I'm asking this mainly as a sanity check and to distinguish between whether I've just done something wrong or need to buy a new one.
I'm using a 65W SMPS power supply. As a load, I'm using a 70ohm power resistor (heating element actually).
Using 19.0V in, when I turn the boost converter all the way down V-ADJ and I-ADJ 100% CCW, I get 18.9V out and a draw of 0.27A, as expected. If I turn I-ADJ all the way CW, nothing changes, as expected. Now, I turn V-ADJ CW so that I get 38V out and draw 0.55A. Everything as expected so far.
Here's the part that seems completely wrong: If I now turn I-ADJ 100% CCW (raising the resistance on the pot to 100K), very little happens. Voltage drops to 37.7V and current remains at 0.55A (probably a slight inaccuracy in my panel meter.) This is repeatable. 38V to 37.7V. This is repeatable at many other voltages as well, for example, something similar happens at 50V and 70V.
What I expect to happen is at 38V, if I turn I-ADJ 100% CCW, for the voltage to drop to 18.9V as it tries to limit the current.
Unless I'm losing my mind, this device used to work and I think I may have caused it to stop working. What I've done is replaced the 100K I-ADJ trimpot with a 100K wirewound pot. It turns out my 100K+/-5% pot is exactly 90K at max, so I've tested all of this with a 10K resistor in series with it, just in case.
So, just to clarify: I've tested the pot independently of the circuit and it's fine. I've ensured its wiring is correct and I've even reversed the wiring just to prove it to myself. Voltage adjustment is working perfectly as expected and it's also using a 100K wirewound pot instead of the default trimpot.
I've verified continuity of the pots to the correct positions on the board. I even verified continuity up to the pins of the chip. Everything is hooked up and hooked up correctly.
I've tried my best to understand how CS works on this board using the datasheet but it is honestly beyond my skill level at this point. I did some measurements anyway in hopes that they may be useful in this question:
At 18.9V, drawing 0.27A with pot set to 100Kohms, the voltage across the pot is 15mV and the voltage across V.CS(pin 3) and GND (pin 5) is 15.5mV.
At 18.9V, drawing 0.27A with pot set to 10Kohms,V.pot is 5.5mV and V.CS(3)-V.GND(5) is 9.7mV.
At 38V, drawing 0.55A with pot set to 100Kohms, V.pot is 319mV and V.CS(3)-GND(5) is 330mV.
At 38V, drawing 0.55A with pot set to 10Kohms, V.pot is 33mV and V.CS(3)-GND(5) is 50mV.
Why does it seem to work just a tiny tiny bit?
What can cause this behavior? Did I fry the LM358 when I was desoldering/soldering? It's pretty far away from the pots but it's the only thing even remotely close to them. Anything else I should test? Anything else I should try modifying?
Update 1 I tested the exact same configuration with a different boost converter and it worked as expected, so mine is broken.
Update 2 I used a 2Mohm potentiometer. At 22V, drawing 0.30A, I was only able to get down to 21.90V with the 100K pot but with the 2M pot, I could get down to 2.80V. Is there anything I can do to fix it or at least further diagnose the problem?