Noob question about designing a power supply circuit for cordless drill

[UltraTrunks]

I have this cordless power drill from Harbor Freight, and the battery is pretty much useless.  I don't know if I can find another either.  But I've gutted the existing battery pack and I want place a AC/DC power supply circuit inside it.  My trouble is, I'm having a hard time finding a DC/DC step down converter that will do what I need.  So far I'll know I'll need a diode rectifying circuit to convert AC to humped DC.  Then probably a cap to smooth that out, but I don't really know how to size it because I don't know if I can accurately measure the impedance of the drill.  I'm sort of hoping a DC to DC converter will do that magic for me.  Problem is, I've been searching on DigiKey to find a 110V to 18v DC to DC step down converter with no luck.  Seems like a may have to build one myself.

Here is what I know.

• Drill is an 18V drill
• At full throttle the drill measures 5 ohms across the battery contacts.
• There seems to be 1 transition point in the throttle where the resistance becomes about 500K before becoming an open circuit.  Now, from using the thing I know the drill has more throttle settings than two throughout the throw of the trigger.  I can't explain why I only see two distinct resistances when measuring it on my DMM. (5ohms and 500k ohms)

So I guess my question is, what exactly should I put after the bridge rectifier?  From my EE classes back in the day, I'm thinking I should go with a inductor step down transformer. Then possibly a capacitor in parallel with the load.  How would I size that Cap?  Do I take the 5ohm or the 500K ohm resistive value when calculating the RC circuit?  Or is there a ready made voltage regulator out there that I'm missing that could do the trick?  Or should I be making a more complicated switching regulator of my own?

Any tips or links I can go check out would be appreciated.

[denelec]

Your power supply will need to output many amps of current.  Not simple nor cheap.
I would buy a new drill...  $18.99 for an 18V 3/8" keyless chuck.
http://www.harborfreight.com/power-tools/cordless-drills.html

[UltraTrunks]

Your power supply will need to output many amps of current.  Not simple nor cheap.
I would buy a new drill...  $18.99 for an 18V 3/8" keyless chuck.
http://www.harborfreight.com/power-tools/cordless-drills.html

Well, that's no help.  I'm sure you knew that though.

As far as amps go.  If the motor is actually ~5 ohms as was measured on DMM.  18V / 5ohms = 3.6A.  So we aren't talking military grade rail gun currents are we?  No, no we are not.  Also, did some experimenting with my bench top power supply and saw that it was fairly workable all the way down at 10 volts.  And at 18V, I didn't see it pull more than 1.5 amps (but I didn't play too much with it.)  In any case, "many" amps translates to "couple" amps.

So for dummies like you and me, yah, it may not be simple.  But there are definitely members of this forum that could do it in their sleep I'm sure.  And probably for < $15.  It'll be fun little project to boot, and maybe you and I can learn something.  Well worth the cost.

[kb0thn]

Now grab the chuck of your drill to stall it. Look at the amps drawn. No longer a couple of amps. Probably 10's of amps, at least ... if your bench power supply does it.

[ajb]

• At full throttle the drill measures 5 ohms across the battery contacts.
• There seems to be 1 transition point in the throttle where the resistance becomes about 500K before becoming an open circuit.  Now, from using the thing I know the drill has more throttle settings than two throughout the throw of the trigger.  I can't explain why I only see two distinct resistances when measuring it on my DMM. (5ohms and 500k ohms)

The reason for that transition is that for most of the speed control range the motor is PWMed via a FET, and then when you pull the trigger all the way back the FET is bypassed by a mechanical switch.  Or at least that's how the speed controls I've seen in cordless drills have worked.  Most quality switch mode power supplies can be trimmed up or down by 10% or so, but unfortunately that only gets you to 16.5V or 21.6V with a commonly available 15V or 24V supply.  There are a couple of options that offer more adjustment range like Meanwell's SPV-300, but of course that'll be more expensive.

[Whales]

I would modify an ATX power supply to do your bidding.  Size might be an issue however.

Alternatively, grab a 2000mAh 3 cell LiPo battery off hobbyking.com, and a charger to suit.  Should cost about $20 to $40 including shipping.

[UltraTrunks]

Now grab the chuck of your drill to stall it. Look at the amps drawn. No longer a couple of amps. Probably 10's of amps, at least ... if your bench power supply does it.

Yah, I did some more tests and when I resist the chuck till its barely moving at full throttle it goes up to 3.5amps.  Its a pretty cheap drill to begin with, so its not that beefy.

[Whales]

Now grab the chuck of your drill to stall it. Look at the amps drawn. No longer a couple of amps. Probably 10's of amps, at least ... if your bench power supply does it.

Yah, I did some more tests and when I resist the chuck till its barely moving at full throttle it goes up to 3.5amps.  Its a pretty cheap drill to begin with, so its not that beefy.

Wow.  Just make sure your DMM is not maxing out at 3.5

[UltraTrunks]

Wow.  Just make sure your DMM is not maxing out at 3.5

The bench supply claims 5 amps, but i only paid $150 bucks for it so..... 

[Richard Crowley]

http://www.mpja.com/18-VDC-Adapter-Power-Supply-34A-Condor/productinfo/31480%20PS/

  US$7.95

There are also online vendors (and even storefront shops) that will rebuild the battery pack with OEM equivalent cells.

But as others have said, any of these extraordinary measures seem overkill for such an inexpensive tool.

[calexanian]

Drill motors have extreme stall current characteristics. That 5 ohm you are measuring is most likely with the rotor not sitting square on the brushes. Just buy a corded drill. They are in the 20 to 30 dollar range. Good cordless ones are about 120 bucks (For a decent makita, porter cable, Hitachi, Milwaukee, etc.) but really worth every penny. Even the black and decker brands (dewalt, craftsman, etc....) have some good sub $100 drills that will last many many years. Your drill is the one thing you don't want to cheap out on.. Angle grinders.. Go right ahead and buy the $9.00 harbor freight special.. Seriously they were blowing the $18.00 one out for $9.00 and i bought 4 of them. One in each car, one at work, one at home! When they break or more common, get stolen, who cares! But drills are different.

[UltraTrunks]

http://www.mpja.com/18-VDC-Adapter-Power-Supply-34A-Condor/productinfo/31480%20PS/

  US$7.95

I'd pay a dollar to see inside that thing to see how they did it!

[Richard Crowley]

I'd pay a dollar to see inside that thing to see how they did it!

Huh?  Its just a bog-standard mains-powered switch-mode power supply (SMPS).  Exactly like a million others out there.
That's probably just a generic photo. They all look more or less the same.
Nothing unusual to see here, folks.  Move along.

[GEuser]

Op , listing just what drill you got is not helping very much but as some has already mentioned under load there may be up to 40 amp or so , the one I purchased recently has a 60 amp fuse in it if I recall correctly .

[Seekonk]

My camp is 12V and I have converted many cordless products.  A drill with bad batteries is about worthless.  I find them at garage sales  for a buck or so, gut the battery case and just put a cord on them.  The 18V and 9.6V work just as well on 12V.   Measuring resistance on a brushed motor is seldom accurate, the resistance could easily be half that.  I load up in Li batteries at the town recycling.  Generally there is only one battery in a pack that has gone south and the rest are good.  Just can't ever fit them in a pack designed for nicd batteries.