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Rotary tool - what battery V and what Vmax for motor?

Discussion in 'Power Electronics' started by FuZZ1L0G1C, May 27, 2019.

  1. FuZZ1L0G1C

    FuZZ1L0G1C

    365
    112
    Mar 25, 2014
    Hi.
    Regarding the 'Speed-Matic' rotary tool.
    I've had this 'As-seen-on-TV' mini-drill / rotary tool several years, rarely used these days.
    I mislaid or lost the original wall wart that came with the kit.
    Did not realize the rechargeable cells have been leaking electrolyte.
    Now want to remove gunky cells and mod to PWM, direct Voltage.
    From researching the device, it appears that similar products use a 3.6-3.9 VDC wall-wart as a battery charger, and users have measured about 500 mA no-load and over 3A stall motor current.
    Motor Voltage unspecified and my motor un-marked.
    The DMM reads DC resistance of 4.3Ω.
    Disconnected cells and 100n cer disc snubber capacitor across motor, for R test.
    So, if 3.5 A is taken as Imax (stall), and Rmot is 4.3Ω, then is max Vcc 15.05 V?
    I plan, if possible, to use 12 VDC, either un-modulated or PWM.
    Thanks, Clive.
     
  2. Bluejets

    Bluejets

    3,439
    696
    Oct 5, 2014
    If it is like below then I'd say motor is max at 6v.

    drill.jpg
     
    FuZZ1L0G1C likes this.
  3. dave9

    dave9

    733
    166
    Mar 5, 2017
    If the original wall wart was 3.6 to 3.9VDC and a regulated switcher, it could have only had two cells in series. They wouldn't have gone to the trouble of putting a voltage boost board in to charge the battery instead of a higher voltage supply.

    If it was an unregulated line frequency transformer/bridge-rect/cap only, it could float to a high enough voltage to finish charging 3x cells.

    Either way this makes it a max design of 4.2V. Maybe the motor can withstand more under certain conditions, but it seems very unlikely it could stay cool enough anywhere near 12V, nor that the bushings would handle that either.

    I would try 5V, maybe see if it'll run off one of those Li-Ion USB power bank thingies, put a barrel jack DC input on the back so if you want to use a regulated 5V AC/DC adapter you can do that too when you don't need portability.

    What were you using something this weak for? I once bought a little rotary tool just to get the bits that came with it, was on clearance at Harbor Freight. It used a 12V, can't remember the current but at least 500mA wall wart and was so weak I couldn't find any practical uses for it, maybe put a sanding drum on to sand smooth pet claws after clipping them where the main benefit is a low power quiet tool to not alarm the pet. Otherwise, it constantly stalled out on anything I'd use a rotary tool for, was no faster than using hand tools.

    If it were high precision I might've been able to use it to drill PCB holes but it wasn't anywhere near that, would have shattered the bits immediately from the runout.
     
    Last edited: May 28, 2019
    FuZZ1L0G1C likes this.
  4. FuZZ1L0G1C

    FuZZ1L0G1C

    365
    112
    Mar 25, 2014
    Thanks for replies.
    @Bluejets #2: identical to your photo, except (Ni-Cads?) which have bigger dimension at 23mm dia and 80mm total length.
    @dave9 #3: counted 3 cells in series, look like shortish D cells, silver-colored.
    ^^^^^ 3 cells 4.2/3 ^^^^^ =1.4V per cell?
    Cells too flat to run motor, (3.2 V total) so have put on 3.47 VDC charge for a few Hrs via 1A wall wart. (3V pos).
    I have a 6V / 12V regulated power supply rated at 6A max, which on 6V may not be too over-rated, especially for PWM.
    For portability, I have a 6 VDC SLA "motorcycle" type battery.
    If 4.2 is the ideal Voltage, I could maybe regulate the 6V to drop 1.8V.
    I agree re weakness - fair for 0.75 to 1mm PCB holes, but needed charge after a few holes.
    Sanding / cutting / grinding stall motor, as you say.
    An "Instructables" member said after chucking the useless cells and powering it (from 12V) it "nearly dislocated his wrist".
    I'll first test it briefly on 6V direct, then if OK, will design a PWM to suit V and I.
    My 650W Bosch pistol-drill works for larger shafts, but for <= 1.5mm, chuck does not grip bits.
    Currently need to cut & grind in small spaces.
    :cool: C
     
  5. dave9

    dave9

    733
    166
    Mar 5, 2017
    1.4V per cell is about what you can expect from NiCd or NiMH at full charge, then rapidly dropping to 1.3x-something and nominal 1.2V.

    If you have a 6V battery, I would just put a couple diodes in series to drop ~ 1.4V, test it, and if the motor isn't getting hot maybe swap (what I defaulted to as an assumption was a silicone diode) one of those to a schottky to drop less voltage, but of course this depends on what you have on hand and I keep forgetting... internet era, you can get an entire, adjustable buck or boost board for $1 delivered from China if Trump doesn't mess that up with tariffs. ;)

    I would ask that instructables member how long it ran like that. Clearly, there is no question that what Bluejets posted, cannot withstand that amount of torque for long and one way or another you'd destroy the tool.

    I'm split on my opinion. On the one hand I love to recover and reuse things. On the other as above I feel like you could just about do the the same amount of work by hand, but I am ignoring certain things like if you have arthritis... it's always easier on the body to have a tool do the work for you.

    However until the trade with China is harmed further, you can get an AC powered dremel-like tool that's AC line powered and some bits for about $20 ( I mean "I" can, it is difficult to make cost:benefit ratio suggestions when I don't know your product market well) and it would unquestionably be multiple times higher performing. It just seems like the tool you have is optimized for battery power and much work changing that, leaves no virtues to it.

    I may be biased. It annoys me how short the lifespan is of these dremel tool bits and I always reach for something else when possible. I discount their usefulness due to the high cost:material-removal rate.
     
    FuZZ1L0G1C likes this.
  6. FuZZ1L0G1C

    FuZZ1L0G1C

    365
    112
    Mar 25, 2014
    I have cancelled the "speed-matic" rotary tool upgrade now, as even on 6 V DC and (briefly) on 12 V DC, the motor's torque is very weak.

    Browsing our hardware store, there is a genuine "Dremel" tool, which may be stronger.
    Maybe the sales staff will allow an in-store pre-test...

    Alternatively, I've bought a chuck adapter for my electric drill, which goes down to 0.5 mm or less, gripping a HSS 0.5 mm drill-bit firmly.
    Another advantage of the shafted chuck adapter is effectively lengthening and narrowing the drill's original friction chuck.
    Allows access to deeper recesses.

    The other portable tool, which I am modifying to work off rectified regulated 12 V DC, at 6 A capacity, is a Bosch cordless screwdriver, not very high speed, but lots of torque available, very difficult to stall.
     
  7. bushtech

    bushtech

    885
    139
    Sep 13, 2016
    I bought a Dremel way back in the 80's and it's still going strong. No question that it's strong enough. And the cut off disks that you can get for it will go through your brass gear in two ticks as per 73's deEdd's suggestion.

    At one stage my neighbour lost the keys to the padlock on his sliding gate. His eyes nearly popped out of his head when I cut the shank off in two places in less than a minute.
     
    FuZZ1L0G1C likes this.
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