On Nov 2, 4:32 pm, eljainc wrote:
Hello,

Does anybody know what size power supply I might need for a typical
cordless drill motor?
For example, I wish to power a 19.2VDC motor from black and decker/
Ryobi drill. Could I get by with a 18VDC supply? What amperage would
I be looking at? I think most of those battery packs are 2.4Amps. If
I put a 4 amp supply would that be sufficient or would that overpower
the motor?

Mike:

A couple of things to understand: the Battery is rated in AMPERE-HOURS
or AH, so a 2.4AH battery is capable of putting out 2.4A of current
for one hour at rated voltage in theory. However, your drill likely
will want something on the order of 15 or 20A of current, so a battery
will last proportionately far less than an hour.

A 4A supply will likely not be adequate to run the drill under any
sort of load. Too low a voltage will greatly overheat the drill if it
turns it at all. Motors are sensitive to low voltage and low current -
Imagine a DC motor as a series of short-circuits creating a magnetic
field that pushes the armature on to the next short-circuit and round
she goes. So, if you have Lots-O-Current but not enough voltage to
turn the motor, all that current will sit across whatever winding
*sizzle*.12V (more properly, 13.6V lead-acid batteries) will do
exactly that.

Do not worry about feeding the motor *TOO MUCH* current IF AT THE
CORRECT VOLTAGE! The motor will only draw what it needs, no more.

Bad Idea overall. A 20A (minimum) 18V P/S will take quite a large
transformer, a heavy rectifier (and that too has implications - a 12V
secondary bridged will yield ~16.5+/-V) and you will need a massive
amount of capacitance to smooth out the ripple. Cheaper & safer to buy
a new (several) batteries and charger and this time take care of them
properly so they do not fail. You could rig up three identical sealed
nominal 6V lead-acid batteries and get reasonable performance, but you
had also better use at least 10gauge wire on the leads and make dead-
sure you do not short anything. Problem is that sealed lead-acid
batteries are both relatively "slow" and do not take repeated deep
discharges very well.

Peter Wieck
Wyncote, PA