Techniques for testing frequency converters
I. Resistor Testing Methods and Experience
1. Testing of fixed resistors.
A. Connect the two probes (positive or negative) to the two leads of the resistor to measure its actual resistance. To improve measurement accuracy, the range should be selected according to the nominal value of the resistor being measured. Due to the non-linear relationship of the ohmmeter scale, its middle section has a finer graduation. Therefore, the pointer reading should be placed as close as possible to the middle of the scale, i.e., within 20% to 80% of the initial radian of the full scale, for more accurate measurement. Depending on the resistance tolerance class, an error of ±5%, ±10%, or ±20% is allowed between the reading and the nominal resistance value. If the reading does not match and exceeds the error range, it indicates that the resistance value has changed.
B. Note: When testing, especially when measuring resistors with resistance values of tens of kΩ or higher, do not touch the test leads or the conductive parts of the resistor with your hands; the resistor being tested should be desoldered from the circuit, and at least one end should be unsoldered to avoid other components in the circuit affecting the test and causing measurement errors; although the resistance value of a color-coded resistor can be determined by the color band markings, it is best to use a multimeter to test its actual resistance value when using it.
2. Testing the resistance of cement. The methods and precautions for testing the resistance of cement are exactly the same as those for testing ordinary fixed resistors.
3. Testing Fusible Resistors. When a fusible resistor blows through a circuit, a judgment can be made based on experience: if the surface of the fusible resistor is blackened or charred, it indicates that it was overloaded, with the current flowing through it exceeding its rated value by many times; if there are no marks on its surface and it is open-circuited, it indicates that the current flowing through it is exactly equal to or greater than its rated fusing value. For fusible resistors with no marks on their surface, the condition can be determined using a multimeter on the R×1 range. To ensure accurate measurement, one end of the fusible resistor should be unsoldered from the circuit. If the measured resistance is infinite, it indicates that the fusible resistor has failed and is open-circuited. If the measured resistance differs significantly from the nominal value, it indicates that the resistance has changed and it should not be used. In repair practice, it has been found that a few fusible resistors are also found to be short-circuited in the circuit; this should also be noted during testing.
4. Potentiometer Testing. When checking a potentiometer, first rotate the knob to see if the rotation is smooth, the switch is flexible, and the "click" sound when the switch is on and off is crisp. Listen for the friction sound between the internal contacts and the resistive element; a "rustling" sound indicates poor quality. When testing with a multimeter, first select the appropriate resistance range based on the potentiometer's resistance value, and then proceed with the testing as described below.
A. Use the ohmmeter function of a multimeter to measure the resistance between terminals "1" and "2". The reading should be the nominal resistance value of the potentiometer. If the multimeter pointer does not move or the resistance value is significantly different, it indicates that the potentiometer is damaged.
B. Check the contact between the potentiometer's movable arm and the resistive element. Use the ohmmeter function of a multimeter to measure terminals "1" and "2" ("2" and "3"). Rotate the potentiometer's shaft counterclockwise to near the "off" position; the lower the resistance value, the better. Then slowly rotate the shaft clockwise; the resistance value should gradually increase, and the pointer on the multimeter should move smoothly. When the shaft is rotated to the extreme position "3," the resistance value should be close to the potentiometer's nominal value. If the multimeter pointer jumps during the rotation of the potentiometer's shaft, it indicates a poor contact at the moving contact point.
5. Testing of Positive Temperature Coefficient (PTC) Thermistors. When testing, use a multimeter on the R×1 range. The process can be divided into two steps: A) Room temperature test (room temperature close to 25℃): Contact the two leads of the PTC thermistor with the multimeter probes and measure its actual resistance. Compare this resistance to the nominal resistance. A difference of ±2Ω is considered normal. If the actual resistance differs significantly from the nominal resistance, it indicates poor performance or damage. B) Heated test: After the room temperature test is normal, perform the second step – the heated test. Place a heat source (e.g., a soldering iron) close to the PTC thermistor to heat it. Simultaneously, use the multimeter to monitor whether the resistance increases with temperature. If it does, the thermistor is normal. If the resistance does not change, its performance has deteriorated and it should not be used. Be careful not to let the heat source get too close to the PTC thermistor or allow it to directly contact the thermistor to prevent damage.
6. Detection of negative temperature coefficient thermistors (NTCs).
(1) Measuring the nominal resistance value Rt: The method for measuring an NTC thermistor with a multimeter is the same as that for measuring a regular fixed resistor. That is, select an appropriate resistance range according to the nominal resistance value of the NTC thermistor to directly measure the actual value of Rt. However, because NTC thermistors are very sensitive to temperature, the following points should be noted during testing:
A. Rt is measured by the manufacturer at an ambient temperature of 25°C. Therefore, when measuring Rt with a multimeter, the measurement should also be performed at an ambient temperature close to 25°C to ensure the reliability of the test.
B. The measured power must not exceed the specified value to avoid measurement errors caused by the thermal effect of the current.
C. Note the correct operation. During the test, do not pinch it with your hands to prevent body temperature from affecting the test.
(2) Estimating the temperature coefficient at
First, measure the resistance value Rt1 at room temperature t1. Then, use a soldering iron as a heat source and bring it close to the thermistor Rt to measure the resistance value RT2. At the same time, use a thermometer to measure the average temperature t2 on the surface of the thermistor RT and then perform calculations.
7. Testing the varistor. Use a multimeter set to the R×1k range to measure the forward and reverse insulation resistance between the two leads of the varistor. Both readings should be infinite; otherwise, it indicates a large leakage current. If the measured resistance is very low, the varistor is damaged and cannot be used.
8. Detection of photoresistors.
A. Cover the light-transmitting window of the photoresistor with a piece of black paper. The multimeter pointer will remain almost stationary, and the resistance will be close to infinity. A higher value indicates better photoresistor performance. If this value is very small or close to zero, the photoresistor is burnt out and damaged, and should not be used.
B. Point a light source at the light-transmitting window of the photoresistor. The multimeter pointer should then show a significant swing, indicating a noticeable decrease in resistance. A smaller value indicates better photoresistor performance. If this value is very large or even infinite, it indicates an internal open circuit and damage to the photoresistor, rendering it unusable.
C. Point the light-transmitting window of the photoresistor towards the incident light. Use a small piece of black paper to wave around the top of the photoresistor's light-shielding window, intermittently exposing it to light. The multimeter needle should swing left and right as the paper moves. If the multimeter needle remains stationary and does not move with the paper, the photosensitive material of the photoresistor is damaged.
II. Capacitor Testing Methods and Experience
Capacitors are commonly marked directly, with the most common units being pF and μF, which are easy to recognize. However, some small-capacity capacitors use a numerical marking system, typically with three digits. The first two digits are significant figures, and the third digit indicates the multiple, i.e., how many zeros follow. For example, 343 represents 34000pF. Also, if the third digit is 9, it represents 10⁻¹, not 10 to the power of 9; for example, 479 represents 4.7pF.
When replacing capacitors, the main thing to note is that the capacitor's voltage rating should generally not be lower than the original capacitor's voltage rating. In circuits with stricter requirements, the capacitance should generally not exceed ±20% of the original capacitance. In circuits with less stringent requirements, such as bypass circuits, the capacitance should generally not be less than 1/2 of the original capacitance and not more than 2 to 6 times the original capacitance.
1. Testing of fixed capacitors
A. Testing small capacitors below 10pF: Because fixed capacitors below 10pF have such small capacitance, using a multimeter can only qualitatively check for leakage, internal short circuits, or breakdown. During measurement, select the R×10k range on the multimeter. Connect the two probes to any two leads of the capacitor; the resistance should be infinite. If the measured resistance (the pointer swings to the right) is zero, it indicates that the capacitor is leaking, damaged, or internally broken down.
B. Check if a 10pF~1000μF fixed capacitor is charging to determine its condition. Use the R×1k range on the multimeter. Both transistors should have a β value of 100 or higher and low leakage current; a 3DG6 or similar silicon transistor can be used to form a composite transistor. Connect the red and black probes of the multimeter to the emitter (e) and collector (c) of the composite transistor, respectively. Due to the amplification effect of the composite transistor, the charging and discharging process of the capacitor under test is amplified, increasing the amplitude of the multimeter pointer swing, making it easier to observe. Note that during testing, especially with smaller capacitors, the leads of the capacitor under test should be repeatedly switched between points A and B to clearly observe the multimeter pointer swing.
C. For fixed capacitors above 1000μF, the R×10k range of a multimeter can be used to directly test whether the capacitor is charging or has an internal short circuit or leakage. The capacitance can be estimated based on the magnitude of the pointer swing to the right.
2. Testing of electrolytic capacitors
A. Because electrolytic capacitors have a much larger capacitance than regular fixed capacitors, the appropriate measurement range should be selected for different capacitance values. Based on experience, capacitors ranging from 1 to 47 μF can generally be measured using the R×100 range.
B. Connect the red probe of the multimeter to the negative terminal and the black probe to the positive terminal. At the instant of contact, the multimeter needle will deflect significantly to the right (for the same resistance range, the larger the capacitance, the larger the deflection), then gradually return to the left until it stops at a certain position. The resistance value at this point is the forward resistance of the electrolytic capacitor, which is slightly greater than the reverse leakage resistance. Practical experience shows that the leakage resistance of an electrolytic capacitor should generally be several hundred kΩ or higher; otherwise, it will not function properly. During testing, if there is no charging in either the forward or reverse direction (i.e., the needle does not move), it indicates that the capacitance has disappeared or there is an internal open circuit. If the measured resistance is very small or zero, it indicates that the capacitor has a large leakage current or has broken down and is no longer usable.
C. For electrolytic capacitors with unclear positive and negative markings, the above method of measuring leakage resistance can be used to determine them. First, measure the leakage resistance arbitrarily and remember its value. Then, reverse the probes and measure another resistance value. The larger resistance value between the two measurements indicates the positive connection, meaning the black probe is connected to the positive terminal and the red probe to the negative terminal.
D. Using a multimeter in resistance mode, the capacitance of the electrolytic capacitor can be estimated by charging it in both forward and reverse directions and observing the magnitude of the pointer's rightward swing.
3. Detection of variable capacitors.
A. When gently rotating the shaft, it should feel very smooth, without any intermittent tightness or jamming. When pushing the load shaft forward, backward, up, down, left, and right, the shaft should not feel loose.
B. Rotate the shaft with one hand and gently touch the outer edge of the moving plate assembly with the other hand. There should be no looseness. A variable capacitor with poor contact between the shaft and the moving plate should not be used.
C. Set the multimeter to the R×10K range. With one hand, connect the two probes to the leads of the moving and fixed plates of the variable capacitor, respectively. With the other hand, slowly rotate the shaft several times. The multimeter pointer should remain at infinity throughout the rotation. If the pointer sometimes points to zero during the rotation, it indicates a short circuit between the moving and fixed plates. If, at a certain angle, the multimeter reading is not infinite but shows a certain resistance, it indicates leakage between the moving and fixed plates of the variable capacitor.
III. Transistor Testing and Experience
Transistors in circuits mainly include crystal diodes, crystal triodes, silicon controlled rectifiers (SCRs), and field-effect transistors (FETs), among which triodes and diodes are the most commonly used. Knowing how to correctly judge the quality of diodes and triodes is one of the key points in learning repair.
1. Crystal Diode: First, we need to know whether the diode is silicon or germanium. The forward voltage drop of a germanium diode is typically between 0.1V and 0.3V, while that of a silicon diode is typically between 0.6V and 0.7V. The measurement method is as follows: Use two multimeters. While measuring the forward resistance with one multimeter, simultaneously measure the diode's voltage drop with the other. Finally, the voltage drop value can be used to determine whether it is a germanium or silicon diode. Silicon diodes can be measured using the R×1K range of the multimeter, and germanium diodes can be measured using the R×100 range. Generally, the greater the difference between the forward and reverse resistances of the measured diode, the better. Generally, if the forward resistance is several hundred to several thousand ohms and the reverse resistance is several tens of thousands of ohms or more, it can be preliminarily determined that the diode is good. At the same time, the positive and negative terminals of the diode can be determined. When the measured resistance is several hundred or several thousand ohms, this is the forward resistance of the diode. The negative probe is connected to the negative terminal, and the positive probe is connected to the positive terminal. Additionally, if the forward and reverse resistances are infinite, it indicates an internal open circuit; if the forward and reverse resistances are the same, the diode also has a problem; if both forward and reverse resistances are zero, it indicates a short circuit.
2. Transistor: Transistors primarily function as amplifiers. How do we determine a transistor's amplification capability? The method is as follows: Set the multimeter to the R×100 or R×1K range. When testing an NPN transistor, connect the positive probe to the emitter and the negative probe to the collector. The measured resistance should generally be several thousand ohms or higher. Then, connect a 100-kiloohm resistor in series between the base and collector. The multimeter reading should decrease significantly. The greater the change, the stronger the transistor's amplification capability. If the change is small or nonexistent, the transistor has no amplification capability or a very weak amplification capability.
Electrode identification method
For germanium transistors, use the R×100 range; for silicon transistors, use the R×1K range. First, fix the red probe in contact with any one pin, and then use the black probe to measure the other two pins. See if you can find two small resistors. If not, move the red probe to other pins and continue measuring until you find two small resistors. If you still can't find two small resistors with the red probe fixed, you can fix the black probe and continue searching.
Once two small resistors are found, the probe used to fix the transistor is the base. If the fixed probe is black, the transistor is an NPN type; if the fixed probe is red, the transistor is a PNP type.
A. Method for determining the resistance of the ce electrode
Use a multimeter to measure the resistance between the two terminals excluding the base. Measure twice, swapping the probes. For a germanium transistor, the lower resistance reading is accurate. For a PNP transistor, the terminal connected to the black probe is the emitter, and the terminal connected to the red probe is the collector. For an NPN transistor, the terminal connected to the black probe is the collector, and the terminal connected to the red probe is the emitter. For a silicon transistor, the higher resistance reading is accurate. For a PNP transistor, the terminal connected to the black probe is the emitter, and the terminal connected to the red probe is the collector. For an NPN transistor, the terminal connected to the black probe is the collector, and the terminal connected to the red probe is the emitter.
B, P-junction forward resistance method
Measure the forward resistance of the two PN junctions respectively; the larger one is the emitter resistance, and the smaller one is the collector resistance.
C. Magnification factor method
With a multimeter, connect the two probes to the base terminal (excluding the two outer leads). For a PNP multimeter, touch the base terminal with your finger and observe the needle's movement on the lead connected to the red probe. Repeat this process, swapping the probes and measuring once more. The terminal with the larger needle movement is the reference. In this case, the terminal connected to the red probe is the collector. For an N-N multimeter, touch the base terminal with your finger and the red probe. Observe the needle movement on the lead, swapping the probes and measuring once more. The terminal with the larger needle movement is the reference. In this case, the terminal connected to the red probe is the collector. Note: The difference between analog and digital multimeters is that in analog multimeters, the red probe is connected to the negative terminal of the source, while in digital multimeters, the opposite is true.
IV. Inductor and Transformer Testing Methods and Experience
1. Testing Color-Coded Inductors: Set the multimeter to the R×1 range. Connect the red and black probes to the first and second leads of the color-coded inductor, respectively. The probe should then move to the right. Based on the measured resistance value, identify the inductor using the following methods:
A. The resistance of the color code inductor under test is zero, indicating a short circuit fault inside.
B. The DC resistance value of the color-coded inductor under test is directly related to the diameter of the enameled wire used to wind the inductor coil and the number of turns. As long as a resistance value can be obtained, the color-coded inductor under test is considered to be normal.
2. Inspection of intermediate frequency transformer eaves
A. Set the multimeter to the R×± range and check the continuity of each winding one by one according to the pin arrangement of the intermediate frequency transformer to determine whether it is normal.
B. Testing insulation performance of cymbals
Set the multimeter to the R port 10K range and perform the following condition tests:
(1) Resistance value between the primary winding and the secondary winding;
(2) The resistance between the primary winding and the casing;
(3) Resistance between the secondary winding and the casing.
The test results above fall into three categories:
(1) Infinite resistance: Normal;
(2) Zero resistance: There is a short circuit fault.
(3) The resistance is less than infinity, but it is difficult to detect: there is a leakage fault.
3. Testing and Experience of Power Transformers
4. Its tendency to cool down is mainly due to internal short circuits. This can be determined by checking the power supply voltage with a multimeter. When the insulation performance of the multi-line output transformer deteriorates or there is a partial short circuit between turns, the horizontal scanning current will surge, causing the switching power supply output voltage to drop. Therefore, measuring the voltage across the resistor can determine if the horizontal output transformer is short-circuited.
5. (1) Check the appearance of the frequency converter to see if there are any obvious abnormalities. For example, whether the coil leads are broken or loose, whether the insulation material has burn marks, whether the iron core fastening screws are loose, whether the silicon steel sheets are rusted, and whether the winding coils are exposed.
(2) Insulation test. Use a multimeter set to the R×10K range to measure the resistance between the core and the primary winding, between the primary winding and each secondary winding, between the core and each secondary winding, between the electrostatic shielding layer and the secondary winding, and between each secondary winding. The multimeter pointer should remain at infinity in all cases. Otherwise, it indicates poor transformer insulation performance.
(3) Detection of coil continuity. Set the multimeter to the R×1 range. During the test, if the resistance of a certain winding is infinite, it indicates that there is an open circuit fault in this winding.
(4) Identify the primary and secondary coils. The primary and secondary leads of a power transformer are usually led out from the two sides respectively. The primary winding is often marked with "200V", while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Identify the coils based on these markings.
(5) Detection of no-load current.
a. DC Measurement Method. With all secondary windings open, set the multimeter to AC current range (500mA) and connect it in series with the primary winding. When the primary winding is plugged into a 220V AC mains power supply, the multimeter reading will be the no-load current value. This value should not exceed 10% to 20% of the transformer's full-load current. The normal no-load current of a typical electronic equipment power transformer should be around 100mA. If it exceeds this value significantly, it indicates a short-circuit fault in the transformer.
b. Indirect measurement method. Connect a 10/5W resistor in series with the primary winding of the transformer, keeping the primary winding completely unloaded. Set the multimeter to AC voltage mode. After powering on, measure the voltage U across the resistor R using the two probes, and then calculate the no-load current I<sub>no-load</sub> using Ohm's law, i.e., I<sub>no-load</sub> = U/R.
(6) No-load voltage test. Connect the primary winding of the power transformer to 200V mains power. Use a multimeter connected to AC voltage to measure the no-load voltage values of each winding (U21, U22, U23, U24). The values should meet the requirements. The allowable error range is generally: high voltage winding ≤ ±10%, low voltage winding ≤ ±5%, and the voltage difference between the two sets of symmetrical windings with center taps should be ≤ ±2%.
(7) The allowable temperature rise of a small power transformer is 40℃ to 50℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
(8) Detecting and identifying the corresponding terminals of each winding. When using a power transformer, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When using a power transformer in series, the corresponding terminals of each winding participating in the series connection must be connected correctly and cannot be mistaken. Otherwise, the transformer will not work properly. Differences in the comprehensive detection of short-circuit faults in power transformers. The main symptoms of a short-circuit fault in a power transformer are severe overheating and abnormal output voltage of the secondary winding. Generally, the more inter-turn short-circuit points inside the coil, the greater the short-circuit current, and the more severe the transformer overheating. A simple way to detect and determine whether a power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced above). The no-load current value of a transformer with a short-circuit fault will be much greater than 1% of the full-load current. When the short circuit is severe, the transformer will heat up rapidly within tens of seconds after being energized under no-load conditions, and the iron core will feel hot to the touch. At this time, it is not necessary to measure the no-load current to determine that there is a short circuit point in the transformer.
V. Integrated Circuit Blocks
To determine the quality of an inverter's integrated circuit (IC), a multimeter can be used to measure the voltage to ground, resistance to ground, and operating current of each pin of the IC. Alternatively, the IC can be removed, and the resistance between each pin and ground can be measured. While removing the IC, the resistance to ground of each pin in the external circuit can also be measured. It is crucial to pay close attention to soldering quality and time when replacing ICs. Generally, it is recommended to use ICs of the same model and specifications for replacement. If the original model and specifications of the IC cannot be found, an IC with similar functionality can be used as a substitute. However, it is essential to ensure that the power supply voltage, impedance matching, pin positions, and external control circuitry are correctly identified during the replacement process.
