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I need help designing an overload protection circuit

  1. Jun 27, 2011 #1
    so I am creating a simple electric motor and I need a circuit that will cut the power if it is stalled (since it can create a short if it is stalled, draining/damaging the batteries) - the problem is that it also creates a short for a small period of time while in regular use

    I would like to avoid any complex integrated circuits if possible, as I plan on building this on a block of wood, not a fancy breadboard, and yes I know that the short is an inherent design flaw of the motor, but I would prefer to keep it simple

    measurements that may be helpful:

    when "shorted" (i.e. the coil is motionless and makes contact), the current is over 15 amps (max of what I can measure) from my 4 D batteries (at 6 volts), so they will drain fast

    when running, it draws over 10 amps at times, but I can't get a better reading as it spins rather fast and my needle has a fair bit of delay lag

    Ideally this circuit would cut the power after about 5-10 seconds of continuous flow, light up an "error" light, and then re-set itself when the coil was no longer making contact (i.e. it was spun by someone), and allow it to power-up at that time

    Attached Files:

  2. jcsd
  3. Jun 27, 2011 #2
    I assume your electronics knowledge is pretty basic judging by the paper-clip motor set-up (early physics lab).
    If not, what do you know/what are you familiar with?

    I would suggest reading about 555 timers - I believe the retriggerable monostable configuration would do a lot of what you want. You will need a current sensing circuit, but that is a pretty simple resistor with an op-amp circuit.
    http://www.doctronics.co.uk/555.htm#retriggering [Broken]
    Last edited by a moderator: May 5, 2017
  4. Jun 29, 2011 #3
    Can you check the current again?
    Anyway, have a look at resettable fuse - PTC here www.littlefuse.com

    The PTC will conduct with very low resistance (milli ohms) during normal current. When a short ckt occurs its resistance goes up (Mega ohms) and limits the current to a small value.
    Once the short ckt goes away, its resistance comes back to milli ohms.
  5. Jun 29, 2011 #4
    It's mostly my meter that is preventing me from getting a good measurement on the current.. I don't want to buy one for just this 1 project

    I am familiar with resistors, diodes, capacitors, inductors, and transistors (pnp and npn), and I was hoping to be able to get something using just those - it seems like that isn't possible...
  6. Jun 30, 2011 #5


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    Staff: Mentor

    To prolong the life of the dry cells you could replace them with rechargeables. :wink:

    Or you could add a resistance in series with the batteries to limit the max current that can be drawn. A piece of nichrome wire, or a suitable car headlight bulb could do. A consequence of this will be that your motor will operate more slowly. You should be able to compensate for this to a large extent by adding a large electrolytic capacitor across the paper clips, and it can supply the rotor's large pulses of current. (Make sure you observe the correct polarity of the capacitor's leads.)

    Your commutator action is not clear to me. Are the horizontal wires projecting from the coil i.e., the rotor's shaft) actually a pair of wires each?
    Last edited: Jun 30, 2011
  7. Jun 30, 2011 #6
    I want the motor to run at full speed with no resistor in its path normally..

    it's a coated wire where half of the coating has been taking off, such that as it rotates it makes/breaks contact
  8. Jun 30, 2011 #7
    A PTC fuse is no more complicated than a resistor.
    Try getting a few free samples from Littlefuse.com
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