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Powering a cordless tool from a plug-in supply?

Posted: Mon Mar 19, 2007 12:40 pm
by PrIsMaTiC
If you're a cordless tool user then you know all the positives and
negatives. I never thought I'd want to add a cord to a cordless
tool, but I've got a 5½" - 18 volt circular saw that I'd like to run
off a power supply instead of a battery. The battery packs are
1.7 AH. Because this saw is lightweight and well balanced I can
rip across a 4 x 8 panel with amazing precision. Most of the time
the rip is so straight it looks like I used a fence.

The problem is the battery packs run down quickly, and I don't
use this little saw enough to justify buying new packs. The most
readily available PSUs are all those surplus switchmode supplies
that are used in copying machines, computers, etc. They put out
low voltage and high current, which is what I need. I'm also very
aware of the problems when you try to convert a device that has
been optimized to run off a battery. I've tried it a few times with
mixed results.

I don't want to waste money, or even worse, waste a lot of time.

Any helpful suggestions would be appreciated.

Posted: Mon Mar 19, 2007 1:27 pm
by Robert Reed
Why not? Volts are volts, and as long you you keep it at the correct voltage it should be the same thing. You may need current limiting though if you have a beefy supply. Don't know about your particular tool, but some devices I have seen in the past require the battery to be removed when operated from an external source (as per manufacturers instructions).

Posted: Mon Mar 19, 2007 1:30 pm
by haklesup
First step is estimate the max current usage. If that 1.7Ah battery lasts 30 minutes then you might be using 850mA. So a 1A, 18V supply should be adequate. If you skimp on current, you will loose torque but it will appear to run fine at no load. Regulation is not a big issue. +/-1V should be OK.

Above is a way to estimate average use. Max occurs when you really force the saw into the wood. May need a direct measurement if you want to be sure. Clamp the blade still and measure the current with the blade stalled, that should really be the max ever.

If you can find a small switcher type (like a notebook PS replacement), I would try to fit it into the battery pack case itself, this way you don't need to modify the saw and you can go back to battery anytime. You also have a nice neat cord without a power supply stuck in the middle somewhere.

Posted: Mon Mar 19, 2007 3:09 pm
by Chris Smith
Switching power supplies can cheat the ordinary transformer power supply.

Ordinary supplies have the inherent problem of internal resistance, always over looked when trying to reproduce the power that a battery can supply.

If a transformer can supply 10 amps, but the internal resistance doing so is no where near the batteries internal resistance, It wont work.

You end up feeding two or three times the voltage in order to make up for the loss of amps.

This is where the switching supply can come in handy, as they dump or spurt large amounts of current over riding the situation, and they still limit the over all voltage.

The second way to over come the internal resistance of power supplies is to add in large caps to the wiring, acting like batteries, but this method can be temporary if they aren’t fulfilling the task of time.

Posted: Mon Mar 19, 2007 5:31 pm
by haklesup
Speaking of large caps. It occured to me that startup and possibly braking of the blade would demand the largest current but only for a short period of time. A large cap might alleviate you from needing a larger PS but only if the surge is short.

Certainly startup currents can be very large on most AC motors (dims the lights when I start my table saw) but I am not sure of a battery powered DC saw. It could be several amps. This startup current should not exceed the stall current I suggested you measure earlier anyway.

Current limiting or fusing (overcurrent protection) would be undesireable in this situation but a thermal fuse for safety would be apporpriate. There probably is already a thrermal fuse on the motor itself, I'm thinking of one on the converter's main heatsink.

18V, 2A is 36W which is possible to find in a small form factor. Small size does push the price up though. I mentioned before the 120W notebook supply that was only 2"x3.5"x0.5" but cost over $100.

18V is an odd value for power supplies but there are some Digikey has a 18V, 670mA wall wart for $17 which is a bit too small but they also have a desktop PS at 18V, 3.3A for $37 (T826-P5P-ND) ~4.3"x2.6"x1.4". That otta work. THere were others but mostly open frame or more expensive.

I said regulation was unimportant because a battery would have higher voltage than rated after charging and lower as it drains down. A battery is generally poorly regulated as it discharges but reasonably well regulated for a short time since ripple is zero and large demands of current do not disturb the voltage too much on a charged battery but very much so on a depleated one. THe actual magnitude of the regulating effect depends on the type of battery, its condition and charge state.

Posted: Mon Mar 19, 2007 8:20 pm
by Externet
When I had to decide which cordless drill to buy, I chose a 12V instead of any other voltage. Installed a connector on it and I can use it in my car powered by the car battery.
You can try if your 18V works acceptable on 12V, and use an automotive battery in your shop.

Posted: Tue Mar 20, 2007 7:08 am
by PrIsMaTiC
I avoided buying any cordless tools when they first showed up in
hardware stores and home centers. They were all flimsy junk.

When Porter-Cable came out with a 12 volt drill I had just started
building a large fence. I thought how nice it would be not to drag
a 100 ft. cord and reel around my yard. This drill was the best
cordless tool I ever had. It could drive three-to-five inch screws
with no problem. The battery packs seemed to last a very long time
before they had to be recharged. The drill itself was dropped and
banged dozens of times. I probably used five thousand screws
during the building of my fence. The drill never stopped running
and the battery packs outlasted the fence project. Since then
I've purchased more expensive battery powered tools, but none
had the ruggedness or reliability of that Porter-Cable drill.

I did think that it might be possible to house a switching supply
inside of an emptied out battery case. Linear supplies are
definitely a bad idea. As you said Chris, the internal resistance
of a linear supply means that the longer the cord is the worse
the voltage regulation. As long as the motor is not loaded
it seems to be running smoothly. Even a slight load will cause
a big voltage drop. You can compensate for this problem to
some extent by using sense leads. I tried this a few years ago
with a much smaller motorized device. Everything I did to
run this itsy bitsy motor off a linear supply was a disaster.
Sense leads, all different types of feedback circuitry, etc. In
the end it still ran better off a battery.

I know this forum is special because I save any replies to my
posts indefinitely. Even a one or two sentence reply can trigger
my brain to think about a problem in a different way.

Thank you Externet, haklesup, Chris Smith, and Robert Reed.

Posted: Wed Mar 21, 2007 5:55 am
by Dean Huster
It occured to me that startup and possibly braking of the blade would demand the largest current but only for a short period of time.
I think you'll find that motor braking is provided by short-circuiting the motor rather than using the power supply in some incredible way. A shorted PM motor that's been geared down provides an incredible amount of braking power.

Be very careful of line-operated supplies in cordless tools. Because they are battery-operated, there's no mandate to provide any kind of safety insulation (e.g., double-insulation) that's necessary with line-operated portable tools. Some switcher designs are especially dangerous if they don't provide any kind of line isolation. Since these tools are used outdoors as much as indoors and since the chuck may be electrically connected to the motor armature, there is a lot of potential danger present, especially if the motor was designed for 24-volt (or less) of insulation.

A better idea may be heavy-gauge cable powering the drill from a standard lead-acid marine/RV battery.

Keep it safe, guys!


Posted: Wed Mar 21, 2007 6:52 am
by ecerfoglio
It occured to me that startup and possibly braking of the blade would demand the largest current but only for a short period of time.
I think you'll find that motor braking is provided by short-circuiting the motor rather than using the power supply in some incredible way. A shorted PM motor that's been geared down provides an incredible amount of braking power.
I think that when haklesup said braking he referred to an external braking, ie the blade being stuck in the wood (or whatever is being cut) and the motor stalling.

In both situations (startup and stalling) there is no (or little) rotation, the armature doesn't generate EMF to oppose the supply and the current is only limmited by the armature's resistance and Ohm's law (and of course the supply's own resistance).

Dynamic braking by short-circuiting the motor, if it is used, shouldn't take too much current - only for the control circuit (if there is a control circuit), or may even use cero current if there is only the switch and the motor.

Posted: Thu Mar 22, 2007 5:58 pm
by Dean Huster
I think that you'll find very few professional-grade cordless drills that DON'T use dynamic braking. I love braking because the drill stops immediately when you hit a jam and release the trigger. I have a hog Ryobi line-operated drill that, of course, has no dynamic braking, and if you get into a jam (like using a larger hole saw that gets jammed), you'll still get an extra 1/2 to one full chuck rotation after letting off the trigger, and that can really hurt!


Posted: Fri Mar 23, 2007 3:48 am
by Bob Scott
I still prefer my basic circa 1965 design cordless screwdriver. It has a shaft and a plastic handle.

Bob :cool:

Posted: Fri Mar 23, 2007 5:48 am
by Dean Huster
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!

As far as cordless tools go, I only own just the drill and it's a crappy 18v Craftsman and I'll never buy another Craftsman cordless anything after this. Batteries have a lousy life. They had lots of user time per charge, but one quit holding any kind of a charge in less than a year. Any other power tool at all and I prefer line-operated models for their power and no worry about battery discharge/charge. This from a guy who now does home repair professionally. And pneumatic tools? A gift from heaven!


Posted: Fri Mar 23, 2007 7:29 am
by Robert Reed
"As far as cordless tools go, I only own just the drill and it's a crappy 18v Craftsman and I'll never buy another Craftsman cordless anything after this."
Craftsman tools are only interested in the bells and whistles that follow up after tool design. trouble is they forgot to put their efforts into good tool design in the first place. They always come in at the bottom of the barrel in tool tests from "Tools Of The Trade" magazine.

Posted: Fri Mar 23, 2007 12:09 pm
by PrIsMaTiC
I've experimented with many different types of motors, so the
the results I got when I tested this 18 volt circular saw did
not surprise me.

The no load current is about 2.3-to-2.6 amps. The current
ramps up dramatically under heavy load. A complete stall
shot past the 10 amp limit of the meter I was using. I've
got a 50 amp current shunt, so if I really wanted to know the
stall current I could have used the shunt. What's the point?

All Electronics (and many other companies) have surplus
"brick style" switching power supplies that seem to be in the
right voltage and current range.

Here's one that All Electronics has in stock: ... PPLY_.html

Under no load conditions this supply would probably be more
than adequate. The big question is what's going to happen when
you load the motor. Will the brick quickly overheat and burn out?

I guess an old fashioned linear supply with thermal and current
overload protection might be the way to go. Before portability
was so important I built some high current linear supplies that
had massive heat sinks. You know the old joke about using an
anvil attached to a block of ice, well, I built stuff like that many
years ago.

So what's the final verdict? Will these small switching supplies
work with my saw? If not, I might as well buy a new saw. Two
replacement packs cost almost as much as a new 5½" saw.

Posted: Fri Mar 23, 2007 12:24 pm
by haklesup
A switcher won't overheat if you overload it. Once you exceed its current maximum, the voltage will begin to collapse (reduce) to maintain its max power. It essentially current limits. Some designs might also have thermal overload protection that would shut it down. Only if you sealed it up tight w/o proper ventilation would you risk that kind of problem

I guess thats why you don't see cordless tool manufacturers selling accessory power cords. and you know they would if they thought it would be profitable. If it cost more than 2 batteries with chargers, who would bother, they just buy more spare batteries or a cheap corded tool.

It's a similar problem to trying to build a reasonably priced and powerful 12VDC supply to run car audio in the home. Once you break that 10A barrior, everything gets expensive fast. You soon realize its cheaper to buy AC powered home audio in most cases or use a car battery on trickle charge.