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Ok, so I perhaps naively thought I was the first person to ever think of this - go figure.

Anyhow I managed to get my hands on a couple of cordless drillers which have an expired battery and thought that if I added a power jack (socket) to one of the drillers I could buy a cheap external "wall wart" power supply to drive it. It turns out that it is not that easy to find such a power supply but I have recently bought a bench power supply which can deliver up to 30V and 5Amps so I thought I could use that given that the driller runs on 18V and draws between 1 Amp and 3.4Amps in operation (the latter when I held the chuck to simulate resistance (and torque current)).

The snag is that some guy at the local electronics shop told me that if I use the bench power supply I would almost certainly damage it (specifically he said that switched mode supplies should NOT be used to drive a driller and that I should use an old style "brick" transformer).

I have in fact already operated a similar driller using the bench supply (to test motors for my robot) but this was only for a few seconds at a time. There wasn't any evident harm to either motor or supply.

But was he right? I obviously would not like to think I had saved a few Euros only to have to throw away my new bench power supply.

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    \$\begingroup\$ I think this guy is, as most sales people do, overly generalising how stuff works. Basically, in the olden days, a battery would "dip" in voltage quite noticeably. Older designs of electronics would rely on that. With heavy duty NiMH and even more so with Li-Ion powered drills the batteries will be able to supply 10 to 50A of stall current with only a reasonably small voltage drop. Maybe if the drill is very crappy it'll get warm, but even very cheap ones these days can often handle a strong switched mode or Lab supply. If you're 'scared': Add an inline resistor for 2V drop at stall current. \$\endgroup\$ – Asmyldof Oct 11 '14 at 1:40
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    \$\begingroup\$ Your current figures look suspiciously low for actual use, but perhaps this is an extremely light duty tool. \$\endgroup\$ – Chris Stratton Oct 11 '14 at 6:24
  • \$\begingroup\$ Thanks guys. As for the current draw I am quoting what I saw on the bench power supply display. It never went up to more than 3.5 amps even when I was trying to stop it. Of course it might go up beyond that if a drill bit gets stuck or something but the bench unit has a current limiter anyhow so I hopefully will be ok. \$\endgroup\$ – Galahad II Oct 11 '14 at 15:10
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Don't know what the guy at the local electronics shop was smoking, but it's very common to drive dc motors with a switch-mode power supply. That's effectively what the variable speed trigger in the drill itself is doing already and it's what millions of radio-controlled modelers do with their motors every day. But even so, the output from you bench supply will be reasonably well filtered and smoothed, so its much the same DC as what you would get from the battery originally fitted to the drills. So by all means crank up you bench supply to 18v, set the current limit up to something sensible and go for it!

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