectification, detection, switching, etc.) and diodes with special properties (such as light-emitting diodes, Zener diodes, photodiodes, etc.). This article first introduces ordinary diodes which are very versatile.

Ordinary diode identification

In the large family of semiconductor devices, the diode is an early member. In the field of modern electronic technology, it still plays a very important role. It is very important for beginners to correctly grasp the identification and use of diodes for the smooth completion of various electronic production and maintenance.

1. Basic structure and characteristics

Semiconductor materials are divided into P-type semiconductors and N-type semiconductors according to their different conductivity types. If a small piece of semiconductor material is made into P-type on one side and N-type on the other side, a PN junction is formed at the junction of them, as shown in Figure 1. Simply put, a PN junction with leads is encapsulated in a glass tube, plastic body or metal casing to form a diode.

The crystal diode has two electrode leads, one is the positive electrode (connected to the internal P-type semiconductor material), and the other is the negative electrode (connected to the internal N-type semiconductor material). Unidirectional conductivity is the basic characteristic of diodes. We connect the battery G, the small bulb H, and the diode in series to form the circuit shown in Figure 2. In the figure (a), the anode of the battery is connected to the anode of the diode, and the cathode of the battery is connected to the cathode of the diode through a small light bulb. At this time, the forward voltage is applied to the diode, and the light bulb glows. In the figure (b), the positive and negative leads of the diode are reversed, and the reverse voltage is applied to the diode, so the small light bulb cannot emit light. The resistance of the diode is very small when the forward voltage is applied, and it can conduct well. When the reverse voltage is applied, the resistance is very large, and it is close to the open-circuit cut-off. This is its unidirectional conductivity. This characteristic can also be understood as: in the circuit, the diode only allows current to flow from its positive pole to its negative pole, and is not allowed to flow in the reverse direction.

The crystal diode detects radio waves in the radio, converts AC power into pulsating DC power in the power conversion circuit, and acts as a non-contact switch in digital circuits, all of which utilize its unidirectional conductivity characteristics.

2. Appearance and type

Ordinary diodes can be divided into germanium diodes and silicon diodes according to the different semiconductor materials used; according to the different die structures, they can be divided into point contact diodes, surface contact diodes and planar diodes as shown in Figure 4; according to different tube uses , Can be divided into rectifier diodes, detector diodes, switching diodes and so on.

The point contact diode is a thin metal contact wire pressed on the smooth semiconductor surface, and a strong pulse current is applied to firmly sinter one end of the contact wire and the semiconductor to form a PN junction, as shown in Figure 4(a) Shown. Because the contact surface of the contact wire and the semiconductor is very small, the point contact diode only allows a small current (below tens of milliamperes) to pass, but it has good performance at high frequencies and is suitable for the detection and detection of high-frequency signals in radios. Rectification of weak alternating current. Domestic germanium diodes 2AP series and 2AK series are all point contact type.

The PN junction area of the 　　 surface contact type diode is larger and made into a plane shape, as shown in Figure 4(b). It can pass a larger current and is suitable for rectifying the alternating current of the grid. Most of the 2CP series and 2CZ series diodes made in China are of surface contact type.

The characteristic of the silicon planar diode is that the surface of the PN junction is covered with a silicon dioxide film, which prevents the surface of the PN junction from being contaminated by water molecules, gas molecules and other ions, as shown in Figure 4(c). The characteristics of this diode are relatively stable and reliable, and are mostly used in switching, pulse and ultra-high frequency circuits. Domestic 2CK series diodes belong to this type.

3. Basic parameters

There are many parameters of crystal diodes, the main parameters of commonly used detectors and rectifier diodes are as follows:

①Rectified current (IFM). This refers to the average current allowed to pass through the PN junction in the forward direction when the diode works continuously for a long time. The rectified current is also called the rated forward working current. In use, the actual working current should be less than the parameter of the diode, otherwise the diode will be damaged. For example, the rectification current of the commonly used 2AP9 type germanium detector diode is 5mA, and the rectification current of the 1N4001 and 1N4007 silicon rectifier diodes is 1A.

②Reverse working voltage (URM). This refers to the voltage that is applied across the diode in the reverse direction without causing PN junction breakdown. In use, a diode whose URM is more than twice the actual working voltage should be selected. If the peak value of the actual working voltage exceeds this parameter, the diode is in danger of breakdown. For example, the reverse working voltage of the commonly used 2AP9 type germanium detector diode is 15V, the reverse working voltage of the 1N4001 silicon rectifier diode is 50V, and the reverse working voltage of the 1N4007 silicon rectifier diode is 1000V.

③ Forward voltage drop (UF). Refers to the forward voltage drop generated at both ends of the diode when it is turned on. The lower the forward voltage of the diode is, the better under the specified forward current. For example, for a commonly used small germanium diode, this voltage is about 0.2V. The silicon tube is about 0.65V.

④Reverse current (IR). It refers to the current flowing through the diode under the specified temperature and reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube. Generally, the reverse current of a silicon diode is 10μA or less, and the reverse current of a germanium diode is about several hundred microamps.

⑤Working frequency (fM). Due to the influence of the capacitance between the PN junctions, the operating frequency that the diode can be applied to has an upper limit, and fM refers to the frequency at which the diode can work normally. When used for detection or high-frequency rectification, a diode with fM at least 2 times the actual operating frequency of the circuit should be selected, otherwise it will not work normally. For example, the commonly used 2AP9 type germanium detector diode has a working frequency of 100MHz, and the 1N4000 series silicon rectifier diode has a working frequency of 3kHz.

The lead of the tube should be as short as possible, and it is not possible to use a long lead or bend the lead into a circle to achieve the purpose of heat dissipation. The rectified current IFM refers to the half-wave average value under resistive or inductive load. If the rectifier diode is working in a capacitive load, the IFM should be reduced by 20%, otherwise the diode may be quickly damaged due to overcurrent heating.

④The crystal diode is generally not repairable after it is damaged, only a new tube can be replaced. When selecting and matching diodes, you should use the same type of diode as much as possible. If there is no diode of the same type to replace, you can try to choose a diode with the same or similar purpose for replacement, but it is required that the material and polarity of the replacement tube must be the same as The original pipe is the same, and the relevant parameter index shall not be lower than the original pipe. For example, the rectified current and reverse voltage limit parameters of the substitute rectifier diode must not be lower than the original tube, otherwise it may be burned or broken down. The working frequency of the substitute detector diode cannot be lower than the original tube, otherwise it will not work normally. Generally speaking, diodes with different materials and polarities are not suitable for each other. This is mainly because the voltage drop of the germanium tube is different from that of the silicon tube. If it is directly substituted, the circuit will not work normally, but the diodes with different uses can be flexibly adapted under the premise of complying with the above principles. For example, a high-frequency switch tube can be used to replace the detector tube, and a low-frequency switch tube can be used to replace the small current rectifier tube.

⑤Under amateur conditions, when there is no high-current rectifier diode on hand, as shown in Figure 8 (a), two smaller current rectifier diodes of the same model can be used in parallel. When there is no diode with high reverse voltage, it can be used. As shown in Figure 8(b), connect two diodes of the same model with a smaller reverse voltage in series. However, in the specific application circuit, you should also decide whether to add an equalization circuit (such as a current-sharing resistor or a voltage-sharing resistor) according to your needs to ensure the safe and reliable operation of the diode.

⑥Under amateur conditions, for a transistor whose collector or emitter pins are all broken, or a transistor with a PN junction damaged, it can be used as a diode as shown in Figure 9. Generally speaking, high-frequency low-power transistors can be used as detector diodes, and low-frequency high-power transistors can be used as rectifier diodes. But note that triodes whose base pins are all broken cannot be used as diodes.

4. Model naming rules

The model naming regulations of domestic crystal diodes consist of 5 parts (the fifth part is also omitted), such as 2AP9, 2CZ54F, etc. Among them: Digits represent diodes. The second place uses Chinese pinyin letters to indicate the material and polarity of the tube, such as A for germanium N type, B for germanium P type, C for silicon N type, and D for silicon P type. The third place uses Chinese pinyin letters to indicate the type of tube, such as P for ordinary tube (small signal tube), K for switching tube, V for mixing detector tube, W for voltage regulator tube, Z for rectifier tube, and L for rectifier stack , S is a tunnel tube, N is a damping tube, and U is a photosensitive tube. The fourth digit (number) and the fifth digit (Chinese pinyin letters) are product serial numbers and specifications, respectively, indicating the differences in parameters such as rectified current, reverse operating voltage, and operating frequency. For details, please refer to the relevant manuals.

The common crystal diode models originated from abroad are 1N4000 series, which are currently widely used in various electronic devices, almost replacing the national standard products. But in fact, not all of these diodes are imported, and most of them are made in China.

5. Shell marking method

Under normal circumstances, only the model and polarity are marked on the housing of crystal diodes, and its main parameters are not marked like resistors, capacitors, and inductors. If you want to understand the relevant parameters of diodes, you have to consult relevant manuals. The attached table lists the main parameters of crystal diodes frequently used by electronic hobbyists.

According to the shell mark or package shape of the crystal diode, the positive and negative polarity of the two pins can be distinguished. The pin identification method of common ordinary diodes is shown in Figure 5. Domestic diodes usually have the circuit symbol (see Figure 6) printed on the case and directly mark the pin polarity. Small plastic packaged diodes usually have a color ring printed on the negative side as a negative mark. Some diodes have different shapes at both ends, the pin at one end of the flat head is positive, and the pin at the round end is negative. To be proficient in the polarity of these flag pins is necessary for the correct use of diodes.

The use of ordinary diodes

1. Recognition in circuit diagram

The symbol of ordinary crystal diode in the circuit diagram is shown in Figure 6. The triangular arrow symbolizes the direction of the current, and the short straight line symbolizes the semiconductor material. We know that diodes have unidirectional conductivity. In a circuit, current can only flow into the diode from the anode and flow out of the diode from the cathode. The "+" and "-" polarities beside the diode symbol are added for the convenience of explaining the problem, and the actual circuit diagram is generally not added.

When looking at the circuit diagram, beginners often don't know which side of the diode symbol is positive and which side is negative. At this time, you might as well use the analogy method to distinguish: the symbol of the diode can be regarded as a funnel (the mouth is big and the bottom is small) , Water can only enter from the funnel with a large mouth and exit from a small mouth. The water flow is the current. The current enters and exits from the anode of the diode, so it is natural to remember that the triangle side of the symbol is the anode of the diode.

2. Detection method

With the help of the electric barrier of an ordinary multimeter, the quality of the crystal diode can be roughly judged, as shown in Figure 7. Set the multimeter to “R×100” or “R×1k”, connect the black test lead to the positive electrode of the tested diode and the red test lead to the negative electrode of the tested diode. Since the positive electrode of the battery in the multimeter is connected to the black test lead and the negative electrode is connected to the red test lead, So the reading indicated by the multimeter at this time is the forward resistance of the diode. The resistance reading is relatively small, generally 500~2000Ω for germanium diodes, and about 3kΩ for silicon diodes. According to the different resistance readings, we can also distinguish germanium diodes and silicon diodes. Then, swap the two test leads and measure the reverse resistance of the diode. The reading should be significantly larger, the germanium tube should be greater than a few hundred kilohms, while the silicon tube is close to infinity, and the pointer generally does not show deflection. This measurement shows that the diode is good. If the measured reverse resistance of the diode is very small, it means that the diode has lost its unidirectional conduction effect. If the forward and reverse resistances are both large, it means that the diode has an open circuit.

The above detection method can also be used to identify the positive and negative poles of the diode. When the test result is low resistance (forward resistance), the anode of the diode is connected to the black test lead of the multimeter, and the cathode of the diode is connected to the red test lead.

Ordinary DT830B and other digital multimeters are equipped with special gears for measuring crystal diodes, which can measure forward voltage drop and judge whether the tube is good or bad. The specific method is shown in Figure 7: First, turn the gear selector switch of the multimeter to Measure the "" position of the diode, insert the red test lead plug into the "VΩmA" jack and the black test lead plug into the "COM" jack. Then, connect the red test lead (note: the polarity is positive "+") to the positive electrode of the crystal diode, and the black test lead to the negative electrode of the crystal diode. At this time, the LCD directly displays the approximate value of the measured forward voltage drop of the crystal diode. Reverse the measurement and display "1", indicating that the diode is good. If both positive and negative measurements show "1", it means that the crystal diode has an open circuit.