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Current limitation circuit

Discussion in 'Electronic Design' started by OBones, May 24, 2005.

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  1. OBones

    OBones Guest

    Hi all,

    I'm considering to build my own PWM power supply for my mini drill. My
    first idea is to use a NE555 driving an N Type Mosfet and that seems ok.
    However, I would like to put some sort of "locking" protection into the
    circuit. If I'm correct, when the effort increases onto the drill, the
    current goes up, and can go very high if the rotor is blocked.
    So this condition can be detected, but I have no clues as to how to do
    that. Any leads are welcome.

    In the end, I'd like to get the functionnalities of this project:
    http://www.web-ee.com/Electronic-Projects/projects/pcb_drill/index.shtml

    but using more up to date components, especially Mosfet(s) instead of
    transistors to minimize the dissipated heat.

    Thanks a lot for your help.
    Olivier
     
  2. Tim Wescott

    Tim Wescott Guest

    A good way to detect that the motor is stalled is to monitor its
    current. The thing that damages the motor when it is stalled is its
    current. The thing that damages a motor that's _almost_ stalled is its
    current.

    So don't try to detect a stall, just limit the current!

    If you want to get fancy then let the motor run over stall current for
    1/2 second or so before reducing the PWM. This will let it do things
    like spin up or get momentarily stuck without shutting the whole thing
    down. You can get real fancy with this, but it's hard to do within the
    confines of analog circuitry -- and a PIC with a cheezy ADC is almost as
    small as a 555.
     
  3. Joerg

    Joerg Guest

    Hello Tim,
    Analog ain't that bad. It's possible with some cheap logic chip like a
    CD40106. I did that for an industrial angle grinder. It would let the
    user go through a tough spot for a second but when the load persisted
    would regulate it down to safe levels, all the way to "off" in a
    complete stall. The circuit also had a "soft start" so if would wind
    back up more gently. The formerly common rapid wind up from zero had
    placed a lot of strain on the wrists of users.

    Possibly something like that might help prevent injuries because a
    sudden and fast revving after getting unstuck can send stuff or the
    whole tool on a rampage.

    Regards, Joerg
     
  4. Tim Wescott

    Tim Wescott Guest

    But your counter circuit is probably the limits of what you would want
    with analog circuitry.

    If I somehow got a contract that was intended for you and needed to make
    a gazillion of the things for less than a penny apiece then I'd take
    your approach. If I needed ten or a hundred, or if I needed more
    features, I'd think of using the PIC.
    Nick Park's "Wallice and Grommit: A Grand Day Out"; the scene where
    Grommit drills a hole...
     
  5. OBones

    OBones Guest

    Hey, I like that approach. Are you allowed to post a schematic of that
    part of the project?
     
  6. OBones

    OBones Guest

    Yeah, but for my project, it feels like using a hammer to splat a fly...
     
  7. Tim Wescott

    Tim Wescott Guest

    Microprocessors are like that. But they're also often the smallest,
    least-cost way to go for a surprisingly large (and growing) set of problems.

    If you can do it reliably with analog circuitry, meet all your
    performance, price and manufacturability goals _and_ not be too big then
    that's the way to go.
     
  8. Joerg

    Joerg Guest

    Hello Tim,

    Most of my stuff is in the gazillion category. Maybe that's how I got to
    be a cheapskate when it comes to electronics. Other times I don't like
    simple apps to return "the program has performed an illegal
    operation...". But that's rare nowadays with uCs, which is why I started
    looking at the MSP430. Before it ends up in a design though they have to
    make good on their promise "starting at 49 cents". At the current Dollar
    something a pop, no chance.

    Hey, I splatted a fly with a wood splitter a couple weeks ago. It got
    sucked into the intake of the engine.
    Problem is, often that's only financially feasible with Chinese
    four-bitters. Anything eight bit seems to be 30c or more.

    Regards, Joerg
     
  9. Joerg

    Joerg Guest

    Hello OBones,
    Unfortunately not. Also, this was not for DC but for 240V AC. Anyway,
    all you need to do for jobs like this is figure out what kinds of ramps
    and delays you need. Then take a Hex-Schmitt and create those. "Poor
    man's logic" with dual-diodes and transistors does the interaction. Most
    of the time I don't even need all six inverters.

    Regards, Joerg
     
  10. Tim Wescott

    Tim Wescott Guest

    For you.

    Most of my customers so far build around 100/year/board and would like
    to be able to do field upgrades or at least have flexible manufacturing
    capabilities; for them the engineering time makes a big factor in the
    amount they have to charge per board. Add that to the fact that with a
    digital system you only have to sweat the precision of the ADC and DAC
    circuits and microprocessors start to look very attractive.
     
  11. Joerg

    Joerg Guest

    Hello Tim,
    Yes, for 100qty the uC makes perfect sense. Sometimes even for large
    qties. Like a strobe that has to double flash at very precise intervals.
    A uC with a versatile timer can run circles around an analog solution
    for such designs.

    Regards, Joerg
     
  12. OBones

    OBones Guest

    I agree with that, but thing is, this is a once in a lifetime project
    (most probably), and this would require me to have a PIC programmer,
    which I don't have. I could build one (I have the schematics), but it
    increases the costs for something that I consider to be simple.
    I don't need high precision, just some sort of a protection. Thanks for
    the help, I appreciate it.
     
  13. OBones

    OBones Guest

    Ok, that's what I thought, NDA and all that stuff coming into play. I'll
    look around and think hard about it.
    Thanks.
     
  14. Luhan Monat

    Luhan Monat Guest

    For the current limit function, I like 10:1 self-adjusting resistors.
    These are sold in many venues marketed as 'light bulbs'. A 100 watt
    bulb has a cold resistance of about 10 ohms, goes to about 100 ohms hot,
    and, oh yea, dissipates 100 watts if necessary.
     
  15. Joerg

    Joerg Guest

    Hello OBones,
    In case you want to learn about micro controllers, and I guess only then
    it would make sense: Check out the Texas Instruments MSP430 series. They
    can be programmed via an RS232 protocol and the interface is rather
    simple. Of course, you'd also have to make sure your computer still has
    one of those. Unfortunately many "modern" laptops don't.

    If you go that route the MSP430F1232 is a pretty decent one because it
    has ADC capabilities.

    Regards, Joerg
     
  16. ehsjr

    ehsjr Guest

    Put a current limiting chip/circuit between the DC supply and
    the mosfet. For complete turn off if too much current is
    drawn (such as would happen in a rotor locked condition), add
    a double pole relay and a pushbutton. One set of contacts
    on the relay is in series with the supply, and, when the relay
    is energized, connects the relay coil to the output side
    of the current limiter. The other set of contacts is in series
    with the input to the current limiter. The pushbutton momentarily
    connects the relay coil to the power supply. When too much current
    is drawn, the current limiter drops the voltage and the relay
    drops out, de-energizing everything. I'll try a drawing:
    __
    +--o o-------------------------+---relaycoil---+
    | | |
    | +--o/ o--+ |
    | | |
    + --+--o/ o--+--Limiter----+-----mosfet-----motor---+
    | | |
    +--------------------PWM---------------+
    |
    - --------------------------------------------------+

    Ed
     
  17. OBones

    OBones Guest

    Oh but I know about microcontrollers, I used them when I was a student,
    two years ago. But I don't have access to the uni resources anymore...
     
  18. OBones

    OBones Guest

    Thanks.
     
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