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Cheap and cheerful DC motor soft start 2010-01-05

I'm planning to make an electronic solder sucker from an old portable vacuum cleaner motor and a 5V switching power supply. Luckily I managed to tweak the supply up to 6V so the motor should in theory run at full speed. The problem was that the supply required a minimum load of 7W and would also current limit if the motor was connected directly to the output so the shaft would just vibrate.

The solution was a soft start, now I did consider a 555 timer or a comparator to monitor the voltage across a series resistor then short circuit it once it's dropped below a certain value but in the end I decided to go with the simple circuit attached because it did the job. I used a 100:mu:F tantalum for the capacitor to minimise leakage but it probably isn't needed and two 1R resistors in parallel for the 0R5 resistor.

By the way the motor draws 6A and the supply powers it fine despite its maximum rating of 30W as it probably just limits the current to 6A and at 5V that's 30W.
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