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I have a generic Ebay DC-DC buck converter, and the current limiting circuit is not working. I find that such devices often suffer from this issue, so I figured I'd reverse-engineer it in hopes of figuring out the problem.

Below is the schematic I came up with by following traces on the board; and by measuring continuity. I believe it to be accurate.

schematic

Unfortunately, the current-limiting (upper) part of the schematic makes no sense to me. Could someone explain what is going on, please?

I see no reason to use a TS431 in full saturation; neither is it clear why the op-amp ground reference isn't connected to ground.

I assume the current-limiter achieves its objective via pin 2 of the XL4015. Perhaps the LED, in addition to indicating, serves to prevent lower voltages from the op-amp reaching the XL4015?

In addition, could anyone offer a possible explanation for the failure of the unit to operate correctly? Where should I begin troubleshooting?

Photos of the board:

top of board

bottom of board

enter image description here

EDIT: Fixed schematic and added the 0.05Ω resistor

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  • \$\begingroup\$ Your tracing of the comparators/op-amps is not correct. The TS431 provides a reference that is more stable than the 78L05. \$\endgroup\$ – Spehro Pefhany Jun 14 '18 at 17:56
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    \$\begingroup\$ The op-amps also can't be connected correctly - inputs and outputs may be mixed. \$\endgroup\$ – W5VO Jun 14 '18 at 19:00
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    \$\begingroup\$ Judging by the lonely via we can see on the bottom side just above the 4 stripes at the bottom-left, that board has 1 or more internal layers which you haven't accounted for in your schematic. \$\endgroup\$ – brhans Jun 14 '18 at 19:11
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    \$\begingroup\$ Where is the 0R050 resistor in your schematic? It appears to be in the OUT- path and would provide current measurement feedback to the opamps. You've also drawn VOUT- as connected to ground when it's pretty clear from the pic that the pin on the screw terminal is not connected to that copper pour. \$\endgroup\$ – brhans Jun 14 '18 at 19:23
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    \$\begingroup\$ Your schematic also omits the connection between VOUT- and the pair of connected opamp inputs which you drew on the top-side board photo. \$\endgroup\$ – brhans Jun 14 '18 at 19:43
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Looks like there are a few connections missing. The low side current must be drawn through the 50m\$\Omega\$ resistor or through a printed trace that does something similar.

Here is a schematic (courtesy of this site) that is very similar, though not exact.

Note how the two LM358 inputs are connected, and of course the dual op-amp power supply is 5V and ground.

Your board has a '431 for the reference and a real current sense resistor, but is otherwise likely very similar.

enter image description here

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  • \$\begingroup\$ Thank you, I appreciate your taking the time to find this. However, the op-amps on my board are NOT wired in this manner. Also, remember I didn't build this board - it is one I bought from Ebay, and the current limiting pot isn't working. Presumably the builders had some reason for what they did, and weren't just totally insane. I'm trying to understand [a] how the board I have works, and [b] why the board I have doesn't work... \$\endgroup\$ – Sod Almighty Jun 14 '18 at 19:52
  • \$\begingroup\$ With your modifications it is beginning to resemble the circuit above much more closely. \$\endgroup\$ – Spehro Pefhany Jun 14 '18 at 20:16
  • \$\begingroup\$ You know, I'm starting to see the resemblance now... Any idea why it might not be working? \$\endgroup\$ – Sod Almighty Jun 14 '18 at 20:32
  • \$\begingroup\$ Nope. Maybe make some measurements under power. \$\endgroup\$ – Spehro Pefhany Jun 14 '18 at 20:35

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