I order a cheap CC/CV board to charge my phone battery. It worked for a few minutes but it blew out; but that is not the point of this post. I am trying to learn something here.
I often try to learn by seeing how others do things. So, I traced the PCB to see how things are done. Attached is the schematic I managed to trace. Looking at the schematic, I cannot see how it possibly limits current.
The 78L05 looks like it is responsible for the CC part. I can see that if I adjust trimpot 2, the virtual ground created by the OpAmp will change the feedback voltage to the LM2596 thus changing the LM2596’s output voltage. That I can understand. What I don’t see is: when current flow increased due to load change, how would the OpAmp or the 78L05 know how much current is flowing out of the LM2596 and change the virtual ground to pull the voltage down. Trimpot 1 looks like the only connection to/from the CC subsystem. The current through Trimpot 1 (V-adj) should be a pure function of the voltage out of the LM2596 regardless of the current supplied to the load.
So, I am missing something. There is something there that I don’t see or don’t understand.
Unless some components are stacked and hidden under another component, all the components are accounted for in my traced schematic; so I am not missing any components that I failed to trace. Just prior to the Output soldering point, the trace on the PCB for the output did a zigzag from the capacitor to the output soldering point. That makes me think that zigzag may be serving as a shunt. But I see no components there. So, I am missing how the 78L05 or the OpAmp detects when current increased beyond limit and pull the voltage down. Before the board blew, I know it worked. When I connected it to a small resistor, the Max-Current LED came on and the voltage came down. The higher the current I attempt to overdrawn, the brighter the LED. So it does work. Something feeds back to the CC subsystem to pull the voltage down. I just don’t see how.
Can someone spot something I am not seeing? How would the current flow get feedback into the CC subsystem? What could I be missing or not understanding here.
I am considering de-solder things to see if a component could possibly be underneath another component. But I need to return the board to get a working one. Failing in 10 minutes is as bad as DOA. Before I do that, I want to first see if someone spots something that can help me understand.
Thanks for any suggestion.
Rick