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So getting to the point. Ive got makita brushed cordless drill with dead batteries as those arent really cheap ive decided to try to power it through a microwave transformer. According to the switch it draws 12A at 12V. So after rectifying(kbpc2510) the 14V AC to around 12.2V DC i though this would run perfectly. But it didn't. The drill didn't work if i didn't press the switch fully and after I applied a load to the motor the bridge rectifier fried. So im asking what i should do to power this with a cable from microwave transformer?

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    \$\begingroup\$ 14V AC does not give a rectified 12.2 V DC. Did you measure it? \$\endgroup\$
    – pipe
    Jan 3, 2016 at 9:41
  • \$\begingroup\$ @pipe sure it does. or very close to that. so long as you don't have a filter capacitor after the bridge. \$\endgroup\$ Jan 3, 2016 at 9:55
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    \$\begingroup\$ buy a cheap electric drill for $10 it'll out-perform any battery drill priced under $200 \$\endgroup\$ Jan 3, 2016 at 10:11

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The internal PWM control on the cordless drill consumes current in trapezoidal pulses most of the time. The low impedance NiCad battery is no longer there to soak things up. What I did was place low ESR caps inside the drill along with some status LEDs. I then used a bridge made from Schottkys and huge 20000 \$ \mu \$F total electrolytic caps. I used a 12V transformer that was thermally protected with a self resetting fuse. I know that not all cordless drills are equal but what worked for me should work for you.

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  • \$\begingroup\$ Thanks for answer! Where did you place low ESR caps? \$\endgroup\$ Jan 3, 2016 at 11:05
  • \$\begingroup\$ I put them inside the handle .Soldered across the conductors that used to go to the battery. \$\endgroup\$
    – Autistic
    Jan 3, 2016 at 11:14
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be aware that the drill has electronic speed control and that's going to want smoothed DC not the all ripple 12V you get from just a bridge rectifier. perhaps you can bypass the speed control. Also motors want higher current when starting, either measure the resistance of the motor and work it out from that, or assume about 10 times the normal current, so get a much bigger bridge rectifier.

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  • \$\begingroup\$ ... and calculate how much heat-sinking you'll need on the bridge rectifier. \$\endgroup\$
    – Transistor
    Jan 3, 2016 at 10:02

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