Powering a cordless tool from a plug-in supply?

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Post by PrIsMaTiC » Sat Mar 24, 2007 6:58 am

Implementing overload protection used to difficult unitil companies
like National Semiconductor started producing integrated regulators
that had everything on a chip. I wonder how many LM317T regulator
chips have been sold? The number must be in the billions.

I'm thinking that I should look for a switcher with a 5 or 6 amp
rating. The tests I ran showed that this midget saw is a current
hog. If I decide to spend the money I'll hunt around for a beefier

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Power Supply Idea

Post by jollyrgr » Sat Mar 24, 2007 10:09 am

I'd say this is an easy task for a larger laptop power supply. Possibly you can get a supply from a computer store that has junked out laptops. You can buy these on the surplus market relatively cheap as well. See this example at All Electronics:
http://www.allelectronics.com/cgi-bin/i ... PPLY_.html

Another idea is to use a supply you may already have. Get a long cable (i.e. an extension cord with the ends cut off) and wire that to the battery pack. Use an automotive battery charger (largish 7A to 10A) as the source. Some even have reserve capacity to jump start a car. Most of the time people have battery chargers that sit on the shelf until needed. Put it to use.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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Post by Dean Huster » Mon Mar 26, 2007 5:34 am

But it's just the batteries, Robert. The drill proper is just fine. Remember that Kodak made inexpensive cameras so that they could sell film. Sears makes cordless drills .....

You're right. Anytime any woodworking magazine tests a particular type of tool from several different manufacturers, Craftsman usually comes in near the bottom regarding quality, precision, accuracy, finish and so forth, but is also one of the most economical. Most people can afford and do very well with a $450 table saw from Sears vs. a $4500 model from some European manufacturer. My cordless drill was a Christmas present, so my complaints have to land far from my house and have no echo! Don't worry. My next cordless drill will probably be yellow --- maybe orange to save money, although Home Depot isn't on my "A" list either. I have three Ryobi line-operated tools (hammer drill, power hand planer and portable table saw) and so far am impressed with the first two, but the saw has some design problems. But I don't have a handle on the performance of their cordless stuff.

Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).


Robert Reed
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Post by Robert Reed » Mon Mar 26, 2007 9:19 am

Well, I guess I shouldn't jump on Craftsman's case to hard. They do provide a tool at a price that anyone can afford . When I first started purchasing woodworking tools, I was faced with the descisions of "a good value'(quality) or a "good buy"(price) and always went for the the "good buy", in which I invariably ended up with a Craftsman (also having a Sears conveniantly nearby helped). But as I improved as a woodworker, it became apparent that these tools were holding me back due to their lack of prescission and durability. From that point on, I started purchasing "good value"tools. First thing I replaced was my craftsman table saw with a Delta Unisaw and life has been sweet in that respect ever since. Funny thing is that Delta (and Ryobi) actually make many Craftsman products, however they are manufactered to Craftsman specs. Still, for the average home owner doing one off or limited work,these tools fill that niche in their economy. Now when I have one off jobs, I usually purchase asian tools from Harbor Freight. My last purchase was an 18 gauge pneumatic nailer for an incredible $19.95. Bought for a trim job on a remodeling project and expected to trash it afterwards. Well that was 4 years ago and it is still going strong and with no complaints. However,when it does crash (and now that I have grown fond of these type nailers) I will replace it with a "good value" tool. As always the old saying of "you get what you pay for" holds true, I guess you just have to weigh cost against the use that the tool will see.

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Post by MrAl » Tue Mar 27, 2007 12:39 pm

Hi there,

You can measure the resistance of the motor itself with an ohmeter.
You might have to open the case though if it uses a speed regulator
or variable speed circuit.
I can tell you though these motors draw lots of current. I would
expect as much as 30 amps.

I have looked into this same problem when one of my drills
battery pack died out. It was a lower voltage drill though
so i could power it from a large 12v battery with a series
halogen bulb to drop some of the voltage. It was more pain
then it was worth so i bought a new drill.

After i bought the new drill as little as 6 months later the battery
packs died (yes it came with 2 battery packs each 9.6 volts).
It was the Ryobie brand.
At the time this first happened i didnt feel like shelling out
another 40 bucks for new NiCd cells so i took out the cells from
one pack and replaced all the cells with a very high current diode
(something like 40 amps, silicon), and connected a 12v automobile
plug with a heavy gauge wire also. Now i can plug this pack into
a 12v 'jump pack' and put the pack into the drill. I could probably get
several hours of use out of it before the jump pack has to be
Point is, the jump pack works as a power supply very well, so
if you have an 18v device it might work for that as well but perhaps
wont turn as fast.

You can build a power supply but it should have a very high
current rating and a current limit circuit to limit stall current.
40v, 50 amp MOSFETS are a dime a dozen (almost) but you
would also need a line transformer which can run 20 bucks
that handles 35 amps.
If you go to an off line switcher (without a transformer) you end
up risking electric shock because the drill/saw was not made to
be run with one side of its power coming from 120vac.
A transformer offers isolation.
They make plenty of switcher controller ic's for very low price
(prob less than a dollar) which will control the mosfet.
Only thing left is a high current schottky.

It's possible to build a power supply for this but it does require
some thought and some bucks. After i considered all that was
required to run my 6v drill from 12vdc (nice high current PS) i opted
to buy a new drill for 30 dollars instead. I got bum batteries with
it though, but if it wasnt for that it would have been cheaper in the
long run.

What i learned from the bad battery packs though was that i could
run my cordless drill off the 12v jump pack, which means i could
go anywhere and drill for hours at a time unlike with the normal
2Ah cells that come with the drills. Because of the 12v auto
cigar lighter plug i could also run it in a car with no problem.
LEDs vs Bulbs, LEDs are winning.

Robert Reed
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Post by Robert Reed » Tue Mar 27, 2007 5:33 pm

But MrAl, aren't you right back to using a "tethered "drill, which was why you bought the Battery drill in the first place. :grin:

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Post by MrAl » Wed Mar 28, 2007 6:06 am

Hi Robert,

I see your point about the 'tethered' drill but from experience i have
found that it is still much better than a plug-in-the-wall type drill in
many cases. For example, i can take the drill and the jump pack
anywhere outside and use it for hours at a time, where i would have
to run a very long extension cord with my plug in drill.
Yes, it's not as nice as having the drill with good batteries but it
still works out in the long run, and believe me the run time from
even a cheaper jump pack is wayyyyy longer than with the cells
that come in a typical drill. A typical drill has 2Ah cells, whereas
my jump pack has a 24Ah battery in it, which of course means
it will run the drill for 12 times longer than the cells (with the series
diode, using a DC buck converter would work better of course but
i dont intend to build one for this high current).

Also, if you happen to have a jump pack already (and not using it that
much for anything else anyway) and your drill is 9 to 14v (havent tried
18 volt drills with it yet but it might work too) and your cells go dead
on you, you may want to try this trick but i warn you you will never
want to go back to the tiny tiny cells (sub C) that are found in most
portable drills.

I did upgrade my drill however with new Sub C NiCds just so i would
work without the jump pack too, but i also ended up designing a PIC
based charger for it because the charger that came with the drill was
crap too! I was lucky in that it came with two battery packs, so i
could convert one to plug into 12vdc (with only the series diode inside)
and fixed the other one with new Sub C cells.
Total cost? I hate to think about it but it's interesting:
Original drill with 2 batt packs: $30.00 USD
New NiCds: $40.00 USD (admittingly bought at the wrong place)
Diode: Had several already, old but still good.
Jump pack: Had already (original cost was about $50 USD)
Wire and 12vdc plug: Had already
So since i had everything already and none of it was being used for
anything else anyway, total cost was about $70.
Oh yeah, the PIC charger:
PIC Chip was about $2 USD, so add that in too.
The board to program the chip i had already.
Total cost $72.00 USD.
If i didnt buy the upgrade Sub C cells it would have only been the $30
for the original drill plus $2 for the PIC chip.
Also i have to say i have learned much from this about battery operated
LEDs vs Bulbs, LEDs are winning.

Robert Reed
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Post by Robert Reed » Wed Mar 28, 2007 9:16 am

well its no doubt that your setup is definately better than loooonnnngg extension cords. Its just a shame that these replacement batteries cost so much. I usually buy 2 batteries with these tools so one is charging while using the other. However, for my moderately priced drills when it comes time for battery replacement its been cheaper to buy a new drill on sale than to replace thje battery. Seems like such a waste.

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Post by MrAl » Thu Mar 29, 2007 4:33 am

Hi again Robert,

Yeah the price is a little high for the NiCds, but i noticed online
there are places that are cheaper. Of course then there is the
shipping charge.
I never considered using NiMH sub C's before but there are some
on the web with a hefty rating now, and a good current rating too,
unlike the older ones. The only problem though is if you switch
your portable drill (or whatever) to NiMH you'll probably have to
design your own charger too, as the NiCd chargers that come with
the drill most likely wont work with the NiMH technology.
I also dont like to have to charge NiMH cells in series (as a drill pack
would be). The best way is to do each cell individually, but in a pack
this is hard to do.
LEDs vs Bulbs, LEDs are winning.

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Bob Scott
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Post by Bob Scott » Sat Mar 31, 2007 12:04 am

Dean Huster wrote:Bob, I'll bet that preference will die quickly for you when you start installing a room full of sheet rock or cement board or build a deck using screws!
But I use an attachment for my corded drill for wallboard and flooring. :)
Dean Huster wrote: This from a guy who now does home repair professionally. And pneumatic tools? A gift from heaven!
I noticed that you list woodworking as a hobby. I hope your work doesn't change that. I definitely know that employment opportunities in electronics are getting as scarce as blacksmith jobs....unfortunately for those of us who love the finer points of analog.


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