I would like to connect an electric toothbrush motor (Braun Oral-B Type 4729) directly to external power supply. So far I have been trying at least 3 different DC power supply unit with appropriate voltage (1-5 V) but nothing happened. However motor works perfectly fine when connected just to a regular battery (AA, 1.5V)...what is happening? The only guess I have is some impedance mismatch between the source and the motor...can anyone confirm this and/or provide any solution how to connect this motor to regular DC power unit? Thank you very much!
If this motor runs when directly connected to a 1.5 V battery, then it will run when connected to any other 1.5 V source.
The reason it doesn't run with your other power supplies is probably because the motor demands more current than the supplies can supply, and they shut down or reduce their voltage.
Note that motors take more current when starting than when running. Your supplies might be rated for the running current, but never get there because they shut down or collapse due to the starting current demand.
You can easily verify this. Put a voltmeter across the supply and see what happens when you connect the motor. You will see the voltmeter go to 0.
The solution is to get a supply that can handle the current requirements. If a AA battery works, then a couple of amps should be good enough. With a benchtop supply, you can set it to 1.5 V, then adjust the current limit to see what it takes to allow the motor to get going.
Another thing to try is to connect a large capacitor across one of your existing supplies before you connect the motor. If the capacitor is large enough, it can source the initial current to get the motor going, then the supply can catch up and source the current to keep the motor running. At this low voltage, you can get sufficiently large caps. I'd try something like 10 mF or more rated for 10 V.
One thing I have seen in some toothbrushes is how the circuit is completed when the cap is fitted over the battery.
If, when you use the external supply, this connection is not completed then it will not work.