5V power supplies are relatively complex overall circuits. Problems in many places will cause no voltage output. For example: fuse broken, rectifier diode damage, oscillation circuit does not work, output power tube burned, high-frequency transformer damage, high-frequency output rectifier burned, etc.
First, observe whether the power supply fuse is damaged.
If the fuse is damaged, there is no need to rush to replace it. You must check if there is has a short circuit in the power supply at first. Method: Use the resistance block of the multimeter to test the AC side behind the fuse（frist test）. Its average resistance should be more than tens of kilohms. If the resistance is zero, it means that the power supply AC side has a short circuit. In addition, we should also focus on checking whether the power supply AC filter capacitor is damaged. At the same time, if there is a pressure-sensitive resistor, should also check the resistor.
Second, the detection of the four rectifier diodes power supply
If the above test results are normal, we should then test the four rectifier diodes of the power supply. In the normal state, the forward resistance of the diode is some k (using a multimeter × 1k block test), the reverse resistance is close to infinity. Test results turn out to be abnormal. They need to be replaced. Down the line, the next thing to do is to test the power supply DC resistance of this part (Third test). Its average resistance is also in the number of k. If resistance to zero, it means that there is a DC side that has a short circuit.
There are many reasons for a short circuit on the DC side. Such as filter, capacitor short-circuits damage, power supply oscillation tube damage, oscillation integrated block, and the peripheral circuit part damage. All of these can cause a short circuit on the DC side. The point to note here is that before replacing the power supply oscillation tube, you must make sure that the oscillation block and peripheral circuits are normal. Otherwise, it will cause damage to the power supply tube again.
Third, the oscillation control circuit, a sampling feedback circuit, and load
After excluding the above problems of the switching power supply, most of the faults exist in the oscillation control circuit, the sampling feedback circuit, or the load.
Fourth, the oscillation integrated block power supply circuit
At this point, we should first check whether the power supply circuit of the oscillation block is usual (fourth test), and its average voltage should be around 10V. Reminder. Since the test voltage is to be performed under power-on conditions. And the power supply board has high voltage city electricity. Need to pay special attention to personal safety, do not try to touch any part of the power supply directly.
If there is no voltage or very low voltage at this point, first of all, check whether the buck resistor is damaged; secondly, check whether the oscillation integrated block and its external power supply circuit are regular. If no fault is found in the peripheral circuit, it is recommended to replace the integrated block. Of course, sometimes the power supply can not be standard output voltage due to the short load circuit to make the power supply protection caused.
Five，Check whether the output voltage is normal
We only need to unplug the output line for this power supply to check whether the output voltage is standard (test point five). You can determine the fault site. After the previous troubleshooting, the last should also check the feedback circuit part. Generally speaking, this part of the fault mainly concentrates on the photocoupler and amplifier circuit to pay special attention.
5v switching power supply standard maintenance methods
Check the large filter capacitor, rectifier bridge, diode, and switching tube on the 300V
The fuse burned mainly. Check the 300V on the large filter capacitor, rectifier bridge, diodes, switching tubes, and other parts. Anti-interference circuit problems can also lead to insurance burn, black. It is important to note that the fuse is burned due to the switch tube breakdown. Generally will also burn out the current detection resistor and power control chip. A negative temperature coefficient thermistor is also easy to be blown together with the fuse.
Measure the start pin of the power control chip
No output, fuse normally no output means that the switching power supply is not working or has entered the protection state. First of all, we should measure whether there is a start voltage on the start pin of the power supply control chip. If there is no start voltage or start voltage is too low, check the start resistor and start the foot’s external components, whether leakage. At this point, if the power control chip is standard, the above checks can quickly find the fault. If there is a start voltage, then measure the output of the control chip in the moment of power on whether there is a high, low-level jump. If there is no change, the control chip is terrible, peripheral oscillation circuit components or protection circuits have problems. Can first replace the control chip and then check the peripheral parts. If there is a change, generally speaking, the switch tube is impaired or damaged.
voltage sampling and voltage control circuit
The output voltage, but the output voltage is too high. This fault generally comes from the regulated sampling and voltage control circuit. In the DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip, and other circuits together form a closed control loop; any one of the problems will lead to a higher output voltage.
the output voltage is too low
In addition to the voltage regulator, the control circuit will cause the output voltage to below. There are also some of the following causes that can also cause low output voltage.
a.The switching power supply load has a short circuit fault (especially the DC/DC converter short circuit or poor performance, etc.). At this point, you should disconnect all the loads of the switching power supply circuit to distinguish whether the switching power supply circuit or the load circuit is faulty. If the voltage output of the disconnected load circuit is average, it means that the load is too heavy; or still, not normal means that the switching power supply circuit is faulty.
b.output voltage terminal rectifier diode, filter capacitor failure, etc., can be judged by the substitution method.
c.The performance of the switching tube decreases, which inevitably leads to the switching tube can not usually conduct, so that the internal resistance of the power supply increases, with the load capacity, decreases.
d.Poor switching transformer not only cause the output voltage drop but also cause insufficient excitation of the switching tube and thus repeatedly damage the switching tube
e.300V filter capacitor is impaired, resulting in inadequate power supply with load capacity. A load connected to the output voltage will drop.
The above is about the 5V LED power supply, no output, and some standard repair methods. I hope it can help you!