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Modifying a Cordless Screwdriver

Discussion in 'Electronic Repair' started by ms, Aug 5, 2003.

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  1. ms

    ms Guest

    I have several cordless screwdrivers with dead batteries. I realize it
    is possible that someoneo loaded the motor and burned it out, don't know
    the history of these units.

    As I also have a working cordless screwdriver, I want to modify one of
    these items to run from a wall adapter. The unit is a Skil 2205,
    normally has(IIRC), 2 nicad batteries, about 3 VDC total. I tried up to
    a 6 VDC 300 MA adapter to the screwdriver switch, the motor did not
    work. I measured across the motor terminals and there is continuity, so
    I assume the motor is OK. I applied the voltage at the motor terminals
    and still no luck. I tried the other identical Skil screwdriver and same
    results.

    The normal nicad batteries can supply a high peak current.

    Does the motor in this unit typically need a high current to run even no
    load, and the adapter I'm using is current limiting so the motor never
    turns on?

    If so, what kind of current does it draw?

    TIA

    Mike
     
  2. Dave D

    Dave D Guest

    Probably need several hundred mA just to overcome the drag of the
    gears/bearings.
    Likely several amps under load.

    Dave
     
  3. Engineer

    Engineer Guest

    It's almost certainly the batteries, not the motors.
    Indeed they do - see below!
    Lots - see below!
    Dead electric screwdrivers are a problem. I have found a
    less than ideal solution - sort of.

    They are 3.6 volts nominally. DC Wall warts don't work at
    the screwdrivers need a lot of current, over 6 amps
    stalled. They pull about an amp or so running free. After
    failing to find replacement NiCads that fitted I built a
    mains power supply.

    This is a 12.6 VAC, 3A, center-tapped filament transformer,
    two 5406 rectifiers and a 0.5 mH choke input to a 4,700 uF
    capacitor. Open circuit voltage around 9 volts, but that
    doesn't matter. Free running voltage is about 5 volts due to
    choke input (does not seem to be a problem - nice fast
    screwdriver on light load!) However, final torque under load
    is limited. Near stall the current goes way up (over 6
    amps) and the voltage goes down to about 2 1/2 to 3 VDC - so
    torque suffers, it's less than with good NiCads.

    As a system it's a bit of a pain - not portable - but I
    leave it plugged in on the bench for light work.

    I actually managed to salvage a good NiCad cell from another
    dead screwdriver (a Skil flexicharge thing, or whatever it
    is called) and used it to replace a dead cell in my Canadian
    Tire screwdriver that died - but it does not keep a charge
    long so I am still looking or a replacement (see
    above.)

    Cheers,

    Roger

    --
    Roger Jones, P.Eng.
    Thornhill, Ontario,
    Canada.

    "Friends don't let friends vote Liberal"
     
  4. Dave D

    Dave D Guest

    They're most often sub-C cells or C cells.

    Dave
     
  5. Engineer

    Engineer Guest

    They are indeed Sub-C's (not C's) but just a fraction
    shorter than the Radio Shack and several other industrial
    replacements. That, and the fact that the originals are
    welded together (and you need to wire replacements) means
    they don't fit. Is this a conspiracy?!

    Who sells 3-cell NiCad direct screwdriver replacements?
    There must be a huge market for them!

    Cheers,

    Roger
    --
    Roger Jones, P.Eng.
    Thornhill, Ontario,
    Canada.

    "Friends don't let friends vote Liberal"
     
  6. ms

    ms Guest

    Thanks, that really explains my results.

    The only reason I want to have a AC supply for a cordless screwdriver is
    the chuck fits many useful bits and has, of course, a slow RPM. Your
    details give me a way to go.

    I will reevaluate my need as building that kind of high current supply
    is more than I planned on. I had hoped to use a wall wart.

    Mike
     
  7. ms

    ms Guest

    I have tried to replace a bad cell in a string with a better cell from
    another unit. But the welded tabs are very difficult to solder to, and
    clip leads have too much resistance for the high current in use.

    Advice?

    Mike
     
  8. nick hull

    nick hull Guest

    Try Josh Sponenberg <>, he has replacement batteries
    for almost anything.
     
  9. Engineer

    Engineer Guest

    Mike, here are the actual numbers for the above P/S on my
    "battery dead" Skil Supertwist (I was too lazy in the above
    post to go to the basement to get them!)

    At stall: 2.15 VDC (too low, of course) but still 6.4 amps
    (amazing!) This is why the torque is a bit disappointing.
    Free running: 5 volts (a bit high but OK), 1.2 to 1.25 amps

    If I did this again (I won't!) I would design a P/S with
    better regulation - the choke input in important to keep the
    reservoir ripple current reasonable low. Ideally, I would
    use a solid state regulator, say 4 VDC, 10 amps minimum, but
    I have not looked for one.

    Yes, it's much more than a wall wart! When the NiCads work
    they deliver scads of current. When they fail they leave us
    with a drawer full of dead screwdrivers!

    Cheers,

    Roger
    --
    Roger Jones, P.Eng.
    Thornhill, Ontario,
    Canada.

    "Friends don't let friends vote Liberal"
     
  10. Engineer

    Engineer Guest

    (big snip)

    I had said...

    Mike said:
    I've done this (see other post.) I identified the dead cell
    by measuring the voltage both on and off charge - it should
    be obvious which one. Then I literally ripped the stack
    apart and cut the thin welded strips (you can use scissors)
    leaving enough on the good cell to solder onto (it can be
    done, use a good flux cored solder and hot iron, tin
    everything first, don't cook the cell!) Then I ripped up
    the target stack the same way and soldered the replacement
    cell in - you need to be careful with the space the wire
    takes up. Then install charge the refurbished battery. To
    date it still works... but I'm still looking to buy a
    complete new battery.

    How we waste our time salvaging stuff!

    Cheers,

    Roger

    --
    Roger Jones, P.Eng.
    Thornhill, Ontario,
    Canada.

    "Friends don't let friends vote Liberal"
     
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