Troubleshooting of lightning strikes on display Power Supply

Faulty model: Great Wall C-I5271 type display;

Fault Cause: Lightning strikes;

Fault symptom: no display. The user reported that when the machine was struck by lightning, it made a "bang" sound.

Analysis and maintenance: the power fuse (f901/3.15a) has been burned out and is black inside the casing. This indicates that the power supply has a serious short circuit fault. Test the Power Supply Adjustment tube drain pole (d) to ground resistance is 0 Ω, the gate (g) to ground resistance is also 0 Ω, indicating that the power supply adjustment tube has been completely broken down.

Remove the irfbc40 from the circuit and test the resistance between the electrodes D, S, and g with a 3 Gbit/s (MF-47) × 1kb Ω block, it has been completely short-circuited. For the ground resistance values of the circuits connected by electrodes ×1kb Ω, × 100 Ω block test D, S, and G, see ^ 50030401a ^ table 1. Based on previous maintenance experience, the positive and reverse resistance of G pole is obviously abnormal.

Check the connection circuit (^ 50030401e ^ 1) of the Power Supply Adjustment tube, drain pole (d) is connected to the power supply pulse transformer, and the source pole (s) is resistor to the ground through 0.2 Ω, the adjusting tube gate (g) goes through the diode d910 (BYT42-M) to the ground. Remove the diode d910 from the circuit and test that the positive and reverse resistance is 5 Ω, which indicates that d910 has been broken down.

The power supply of this machine is composed of current type Pulse Width Modulation Circuit uc3842b and MOS tube (Power Supply Adjustment tube) irfbc40. When the Power Supply Adjustment tube breaks down, the uc3842b is also damaged in general. Check the ground resistance of each uc3842b pin. No obvious exception is found. Check the DC resistance between each pin in the uc3842b offline status and check the uc3842b values. The normal values in Table 2 are met. For the normal resistance values, see ^ 50030401b ^ table 2, ^ 50030401c ^ table 3 shows the typical voltage parameter values of uc3842b ). Check other parts of the power supply circuit for no short circuit.

Change Power Supply Adjustment tube, diode d910, fuse f901. To prevent short-circuit (especially the Line Scanning Circuit) of the power supply load, + 80 V output end of the line scanning circuit is disconnected, and a 40 W/V light bulb is connected from the power supply output end to start up, the power indicator of the monitor is on, indicating that the short-circuit fault of the power supply is ruled out. Field Scanning Circuit Power Supply + 50 V, digital circuit + 15 V, control circuit + 25 V, video amplification circuit + 80 V are normal.

Detects the line scanning circuit and the picture tube circuit respectively. If no obvious open or short circuit fault is found, remove the light bulb, connect the Line Scanning Circuit + 80 V, and start the system again. The display power indicator is on, the line scan circuit or line output circuit is normal. The collector + 80 V of the output tube is normal, and + 50 V, + 15 V, + 25 V, and + 80 V are normal respectively. Enable the host, add a signal, and display the display. By changing the display refresh rate on the host, changing the display mode, and adjusting the screen brightness, contrast, and other items are normal. If no problem occurs after two hours of continuous boot, the display fault has been ruled out.

Summary: 1. A display is struck by lightning, which often results in a large area of damage to the circuit, especially power supply circuit, line scan, field scan circuit, and interface circuit with the host. When the display is struck by lightning, there is a sound of "lightning", which roughly indicates that the power supply has a serious short circuit fault, and its repair focus should be placed on the power circuit. Because the lightning monitor has no rule to follow and does not know the circuit that may occur due to a fault, we should establish a correct analysis and determination of the fault range. Because the protection circuit is relatively complete in the power supply circuit of the general display, when the power supply load (scanning circuit, etc.) circuit appears short or open circuit, can be well protected, the power supply circuit will only stop working, no output voltage, the power supply fuse will not burn. If the power fuse is found to burn out, the fault should be in the power circuit itself. Starting from this idea, first check the power supply circuit to quickly check the fault location.

2. when lightning instantly enters the machine, its powerful pulse current will generate an instantaneous strong reverse induction pulse voltage on the power supply pulse transformer, because the pulse voltage is far greater than the reverse breakdown voltage of the Power Supply Adjustment tube, thus, the reverse breakdown of the Power Supply Adjustment tube causes the power fuse to burn out.

3. because the power supply circuit of the monitor is a switching power supply, the load of the power supply cannot be opened. When checking and judging the fault of the power supply circuit and scanning circuit, the power supply output end (high voltage output end) should be connected to a false load, generally, the light bulb is used. The light bulb size should be determined based on the output voltage of the display power supply or the size of the power load. The value ranges from 14 inch ~ 17 inch display generally uses ~ 60 W/V bulb.

4. generally, when you enable the power of the monitor, it is synchronized with the system (Win95/98), that is, when the host power is turned off, the power of the monitor will also be turned off, however, it is a soft shutdown. Although the power supply is turned off, the power switch is not really disconnected and remains closed. Therefore, it provides conditions for the insertion of lightning or other strong pulse interference. It is recommended to develop a good habit: each time you shut down, the power switch of the host, the monitor is disconnected. If you are used to soft shutdown, you should turn off the total power supply to avoid unnecessary troubles.

5. irfbc40 is difficult to buy after it is damaged. You can use irfbc40a or irfbc80a for replacement directly or use 6n60 for replacement directly. For performance parameters of irfbc40, see ^ 50030401d ^ table 4.