Repair of a switched-mode power supply of an air conditioner

This is an example of how you can repair a power supply in a short time and with inexpensive materials. It is a ducted air conditioner. It is much more expensive equipment than the typical console air conditioning. If you don’t have any idea about the electronics’ than it’s best to hire Tampa AC repair and get it done without any problem.


I had measured the voltages, and the problem was in the power board, because it had a voltage of 230Vac in the input, and did not mark anything in the output.


As the circuit was so simple, I devoted myself to measuring the components that usually fail.

First I unbolted the power transistor (the one with the heatsink) and checked it with a semiconductor meter. The device told me that the component was in poor condition, so I had already located the fault.

However, before looking for the spare, I had to check if there were more damaged components. When the transistor fails, the input fuse usually melts. In this case, the board did not have a fuse, so I measured the components from the input to the transistor. Effectively, the ceramic resistance (white) was cut off. The other components were fine.

I did not have the same transistor, and the usual providers did not either. Apparently, it was obsolete.

Investigating the data sheets, I discovered that it had an equivalent component. The original was a 2SC4420, which is an NPN transistor of 800V and 3A. I replaced it with a BUL216, which has similar characteristics, and it is also used in switched sources, so the response speed is correct.

Another problem is that the transistor had a different encapsulation. The original was isolated, while the new one was not, so I had to mount mica insulation. I also applied some silicone paste to improve thermal conductivity.

If you do not master the technical characteristics, you can always look for the identical component, although sometimes you will have to search among suppliers or ask directly to China, being a bit slow.

The original resistance was 2W. As I did not have the amount of 5W, with the same ohmic value, that is 10Ω.

Before connecting the voltage, I made sure to mount my test lamp in series with the circuit, to avoid damage if there was a short circuit in some component.

The output was correct, so I connected it directly, without the lamp. It worked perfectly.


This type of faults is very common in switched power supplies. The transistors are cutting and letting the current pass thousands of times per second, so the electrodynamic effects that occur end up damaging the silicon. It is difficult to prevent this problem because the component is already quite oversized.


The total of the materials used is a few dollars, so the main cost is that of labor. Being a small plate with few components, it is easy to test it. Also, in this case, the fault was where it was supposed, so it was still faster.

The most important cost was to investigate to find the equivalent component. However, it is worth losing one or two hours, as long as you can leave the equipment running. If you need to place an order with a supplier, you will also lose some time, and the repair will last several days.

Danor Aliz
Danor Aliz
Danor Aliz is a lifestyle journalist who enjoys writing about everything luxury. Her favorite subjects are luxury travel and everything that has to do with fashion. In her spare time, she loves to paint and also enjoys her time walking her dog Daisy.

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