1000 Watts DC to AC Power W/Charger Inverter Repaired
MY confreres working in Mapourdt, South Sudan brought two inverters last year December to be repaired here in Nairobi, Kenya. One was just an inverter and the other a combination of inverter and charger. In Mapourdt, the main source of power is solar panels. When they asked one of the electronic repairs shops in Nairobi, they advised them to buy a new machine rather than having them repaired. According to the technicians the repair of the two machines would cost more than the new ones. The guys from S. Sudan brought the machine back to the house. They bought a new inverter since they were in dire need of the machine. I felt bad because the new inverter was purchased at a much exaggerated cost.
Before they leave I asked them to leave the two failed inverters with me as they go back to their working place in Sudan. I was perplexed by the smoked components inside and the burning smell the gadgets were producing. When I opened inside I almost gave up!
In the smaller machine all the MOSFETs were smoked! Almost everything was under fire! I put it aside and I opened the other bigger inverter/charger, almost similar tragedy. However, the inverter/charger machine had some components intact. So I decided to try to fix the power inverter/charger and use the smallerinverter as a scrap.
With a daring spirit, I told my comrades that I will look into the machines but they should not give me a deadline. In fact when it comes to electronics repair work I don’t like to work with deadlines.
Through visual inspection, I found that the transformer was tampered some time back. I also found some flying wires from the Printed Circuit Board (PCB) that were supposed to be connected to the Light Emitting Diodes (LEDs), some fuses open and removed from the contacts. Theinverter/charger had a separate PCB with relays and some components. One electrolytic capacitor on this PCB was exploded completely.
Using my meter I started testing the components one by one. On the separate PCB a transistor MJE 2955 (NPN complementary silicon power Transistor, 10A, 60V 75Watts) was shorted between two leads. On one side of the mother board I found four IRF 840 MOSFETs dead short as well. On the other side of the machine I tested two IRL 8113 transistors shorted and two TP 80N10T reading low resistance between leads. There were also a number of 10 ohm resistors that suffered the fate.
I recorded the part numbers of the faulty components. Any strange number I was googling the datasheet and check their equivalents (www.irf.com).
I left the machine unattended for sometime not only because I was entangled in examinations at the college but also that I did know where to buy spares in Nairobi city. Thanks to Mr. Humphrey Kimathi (my friend and author of electronics books) who came to my help and advised me the direction where they sale electronic components.
The first time I went to the shops I managed to get some components, but IRL 8113 was not found. I came back to fix them. After some time I went again to the market I get the equivalent. I replaced IRF 8113 with STB 180N10F3 since I saw that their internal arrangement is similar and the latter is (www.st.com) is bigger in capacity than the former.
I replaced all the burnt components and tested with my 12 DC Volt source. There was no response at all.
I stated suspecting the two relays on the small board. I removed from the circuit one by one and apply 12V Dc separately. To my surprise they clicked and they showed continuity on my meter. I put them back but the inverter/charger couldn’t work at all. I noticed that a transistor and 100 ohm resistor were becoming hot seconds after connecting DC supply. I replaced them. I tested with the 12V DC again. This time a low voltage alarm gave out the noise, but there was no ACvoltage output. I abandoned the machine again for some time.
A month later I took the inverter and scrutinize it thoroughly. I don’t have the capacitor meter so I used my multimeter to test the resistance of the caps. Six of the 2200uf/25V caps showed low ohmic value together with the two big 150uf/400 volts capacitors. I replaced them all.
I tested the machine with my 12 V DC source; the alarm still gave out the sound. This time it was my adapter that was not able to supply constant 12V DC because when I connect it and switched on the voltage was reducing to 10V DC. This inverter has 10.5 V DC low voltage alarm. When 12 DCV is fed to the machine, 240 VAC is expected on the output and vice versa so since it failed with low DC supply I tested with a 240V AC source to see if the machine can work as a charger. Whoops!! 19 to 20 Volts DC came out from the DC output terminals! Few seconds later, Hey there is smoke coming out from the main board!!!! It was a poor 10 ohm resistor burning and smoking quietly on Charge testing mode! My God!
I traced the line connecting to this resistor. I found two pairs of transistors 2222A and 2907. On the first pair it was the 2907 transistor that was shorted and on the other pair it was the 2222A that was a culprit. I got them from the other small inverter that I regarded as scrap they were good despite the fate of the machine. I replaced them. I connected 12 V DC the sound remains. Note that every time the sound came out the low voltage indicator LED was glowing.
When I connected the 240V AC and the DC voltage terminals showed 19V DC on no load. This time nothing was burning. Yeah! As a charger it works but not as an inverter.
I did not have a 12V battery therefore, I asked a mechanic if I can use the one of the car batteries. I connected the inverter to the heavy duty 12 DCV battery. I put a 100 watts bulb as a load on theAC output. Connected DC voltage, switched on … tell you what, guess… the bulb glows brightly and tested the output it was between 237 to 240 Volts AC! Glory to God!!! It worked!!
The Power inverter/ charger came back to life after six months of attempts! I never gave up totally! However, am glad that people in Mapourdt hospital in S. Sudan will find happiness in using the machine as both a battery charger and inverter.
Luciano Francisco Thomas Khware (Malawian) studied Electrical and Electronics at Comboni Technical College and at Polytechnic (University of Malawi). Currently, he is a student at Tangaza University in Nairobi Kenya.
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By the way you can read his previous article below: