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RPM of small DC motors

Discussion in 'Electronic Basics' started by John Riley, Apr 17, 2006.

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  1. John Riley

    John Riley Guest

    I have an automotive DMM with an inductive clamp to clamp a spark plug lead
    to measure RPM of an internal combustion engine.
    I want to measure the RPM of some small DC motors with and without
    gearboxes. Range up to about 3000 rpm.
    I was wondering what would be the simplest way to do this. Perhaps a tiny
    strong magnet attached to a tooth of the sprocket I want to measure?
    I was thinking about a small coil near the spinning magnet and clamping the
    wire in this continuous loop, but then I wondered if I set the sprocket up
    to rotate between the open jaws of my DMM's induction clamp, would this
    likely give me a result? I realise that I should try this to see, but as I'm
    not able to get to an electronics shop to get magnets etc for several days,
    I want to try to find out more of what I should buy when there. Someone
    suggested a Hall Effect switch, but I have no idea how to set this up for my
    meter to read RPM. Thanks in advance for any help in this area.
     
  2. Chris

    Chris Guest

    Hi, John. If you need to measure with an inductive clamp, you're
    probably going to need at least 100mA, and possibly twice that, going
    through that clamp in order to get a reading.

    Also, a magnet will be an eccentric load, which might not be great for
    your motor bearings.

    The best way to do this might be to make a small card stock disk with a
    good sized slot in it, mount it to the motor shaft, and use an
    opto-interrupter to drive a high current transistor switch. You can
    then put the inductive clamp around the high current wire, and you're
    good to go, without having to wait for the magnets. Of course, you
    might be able to get a fast hall effect switch to drive a high current
    transistor switch, too.

    To make it simple, order an H21LOI opto-interrupter (Mouser 512-H21LOI,
    $1.89 in stock) and a TIP105 PNP Darlington transistor (Mouser
    511-TIP105, $094 in stock) and use the [email protected] or greater wall wart
    you've avoided throwing out. Put together something like this (view in
    fixed font or M$ Notepad):

    |
    | VCC
    | +
    | |
    | .-. VCC
    |470| | +
    | | | |
    | '-' .-. VCC
    | | VCC 1K| | +
    | '-------. + | | |
    | 1| |3 '-' |
    | | | \ ___ | |<
    | V ~ | H )--|___|-o--| TIP105 (PNP Darlington)
    | - ~ |__/ 4 1K |\
    | 2| |5 |
    | | === |
    | | GND | Inductive
    | === |<---Pick-off
    | GND | Here
    | |
    | .-.
    | | | 33 ohm
    | | | 2 watt
    | '-'
    | |
    | ===
    | GND
    |
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Just have the wire extend off your protoboard, and clamp around it.
    Start out with a small slot in the disk, and open it up if it's too
    brief a pulse for your meter at higher speeds.

    The H11 and H21 series of optocouplers is a great choice for contrivers
    of contrivances, because they have built-in hysteresis to square up
    iffy signals, and open collector outputs that can easily sink 10mA if
    you need it. Be sure to use a pullup if you require logic level, just
    like with any open collector output. These ICs work on a 5 to 15V
    supply.

    Good luck
    Chris
     
  3. BobG

    BobG Guest

    If you have a scope (most new pc usb scopes have a freq button!) the
    opto technique cant be beat. Even with an old scope, the period is div
    x time/cm
     
  4. John Duffus

    John Duffus Guest

    Hi John,
    My first inclination would be to use a stroboscope if you can get your
    hands on one. I see some on eBay.
     
  5. D. Akers

    D. Akers Guest

    John wrote:
    "I have an automotive DMM with an inductive clamp to clamp a spark plug
    lead to measure RPM of an internal combustion engine. I want to measure
    the RPM of some small DC motors with and without gearboxes. Range up to
    about 3000 rpm." <snip>
    ______________________________________
    Re;
    Just a reminder: if you do get the automotive "inductive-spark" tach to
    work, don't forget you'll need to divide the indicated RPM by two to get
    the actual motor RPM since 4 cycle engines only spark on every other
    revolution.

    Good Luck,
    Dan Akers
     
  6. Phil Allison

    Phil Allison Guest

    "D. Akers"

    ** Err - that would be *multiply* the indicated rpm by 2.

    Plus beware - some 4 cycle engines spark every revolution.



    ........ Phil
     
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