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It works, but how? There is something I am missing

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Rick Law:
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

c4757p:
I am 99.999% certain something is missing there. The entire circuit around that op amp/comparator is bizarre. Even ignoring the fact that there is no visible sense mechanism, the comparator is set to activate for a negative input - but there's no source of a negative voltage in that circuit.

Edit: Not a comparator, it's an op amp configured as an integrator, if you drew the schematic right. What the hell it's integrating is beyond me, though. Looks like it's just integrating a constant DC, which makes absolutely no sense.

peter.mitchell:
Thin zigzag traces are often used as fuses.

Rick Law:

--- Quote from: c4757p on March 31, 2013, 03:31:21 am ---I am 99.999% certain something is missing there. The entire circuit around that op amp/comparator is bizarre. Even ignoring the fact that there is no visible sense mechanism, the comparator is set to activate for a negative input - but there's no source of a negative voltage in that circuit.

Edit: Not a comparator, it's an op amp configured as an integrator, if you drew the schematic right. What the hell it's integrating is beyond me, though. Looks like it's just integrating a constant DC, which makes absolutely no sense.

--- End quote ---

Thanks - Good to confirm that an experienced hand like you also think there is something missing.  I thought there may be something I grossly misunderstood about how the stuff works.

I think I got it - perhaps:

The non-inverting input connected to a trace that went under the chip.  It may not be a clean pass-through to the other side of the board which is ground there.  I surmised that it was ground by measuring the resistance.  Given how close to the start of the ZigZag (that ends at the output solder join output and I hypothesize it being a shunt), that may be that the feedback mechanism.

So, perhaps your first thought is right - it is a comparator, the non-inverting input of the Op Amp could be the voltage of the shunt to the constant voltage from the 78L05 verses the voltage across the shunt.

See the modified schematics with the shunt in RED.  That could have worked?  Don't you think?
(Perhaps only the capacitor is tied to the shunt, the diode directly grounded and the LM2569 may be direct-ground)

Oh...  and... there must be different configurations for this board that the Op Amp could have integrated.  R1 is not installed.  R1 is next to another two un-labelled open spaces with component holes but no component.  R1 would have connected to the OpAmp which would have given it something to integrate.

I have to decide now, should I keep this board for return, or should I totally strip it just to learn and confirm.

Thanks for your input - it is educational and thought provoking!

Rick

Rick Law:

--- Quote from: peter.mitchell on March 31, 2013, 04:08:21 am ---Thin zigzag traces are often used as fuses.

--- End quote ---

Thanks!  That is something I didn't know!
 (Thanks for the knowledge - always glad to learn)
Rick

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