Intro: USB Powered Electric Shaver
"Refurbish and Reuse Your Electric Shaver"
My Philips rechargeable electric shaver was supplied with a 8V 100mA charger. Chargers of this type avoid a AC mains cord and reduction of voltage within the charger for increased safety. The external charger is not rated for operating the shaver when the battery is discharged. Consequently we need to ensure that he shaver battery is charged before use as it does not switch on of the battery voltage is low.
Initially a 8 hour charge provided for a full week of operation. But, over a period of two years the battery charge retention became poor and came down to a couple of shaves at best.
As the cutting head and the rest of the machinery were still working fine I decided to open up the shaver and find out a simple way to overcome this tedious requirement for charging almost every day.
Step 1: Disassembly and Study
I carefully disassembled the charger. I found that a small DC Motor was powered by two AAA NiMH batteries.
There was a circuit board with complex electronics circuitry. This circuitry probably provided for charging the batteries, preventing overcharging and additional safety circuits. I checked out the DC motor at 2.5V and found that it had as starting current of ~750mA and ran at 250mA.
I had two options: either replace the AAA batteries, or work out an alternate scheme to provide the DC motor with the rated voltage and current.
I went with the second option.
Step 2: I JUST REPLACED THE ELECTRONICS WITH 3 DIODES
I realised that a simple modification could convert this shaver for USB operation and extend its life.
Adding three 1N4007 1A diodes in series with the motor drops 3 X ~0.8V. Powering from the USB 5V provides a motor voltage of ~2.5V and it starts-up and runs smoothly.
Although I have eliminated the rechargeable option it runs from my computer/laptop/USB-chargers which always seem to be around.
This scheme could also work with electric-toothbrushes which use similar DC motors.