# Power for 18VDC Drill

Discussion in 'The Projects Forum' started by blueknight, May 3, 2009.

1. ### blueknight Thread Starter New Member

May 3, 2009
2
0
I would like to use a laptop switching power supply rated at 18VDC 4.5 A to replace the 18V 2400 MAH NiCad battery that powers a Dewald 18V drill. I would like to have a 120VAC source to power the drill when the battery dies. Has anyone tried to do this and was it successful?

Last edited: May 3, 2009
2. ### tkng211 Well-Known Member

Jan 4, 2008
65
2
Suggest you measure the coil resistance of the motor first. You can then calculate the current when the motor is at stall. If the calculated value is much more than 4.5A, you'd better add a serious resistor to the circuit to limit the maximum current to approx. 4.5 to 5A. Besides, a heavy duty diode must be connected to the drill power supply terminal to bypass any unwanted reverse back emf generated by the drill motor, so it won't cause any problem to the switching power supply.

3. ### mik3 Senior Member

Feb 4, 2008
4,846
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It is possible.

4. ### blueknight Thread Starter New Member

May 3, 2009
2
0
Thank you tkng211 for your reply identifying the factors to be considered in constructing the power supply. Your further assistance would be greatly appreciated since I only have a basic knowledge of electronics. Using a multimeter, I measures the resistance across the battery terminals of the drill with the trigger fully depressed and got a reading of 0.5 Ω. With a 18Vdc supply, the stalled rotor current would be 36.0 A. How do you calculate the value of the current limiting resistor and its associated wattage? Also would you recommend the current and PIV rating of the diode connected to the power supply terminals. Would the diode be reverse biased with respect to the positive terminal of the power supply?

5. ### gerty AAC Fanatic!

Aug 30, 2007
1,154
304
Is the drill variable speed? If so, when you measured the resistance you also measured the drive circuit for the motor. That will throw your calculations off, how much,I don't know. I have on my bench a motor from a small,cheap drill. The motor draws 2.75 amps with no load @ 7.2 volts.