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Circuit design

  1. May 10, 2009 #1


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    Gold Member

    I started to design a simple circuit and want to ask you what you think about it.
    http://img152.imageshack.us/img152/8508/circuit.th.png [Broken]
    Basically what I have is a motor that can be turned on with a switch and can run for 5 more seconds when the switched is turned off. The coil (electromagnet) can run for 5 seconds when the other switch is turned on. Since I don't need the timing to be precise I've decided to try the capacitor approach instead of a time delay relay since it's easier to implement (do you think that I should start with the relay system anyway?).
    I'm using a power supply of 5.7V and 800mA. So R3 =V/I = 7.125 ~ 10ohm. R1*C1 = R2*C2 = 5 so:
    R1 = R2 = 5Kohm and C1 = C2 = 1mF (10^-3).
    Did I do that right? Am I missing something important here? Would it be better to connect the motor and coil circuits to R3 in parallel instead of in a series?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 10, 2009 #2
    Are you expecting the motor to run at full speed for 5 seconds after the switch is opened? Is that realistic, given the way that a capacitor discharges?
  4. May 10, 2009 #3


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    I don't need it to run at full speed for 5 seconds, 1 or two will be enough - it'll be ok if it runs for more than 1 second and less than 5. Is that feasible with the capacitors?
    Any other suggestions?
  5. May 11, 2009 #4


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    Staff: Mentor

    You will not keep the motor running with the power off for anything like a few seconds. Instead, you should look into using a simple timing circuit to keep the power on for the motor for a couple seconds after the "off" switch is pressed.
  6. May 11, 2009 #5


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    It looks as if you are assuming your motor has negligible resistance compared with 10 ohms.
    This seems unlikely.

    I think it might be better to put the capacitance in series with the coil of a relay which has predictable resistance. The contacts of the relay could then be used to switch the motor on and off.

    There is another consideration. If a relay closes completely (magnetically), it only takes a very small current to keep it closed. The leakage current of electrolytic capacitors might be enough to keep it closed. You also have to have some way for the capacitor to discharge and this path might also keep the relay from opening again.
    You might have to put a bit of paper or insulation tape on the iron part of the relay that is attracted to the core of the coil, so that it can't close completely.

    Another, better, way is to use electronics to provide a time delay.
    A NE555 in monostable mode can switch a transistor to turn on a relay and switch on the motor.
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