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How to use the dummy load switch power supply circuit for repair?

by:Fuyuang     2020-05-25
Control of switch power supply circuit in the switch tube, opened and shut off time, it can continue to the stability of the output voltage of the circuit. In recent years is developing a more mature technology. Dummy load is to point to in a circuit or circuit, the output of the mouth can accept electrical components called dummy load, dummy load in the middle of switching power supply and the ability of error detection circuit, so how to use the dummy load to check? Electronics will focus on this problem. When a short circuit of switch power supply load, will make the output voltage is reduced, in the same open load or no-load output voltage will rise. Commonly used in repair dummy load replacing method, is to distinguish the power part defective or malfunction with the load circuit. About the selection of dummy load, generally choose the 40 w or 60 w bulb false load, convenient advantage is intuitive, according to whether the bulb light and the brightness of the light source to see if there is the output voltage and output voltage. Compared with the advantages, disadvantages are also very obvious, mainly reflects on the problem of resistance, for example, 60 w bulb its thermal resistance 500 q, and the cold resistance is only 50 & Omega; The left and right sides. Assuming that power the main voltage output is 100 v, when use 60 w bulb false load, power source current of 200 ma at work, but at the start of the load current is up to 2 a, the United States was 10 times higher than the normal working current, therefore, bulb with a false load, easy to make the start-up difficulties, because the lamp power, the greater the cold resistance is smaller, therefore, high power bulb starting current is bigger, the start-up is more difficult. When calculating the starting current and working current of the power, can make use of the formula I = U/R calculated: power load current when starting for 100 v / 50 & Omega; = 2 a, the power supply work when load current is 100 v / 500 & Omega; = 0. 2A。 But it is important to note: the above for the theoretical calculation and actual may have come in and go out. In order to reduce the starting current, soldering iron false load (50 w can be used Cold and hot state resistance are 900 & Omega; ) Or 50 w / 300 & Omega; Resistance, it is more accurate than using 60 w bulb. Some power supply can be directly pick up dummy load, some of the power supply is not ok, specific to the analysis, the following according to the explanation of three kinds of situation. The first category: line pulse synchronous switching power supply, can break operation load directly the dummy load. This kind of switch power supply is autonomous switching power supply, the switch tube base introducing positive line flyback pulse's purpose is to make the switch tube line excited oscillation and pulse synchronization, the switch power supply of pulse radiation Angle of screen article interference limits in line scan flyback, thus no interference on the screen. Add on the switch tube the base line of pulse, just make the switch tube in ahead of the deadline conduction, basic does not constitute a auxiliary incentive function, so called pulse synchronous switching power supply. To judge whether belong to this kind of power supply, fault operation flyback pulse switch power only appear when cry ( Due to the lower oscillation frequency) And the output voltage does not drop. Therefore, the power supply can disconnect the line scan circuit, maintenance by the dummy load method. The second category: he dared type of switch power supply. For no conduct synchronized separately excited type pulse generator can break operation load directly the dummy load. For a row lock pulse frequency and indirect sampling separately excited type switching power supply, when answering directly dummy load ( Especially after power larger bulbs 150 w) , the output voltage may fall more or no output, because this kind of power supply, while the line and lock the frequency of synchronization pulse to join only, and not participate in the oscillation, but line sync pulse can make the switch tube conduction time in advance, then the strongest power load capacity, if the broken line loads, line sync pulse is useless, the power load capacity will be lower, and low power supply voltage sensitivity of indirect sampling, output voltage must be reduced. But if such power regulating circuit adopts direct sampling ( Sampling the voltage from the switch transformer secondary) , is due to the voltage stability sensitivity is higher, but to take off the operation load and direct answer the dummy load even light for maintenance. Third class: line auxiliary pulse excitation switch power supply. This kind of switch power supply line flyback pulse, not only complete the switch power source excited oscillation frequency synchronization, and constitute the switch tube is an indispensable part of the feedback network. This kind of switch power supply work process is: after startup switch tube produce self-excited oscillation, the feedback network under rated load can only make the output end produce 40% below normal output voltage, the voltage starting line scan, line by pulse feedback to switch tube to aid incentives, to achieve the rated voltage output. Doing so has two purposes: one is a step-down protection function, once the line scan circuit fault, open circuit and short circuit, switching power supply output voltage is reduced to 60% of the original value, narrow the area of damage. Second, the power supply and line scan has soft start-up process of a very short time, reduce the failure rate of power supply and line scan. If this kind of power supply, remove the feedback pulse circuit, the power supply output voltage by 40% ~ 60%, and even the output voltage is very low. Obviously, this power cannot directly on a false load disconnect line scan method to repair, because even if the power supply circuit is normal, it is impossible to output voltage rating. Distinguish between power supply and fault line scan method is to use the power converter separately to the line scan circuit of power supply, if the line scan circuit works well, bad switch power supply.
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