Electronic – How does this current limiter work? – and why doesn’t it


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.


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

Best Answer

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