12vdc to220 commercial chinese inverters over-current protection

i fixed many of those Chinese commercial Chinese 12vdc to 220ac inverters i know that

the lm324 compactor is the one responsible for all sorts of protections in these units

a 5vdc output from the compartor due to any sort of fault...like lvp ovp ocp etc etc

the 5vdc will go to the 4th pin in the tl494 this pin will shutdown the tl494 if logic high

reach it and once the tl494 goes down the entire inverter will shutdown...


ofcourse i am talking about tl494 in the primary side that is responsible to turn the 12vdc to

250vdc not the the tl 494 at the secondery which is gonna turn the 250 vdc into 220vac

when we look at it ..it is easy for lm324 to detect a low voltage case ...just a voltage

divider on the 12vdc bus and the mission is done


over voltage also can be easily detected with the same method "voltage divider"

however i cant see the notorious current sense resistor in any of the primary fets ...those

resistor ARE easily noticed in any uc38xx based smps used for current sensing and thus

over current can be calculated ....but in these inverters i can not see any

way for the tl494 or the lm324 for measuring an over current state??????????

i posted an inverter schismatic for discussion and ideas

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ok here is the image

Sorry, no image visible.

Please clarify, what is the normal method of overcurrent detection you have seen in units from other manufacturers?

BradtheRad said:

Sorry, no image visible.

Please clarify, what is the normal method of overcurrent detection you have seen in units from other manufacturers?

Click to expand...

it is not about other manufacture it is about different ic used ....uc3842 is current mode pwm controller power ic ....it is uses current sensing resistor between the source and ground based on the voltage on this resistor the ic can "guess" what is the current and can shut down its output if the current passes a pre-determind threshold

Sorry, still no image visible. Try the 'Add an Image' button, it is reliable and straightforward.

add image is never at all easy in this forum once i add and upload and back to click on the quick post it is never happen ....

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here i add i think

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libyantiger said:

add image is never at all easy in this forum once i add and upload and back to click on the quick post it is never happen ....

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here i add i think

Click to expand...

when i start analysing the circuit :

over voltage protection
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r2 and r9 are forming a voltage divier which divide down the 280vdc at the secondery

side to safe lever and fed to the compartor via r39 which if raise to high level compartot will produce 5 volt at its output at pin 12 to pin "4" of the tl494 which will let this ic shutdown the inverter

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high battery voltage is done by the zener zd15

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low volltage is done by r4 r19 r22 voltage divider and a high logic out pin at pin 7
to pin4 tl494 and thus shut down

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high temp sensor is done by ct1 ntc which will kick in p1 pnp transistor into operation if
tempertaure is high which will in its turn send 12 volt pulse to pin 10 of the comparator

which will int turn turn the n4 npn transistor which at last operate the fan


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cant see any over current protection...


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You are correct, there is no over current protection.

it is not that easy hehehe ...there must be one because when any of the secondery fets are

shorted that inverter shuts down ....it is up to real expert to find it

here is one that does contains an obvious current sensing resistor r63 0.39 ohm and it is fed to one compartor that seems to handle both over voltage and over current protection ...but stilll i find i cant understand how it is done
i will show where is saw contradiction when some one answer me