Thiiiiiiiing

The charging is very minimalistic. A shottky diode and a 2-ohm resistor, that's it. Overcharging is prevented by protection chip, which I transplanted from another Li-ion cell. It switches off charging, as the voltage reaches 4.27 V, which is just a little bit too much. But as soon as it kills charging, cell voltage drops to about 4.17 V. So not actually overcharged.

Charging is quick! Charge current ranges from 0.5 A in depleted state to 0.25 A at the end. Cell capacity is 0.15 A*h, and full charging takes about 25 minutes.

A problem I had to solve was heat. The two 0805 resistors would heat up to self-desoldering temperature in free air. Would have melted PLA. So I managed to cram a tiny PCB into the USB connector. The heat is transferred to the metal shield of the connector, and some of it is conducted into the charger. The connector heats up to 48 deg.C, which is something PLA can withstand.

A new Thiiiiiiiing is being made. Stay tuned...

This Thiiiiiiiing is based on a motor and a Li-ion cell I bought in RC hobby store. The motor spins at 39kRPM (650 Hz), which is faster than regular Dremel's max of 30kRPM. It also has quite a bit of power. Not nearly as much as regular Dremel, but much more than the old Thiiiiiiiing.

A few thoughts.

1. Charger. It begs to have a full-sized USB connector at the back. Not that I was to lazy to put it there - I didn't think about it back then. Right now, battery terminals are exposed at the back as the wires, and I hook up my universal charget to charge it.

2. Motor is definitely weak. I am considering visiting an RC store to find a new one (will need a new battery, then). Nevertheless, a weak motor means that the cut-off wheel is going to last forever (until I break it; I've broken one already).