converted cordless drill to corded drill - only works once

Question

I have read several articles about converting cordless drill into corded drill.

I have an old Craftsman 10.8V NiCd battery. It comes with 2 batteries. One is completely dead and the other is 90% dead. I opened the battery, it is basically a bunch of NiCd cells connected in series. No circuits whatsoever. I decided to do away with the cells and convert it to corded.

I found a Switching Power Supply that outputs 12V/12.5Amp. I thought 12V is close enough and proceed to convert. But when I put it together, it didn't work. When I first press the trigger, the drill works great. But as soon as I release the trigger, it stops working. I have to unplug the AC and plug it back in only to repeat the cycle.

I read several similar posts but none reporting the same issue. I figured maybe it is because of the safety feature of the Switching Power Supply but I can't see why it fails only when I stop the drill. It is hard to find a P/S that gives out high current. I wonder if there is an explanation to the behavior and if there is a simple way to fix it?

Answer 1

For something like this you really need to go old school and not use a smart PSU.

Drills depend on the resistance of the battery to limit the current taken when there is a high torque demand. This limits how much force can be transferred back into your wrist and provides the motor and driving electronics with some protection from over-heating.

A SMPS is not the right source for this kind of tool and can actually be dangerous. For one the SMPS needs to be rated to supply the max start-up current of the drill so it does not go into current limit. This will be a LOT higher than the battery can deliver. The SMPS required will then be over-sized. The SMPS also has to be able to withstand the nasty voltage transient fed back from the motor.

A simple, suitable sized, 12V transformer (9V RMS) with a fuse, bridge rectifier, and storage capacitor is really all you need to power the drill and will work more like the battery did.

^{simulate this circuit – Schematic created using CircuitLab}

The bridge rectifier and fuse of course need to be rated for the max current under full torque with the transformer size you chose.

Answer 2

Possible causes are either the drill's electronics or the power supply's electronics.

The Drill:

• You're supplying a fixed voltage and a fixed current limit and the drill measures the voltage, sees the voltages fall of the cliff due to the high current draw, assumes it's a low battery and turns the motor off to stop over discharging the cells
• Your supply is too good from the PSU, so the current is high and the voltage is high, this confuses the drill and causes some safety lockout

The Supply:

• The supply isn't used to getting so much noise back from the supplies, it's erroring out with all the back EMF from the motor
• The current draw is too high, safety factors cut in, such as over current lock out etc (though this would probably happen as you turn the drill on and the motor tries to spin up)