UP300AC Charger with defect temp sensing circuit

[JuculianD]

Hello dear EEVBlog enthusiasts,
I am Julian, 15 years old and I recently bought a UP300AC charger from Ultrapower.
The device worked for 10min but after I brought it to germany and plugged it in, there were some black lines on the display for only a second and then the fans went like  .
Now the charger reads a temperature of 97°C all the time and doesn't want to charge anything.
I tried a new thermistor but that doesn't help. I don't have electronic knowledge  .
Can someone maybe help me?
Is one of the diodes broken? (I have a multimeter and checked them, they seem okay, but I only measured them in the circuit) or the opamp?

Thank you for any help!

[KhronX]

That SOT23-packaged device near the thermistor terminals kinda-sorta looks like it has a crater in it. Or is it just the angle of the photo, or the lighting?

[JuculianD]

Good spot, I will take a look now.
Thank you very much for the input. I will check back in 5 minutes 

[JuculianD]

 
Looks okay.

Second thought:
Do you know how I can fake the reading somehow or deactivate the safety function to let it charge? I am smart enough to build my own temperature controller, but not to repair it .
Would be compromise solution.

Thanks for any help.

[Armadillo]

Hi,
The sensing thermistor works like a voltage divider. You can see it leads into LM358 opamp.
therefore, it needs a voltage supply source to work, form a voltage regulator, I suppose.
There are 3 situations.
There is no voltage supply, or the voltage regulator has failed or the LM358 is faulty.

Confirm there is voltage supply to the thermistor and voltage drop across the thermistor.
Confirm that the regulated voltage is correct. Check the voltage regulator. Trace the copper to the voltage source.
Measure each of the pins of the LM358 with respect to ground and post it up. There are only 8 pins to measure.

Above are merely suggestion to start troubleshooting.

Take picture of the whole board and trace. High temperature may not necessary be the thermistor problem.

[JuculianD]

Thank you for the answer. I need to say, that I don't have much knowledge. I kinda know the ohmic law and basically what a mosfet is, but that's the end already. What I know, is, that it the measured temperature is wrong. Even after 5minutes idle, it still reads 97°C while the heatsink where the mosfets are is completely ice cool.

I know that between the two pins are around 2.?? Volts. I measured that once, without any thermistor attached.
I will measure the LM358 between ground now.
What should I do next?
Thank you!

[JuculianD]

When I am measuring from the XT60 input GND, then I am getting overload on my multimeter between all the pins.
When measuring from GND from the chip on the right (CM6900G) I am getting Overload anywhere except on Pin 8 (bottom right), there is 0Ohms.

[Armadillo]

For the LM358, you will need to power it on, do not attached any lipo.

Measure the rest of the LM358 Pins [voltage] with respect to Pin 4 which is the ground.

Let me complete relate the circuit so that you may know the operation [base to the best of my knowledge..could be wrong]..

The thermistor develop a voltage in accordance with the temperature and is conditioned by the LM358. This is led to one of the microcontroller ADC pins to convert from analogue to digital. The digital info is compare to the memory setting and the microcontroller send this info to the display and output pin which goes to a driver to drives the fan.

Take picture of the microcontroller so that we can see the IC number.
Take picture of the whole circuit so that we can see the orientation and order of things.

What could be wrong;
1. The LM358 conditioner IC.
2. The microcontroller Reference voltage source or the microcontroller itself [unlikely].
3. The voltage regulator to the thermistor.

[JuculianD]

Thanks for your help, again.
Should I power it on by giving mains in the input? I do not feel well, I don't have a galvanic isolation transformer or something similar.

[Armadillo]

Oops! better not the switch mode power supply is exposed.
Too dangerous for you.

Do not power on.
Let's see others may have more wise and brilliant ideas.

[Zucca]

Congratulation JuculianD,

You are much more skilled than a lot of beginners here. I will now think on your problem and reports.

How about to solder some wire to the point you want to measure and then connect those wire to your DMM multimeter?

Then step back, plug the equimpment to 220V and read your DMM. you can unplug the 220V and then remove the wire.

BTW there are equipment which holds a lethal voltage after being unplugged. So may be you need  to call for help. Where are you located in Germany which area/region?

[Armadillo]

Did you plug the lipo before you turn the unit on? Just curious to the extents and where to look.

[JuculianD]

Thanks for the compliment .
I don't have anybody in my family and I am 15 years old, so I am learning by doing  .
I know that capacitors can be lethal.
I don't have a soldering iron now, that is the problem.
I am living in Kassel, that is in the center of Germany.

[Zucca]

I am in Munich too far away....

Well, you need to buy a soldering iron if you like EE stuff!

Even if we found the problem 99% you need to solder or desolder something so back to the iron problem.

[JuculianD]

I have one! Obviously, I have some multimeters, resistors, caps and a soldering station. But its at my fathers house who I in holiday  . And on sunday, I will fly to my school in Great Britain again.
So I will postpone that project to then, because I have a soldering iron there.