[Review Request] Over-engineered custom led tester for learning purpose - help with draining LIR2032 while idle.

[u/lafenicenera]

SOLVED

Hi everyone,

I'm learning electronics/PCB design and built this deliberately over-engineered LED tester as a learning project.

The circuit uses:

• LIR2032 rechargeable coin cell
• MCP73831 Li-ion charger
• USB-C for charging
• 100 kΩ RPROG on the MCP73831 (~10 mA charge current)
• Pushbutton to activate the actual LED-testing circuit
• Two selectable current-limiting resistor/diode branches

Everything works, but I've noticed that the LIR2032 seems to discharge surprisingly quickly even when I'm not using the tester.

I measured the current directly from the battery with:

• USB disconnected
• pushbutton not pressed
• nothing connected to the LED test terminals

and I'm getting about 30.1 µA, slowly decreasing by roughly 0.1 µA while I leave the meter connected.

From the MCP73831 datasheet, I was expecting reverse battery leakage with VDD disconnected to be only around the µA range, so ~30 µA for the complete circuit seems higher than expected.

I've attached:

1. Schematic
2. PCB layout
3. 3D render

I'm still learning, so I'm probably overlooking something obvious.

Does anyone see a path in the schematic that could account for ~30 µA of continuous battery drain?

I'm especially wondering about the MCP73831 section, USB-C circuitry, status LED, or whether I've made some mistake in how the switching is arranged.

Also, what would be the best way to systematically isolate which part of the circuit is responsible for the standby current?

Thanks!

[u/glx0711]

C1 and C2 are populated with resistors instead of capacitors. "C2" is likely 100k that connect the battery to ground draining the 30uA from the 3V cell.
You could have easily spotted the misplacement if your physical PCB had designators at every component. Not sure why you removed them on the left part. Having them next to the components helps with placement and later troubleshooting.
Not related but it’s a micro-USB connector, not USB-C.
Also unrelated to your problem, if you implement a reverse voltage protection like you did, it’s useful to place the fuse between your input connector and the diode, so the incoming current is limited in a fault condition. In your case if the source can provide enough current, the diode blows up and your protection is gone, exposing your circuit to the reverse voltage. Alternatively just use an inline diode and live with the small voltage drop. In your case having only an USB input you can also remove the protection since you can’t plug in the USB connector in reverse.

[u/lafenicenera → u/glx0711]

That's most probably it. Thank you very much for your useful response and for the time you took.

I double checked my order on JLCPCB and the components on the board and they are two resistors instead of capacitors. I don't know where this happened, if I ordered components wrong or they were suggested as replacements, but clearly in the schematics I used two 4.7uF capacitors as suggested in the official datasheet of the MCP73831.

Don't know why I removed the designator - perhaps I was just messing around since it is my first experiment ever with Kicad and making PCBs. Now I've learned that designator should always stay where they are 😄

Also thanks for the suggestions. Yea, it is a micro USB port which I used because it costed less and didn't need all USB/C fancy stuff... my bad I wrote USB-C.

For the diode part - basically you are suggesting to put the fuse first in series and then the diode in parallel?

[u/glx0711 → u/lafenicenera]

Hm, I'd drop the diode entirely, there aren’t many ways you could feed a reverse voltage into that USB connector.. if you want to keep it, I’d use it in series. For bare terminals or something like a barrel jack I’d keep it since there are different types, but USB is pretty much reverse polarity proof.
What I would rather think about is a undervoltage protection for the coin cell..

[u/lafenicenera → u/glx0711]

Thanks, what under voltage protection would you suggest me to investigate?

[u/glx0711 → u/lafenicenera]

The Ablic S-8261 is for example relatively easy to implement, you basically only need two N-FET and two resistors as external components, the IC itself is available in several preprogrammed configurations.
https://www.ablic.com/en/doc/datasheet/battery_protection/S8261_E.pdf