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Inspecting split AC control board not running the fan

eagle1109

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Hi,

I have a split AC control board that isn't working.

I tested the voltages, there are two main caps holding 313V DC, but still the fan isn't working.

First I inspected a relay that could be faulty, tried to desolder the relay, but one of the pins broke as I was taking it out. It's 4-pin relay, so I replaced the power line of the relay with a wire for testing and ordered this model of relay.

But again powered the board with the fan, but the fan isn't working.

Could it be this chip ?

20210328_142419.jpg




20210328_144428.jpg



20210328_142409.jpg



The output has 3 pins, and the fan has 3 wires and one ground but didn't know where to connect the ground pin so I left it floating.

Could the fault from the following:

1. In testing I left the ground wire of the fan floating, so the motor is DC and has 3-ph so the ground wire is a must since it's a DC circuit. Or it's balanced 3-ph PWM that doesn't require a ground wire.
2. The chip with the heatsink.
3. The relay is connected with point 1, in case the ground wire is a must, then I have to reconsider the relay is faulty and the chip is OK.
 

c_mitra

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Could the fault from the following:

1. In testing I left the ground wire of the fan floating, so the motor is DC and has 3-ph so the ground wire is a must since it's a DC circuit. Or it's balanced 3-ph PWM that doesn't require a ground wire.
2. The chip with the heatsink.
3. The relay is connected with point 1, in case the ground wire is a must, then I have to reconsider the relay is faulty and the chip is OK.

1. The ground wire need to be connected to the body (earth point) and it is not a electrical connection. It is there for safety. You are right that the output is p-phase balanced and the PWM is used to control the amplitude of the three phases.

2. Generally these chips are rugged but this is only culprit (apparently) and not the relay. As the relay is broken, you need to ascertain the type of the connection and manually test the relay replacement.but are not equal,

3. Test with a meter that the three points marked U V and W on the PCB are having voltage. Test very carefully the voltage between UV, VW and WU. They should all be equal.

If you see no voltage at all, we need to go back to the relay. If the voltage is there but are not equal, then the driver chip need to be looked into.

Also look for some high voltage caps near the output pins (motor connection pins U, V and W and see they are healthy.
 

eagle1109

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1. The ground wire need to be connected to the body (earth point) and it is not a electrical connection. It is there for safety. You are right that the output is p-phase balanced and the PWM is used to control the amplitude of the three phases.
OK got that.

2. Generally these chips are rugged but this is only culprit (apparently) and not the relay. As the relay is broken, you need to ascertain the type of the connection and manually test the relay replacement. but are not equal,

I ordered the same relay model, but put a wire for now until the replacements arrive.

3. Test with a meter that the three points marked U V and W on the PCB are having voltage. Test very carefully the voltage between UV, VW and WU. They should all be equal.

Yep, you're right, it reads 0V, all the pins.

If you see no voltage at all, we need to go back to the relay. If the voltage is there but are not equal, then the driver chip need to be looked into.

Yeah there's no voltage at all, but the relay function is to open close the mains line

Also look for some high voltage caps near the output pins (motor connection pins U, V and W and see they are healthy.

I don't know if you mean this cap, because it's not high voltage.

20210330_111208.jpg
 

c_mitra

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No, not this capacitor. Please test the capacitor marked 100uF, 450V (they are also big in size) for DC voltage. The voltage should be around 300V with the power turned on. This says that the DC mains is coming all right. There are two of them, side by side. You need to check both.

Be careful because you will be testing live circuits. Make connections to the meter (set in DC high range say around 1000V) and then apply power. After taking the reading, disconnect power and wait for the capacitors to discharge (or discharge them with a load resistor manually).

If the HV power is available, we need to know the driver IC part number.

In the meantime, please clean the board because dirt can make a mess.

Use ethyl alcohol or isopropyl alcohol (both are easy to get) and a used toothbrush. Scrub gently but surely. You can dilute the alcohol with water if it evaporates too fast to clean. Use a hair-dryer to dry the board thoroughly. Also get a photo of the solder side of the PCB. Because we need to know where the power is going and where it is getting blocked.
 

eagle1109

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the big caps have 320V on them and they discharge very quickly less than 1 min.

Is the quick discharging a safety feature of new boards or it's a sign that there's something shorting output current ?
 

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