power inverter not working

Thread Starter

kyu9971

Joined Mar 28, 2015
90
Hi everyone,
i have a problem with my 12vdc / 220 vac power inverter that a certain moment, not working properly.
Some days ago it is stopped completaly.
I opened it and found a little track broken.
this track carry the 12v to second stage oscillator (TL494). After repaired this track, it started to work again, but now it has few power . From feature it has an 1200w continuous and 2400 w peak power and before this problem it succeeded to supply a refrigerator.
now instead to turn off with an absorption of about 100 w. It does not sound, but it turns off. If i disconnect the load it turns on automatically.
this power inverter have 2 oscillator stages with 2 tl494.
the first stage raise the low voltage (12v) in high voltage through use of four trasformers.
the second stage use h-bridge to generate the vac 220.
the first stage have 8 mosfet irf3205 , while the second stage have 8 mosfet irf740.
you have any ideas about thing i can check?
 

dendad

Joined Feb 20, 2016
4,451
Check all the FETs and diodes.
Also the electrolytic capacitors. Look carefully at them for swelling or cracked tops. The top should be flat.
Electros dry out and cause many troubles.
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
At the beginning there were some electrolitic capacitors on low voltage side swollen but i replaced all them (8 x 2200 uf /16v).
I added two big capacitors on low voltage side ( 4700 uf/25v and 3300 uf/25v) also, but i have always the same problem.
on high voltage side there are three electrolitic capacitors from 47uf/400v and i tried to replace them with an big capacitor 220 uf/400v, but i not solved still.
i checked with my oscilloscope the voltage on output and it seems good with 300 volts peak.
I checked the diodes and fets and not found any short circuit, but you think i must remove from board and checked separately?
i attached an schematich that i extracted from circuit some years ago.
thank you for any help

 

dendad

Joined Feb 20, 2016
4,451
One thing about the capacitors, did you use low ESR types?
When running, does the 300VDC fail or just the AC out?
If you can, see if you can isolate the failure to one half of the circuit. DC or AC.
Often, high value resistors fail too, particularly those in the high voltage area.
 

Alec_t

Joined Sep 17, 2013
14,280
Have you checked the high voltage KS44 transistors Q-green and Q-gray, also electrolytic C23 and its unlabelled counterpart near those transistors?
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
IMG_20171008_140502.jpg 081020176883.jpg 081020176872.jpg I used the capacitors as those the original mounted, but i think they are normal not esr.
Anyway, in output i have an sine modified wave and i believe it is correct.
I attached the images for see to you.
i have the problem when the load exceed about 100 watt, while before it worked with more power without problems
 

dendad

Joined Feb 20, 2016
4,451
I can see the FETs are not clamped to the heat sink. Have you still got the clamp?
That may be the problem.
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
in the immage there are not the clamps on the FETs, but during the tests i mounted all them.
i don't understand where is the problem.
seems working ok, but when I ask for more power it turn off and immediatily turn on again.
the odd thing is that it not emit nothing alarm sound.
I thought the problem was the capacitors but so is not.
 

dendad

Joined Feb 20, 2016
4,451
What are you running it from?
Put your meter on the incoming supply to the board to check it is ok under load.
If it is, than meter the 300VDC.
Try to figure out what is failing.
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
Before the break, I only used it for the fridge.
This, although having a power of 150Wh, has a remarkable start for this I needed 1200 / 2400wp.
if I now try to connect the fridge to turn off.
I'll tell you the tests you've recommended me, that is to see the voltage on the high-voltage capacitors under load.
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
hi everyone,
in these days i did some test.
i connected the inverter on the big batteries directly solar panels so i sure that the problem is not the few current.
without load i readed 315 volts on big capacitor after the diode.
when i have connected the fridge the voltage drop about 100 volts in same point.
after some seconds four mosfets and one pnp transistor (Q15) of first stage are burned.
 

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Thread Starter

kyu9971

Joined Mar 28, 2015
90
I replaced all irf3205 and the pnp transistor also if i found broken only for of these
now the power inverter working correctly.
It's possible that some mosfet were broken but not short-circuited?
 

Thread Starter

kyu9971

Joined Mar 28, 2015
90
I think you need an inverter that is current base energized not voltage base energized. We manufacture the inverter Systems in Africa, Nigeria.
I have this inverter from five year about and i like it.
it is true sometimes it is broken.
for this motive some years ago i have passed a lot of time for obtain from the board the schematic.
If you want sell your item i am sorry. I am not interested.
i like this forum because with it i am learning a lot of thing about electronics world.
i am not searching something from to buy !
 
I have this inverter from five year about and i like it.
it is true sometimes it is broken.
for this motive some years ago i have passed a lot of time for obtain from the board the schematic.
If you want sell your item i am sorry. I am not interested.
i like this forum because with it i am learning a lot of thing about electronics world.
i am not searching something from to buy !
It is important you know about things that give you long lasting service, irrespective of buying or selling. Be informed.
 
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