FIXING THE IKEA FIXA

I bought the little power drill and used it several times. The small battery drains out before you finished work and the recharging with the dedicated charger takes up 8 hours according to IKEA.

If you open the case, you will find a standard industry Li-Ion cell next to an oversized circuit board. The electronics protects the accumulator from undervoltage during use and overvoltage while charging with a limited current.

 

IKEA FIXA drill open

I used the drill away from mainspower thus an USB charging ability would be handy. With the original circuit 5 Volts was not enough to charge as there are two bipolar transistors in series, a diode and a 2.5 Ohm resistor.

Why not to use the USB current limit for charging? Why not to use the same MOSFET which switches the motor off in case of undervoltage also for charging control?

A new circuit, easier and smaller, would do.

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The board fits on a fingertip. The logic level FET was recycled. A NPN transistor drives the gate and controls cell voltage during use and charging.

While the motor is on and the battery voltage drops below 2.5 Volts, the MOSFET will not conduct that good any more. The Voltage across it will drive the base of the NPN transistor which shuts down the gate und the MOSFET switches fully off. When you release the motor direction switch, the voltage across the MOSFET drops and the transistor opens, which will raise the gate voltage again. The red LED indicates that the undervoltage protection is active.

When charging the current flows in reverse through the MOSFET. The cell voltage rises until current through the two green LEDs drives the Base of the bipolar. The Gate voltage drops and the MOSFET opens. With 5 Volt USB voltage and 4.3 Volts cell voltage, only a marginal current remains through the MOSFETs internal reverse diode. The two green LEDs act as a Zener-diode with about 3.5 Volts and light up if fully charged. Again the red LED lights up brightly.

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