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Running a DC motor from BS2px

Discussion in 'Electronic Design' started by QWERTY, Apr 26, 2007.

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    QWERTY Guest

    Hey group

    I have bought a small hobby motor with the following characteristics.

    Torque: Low
    Operating Volts: 1.5- 4.5V
    Nominal Volts: 3V
    Speed (no load): 12000rpm
    Current (no load): 0.28A
    Speed (max eff): 8540rpm
    Current (max eff): 0.69A
    Torque (max eff): 7.94gcm
    Size dia: 20mm
    Size length: 25mm
    Shaft length: 8.1mm
    Shaft dia: 2mm.

    I would like to run this of a Basic Stamp, BS2px in particular. The
    datasheet for the BS says that the I/O pins each can source and sink
    30 mA. Therefore I believe there is not enough current to run this
    motor direct from the ic.
    I only want to be able to turn the motor on and turn it off. It will
    be used to create vibrations. ie like a mobile phone.

    Is a mosfet the best way of isolating the motor from a chip or is
    there another simple solution?

    Thanks for any help
  2. Here is a document that might interest you:

    My suggestion is to start with a very simple circuit using a 2N2222 or
    2N3904 and a 5K base resistor. For maximum learning potential be prepared
    to cook a transistor or two, they're cheap. If you have access to an
    oscilloscope, that would be great since you could see the actual voltages
    showing up on your motor terminals. You will want to experiment with
    clamping diodes as well. Motors are great for learning the trials of
    connecting your "fragile" PIC to a real world inductive load.

    MOSFETs are great too. 2n7000 is a nice logic level input low-power MOSFET.
    For higher power, use one of the IRLxxx types as these have so-called "logic
    level" inputs. This means that 5V is usually enough to turn the MOSFET on
  3. A little more costly, but easier approach would be an Internationar
    Rectifier PVN012 Solid State Relay. Just put a 200ish ohm resistor in series
    with the LED, and you can switch AC or DC. Digikey has them for $5.85 in
    single qty. Considering the BS2PX runs about $79 each, I'd certainly prefer
    the electrical isolation of an SSR.

  4. ehsjr

    ehsjr Guest

    The suggestion to start simple is darn good, but needs
    a little more said. Those transistors are great for
    experimenting, cheap, and so what if you fry them.

    The transistors mentioned are not the best choices for
    connection to the stamp or PIC. So the experiemnts should
    initially be done without it. When he goes to the stamp/PIC/uC
    the 2N3904 & 2N7000 are way too small in terms of continuous
    collector/drain current, with absolute maximum ratings of 200 mA.
    He might squeek by with the 2N2222 for a short time before it
    overheats and dies, if the motor is loaded. If the transistor
    is connected to the stamp and shorts, the stamp potentially dies.
    A TIP33 would work well. It has sufficient gain, continuous
    Ic and power dissipation ratings to switch the motor current

  5. You're right, I should have checked first. I thought the 2N3904 would
    handle 500mA or so, oops.

    As for the 2N7000, I didn't intend for that to be interpreted as a
    recommendation for the motor. I only said it was a handy part for low power
    switching. ;-)
    On that I must disagree. Even if the transistor shorts to ground, the base
    resistor will limit the current protecting the STAMP.
    My personal favorite is a TIP120 for heavier loads, probably because radio
    shack used to carry them. They will handle the OP's current requirements
    easily and the off leakage is very small. Being darlingtons, they don't
    need much drive either (Hfe is above 1000).
  6. ehsjr

    ehsjr Guest

    Yabut - it's really a very minor oops - it could be a
    valuable learning experience, which you stated at the outset.
    Frying a few transistors - which you warned him of - can really
    make the lesson sink in.
    Don't get me started.
    I am drooling for the day when they make jellybean Mosfets
    that work well as switches with 1 volt on the gate - or
    (grudgingly) 3 v on the gate - and can carry heavy current
    with low Rds on. Give me some 10 amp, 6 cent, low Vg, <.1 ohm
    50 Vds etc Mosfets! Yeah, yeah, I know - you can get them at
    Unobtanium Incorporated.
    Yup, you're right. I forgot the base resistor.
    Good point. I like them, too, but you pay a Vcesat penalty with
    darlingtons. It (the penalty) might work real well for him in
    this case. He could use a 4.5 or 5 v supply, and the Vcesat would
    drop it near nominal.

  7. It's "only" about .6V at 1A. Logic-level MOSFETs are much nicer though IMO.
  8. ehsjr

    ehsjr Guest

    The datasheet shows Vcesat as .75, minimum. That
    moves his supply near enough nominal, even with
    a 5V supply, so the Vcesat "penalty" would work in
    his favor. His maximum is 4.5, and the darlington
    would lower the 5V to 4.25. The TIP120 would be a
    good choice. If he wants to go closer to nominal he
    can add a diode between the transistor and motor.

    The logic level mosfet is another good option with
    that series diode and a 5V supply.

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