William G Darby wrote:
A couple of questions that I thought I'd ask:

The first is as per the subject line: What would I need to construct a PSU
for my 18 volt cordless tools. My batteries just will not keep up. ( I was
thinking only of a transformer and a full wave rectifier, and wondered if I
needed something special to allow the variable speed function to work)

One option that may be stupid or workable.
A 12 and 6V lead-acid battery in series, and a couple of chargers for
them.
Ensure they are of the same capacity, and never discharge too much.
(below 10 and 5V)

Depending on how the thing is constructed, you may or may not need
a smoohed power supply to get the variable speed working.

I'd start off guessing that you need at least 20A at 18V (360W) to
equal the torque on batteries.
Do you have the means to test the current when the tool stalls?

--
http://inquisitor.i.am/ | | Ian Stirling.
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He who lives in a glass house should not invite he who is without sin.