Designing a Simple Circuit - Ask for Feedback!

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Discussion Overview

The discussion revolves around the design of a simple circuit involving a motor and an electromagnet, focusing on the use of capacitors for timing instead of a relay system. Participants explore the feasibility of the proposed circuit design and seek feedback on its components and functionality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes a circuit design using a motor and an electromagnet, proposing to use capacitors for timing instead of a relay due to ease of implementation.
  • Another participant questions the expectation of the motor running at full speed for 5 seconds after the switch is opened, suggesting that this may not be realistic given capacitor discharge characteristics.
  • A participant clarifies that they do not need the motor to run at full speed for the entire duration, indicating that running for 1 to 2 seconds is sufficient and inquires about the feasibility of achieving this with capacitors.
  • Another participant suggests that the motor's resistance may not be negligible compared to the circuit's resistance, proposing that capacitance should be placed in series with a relay for more predictable behavior.
  • Concerns are raised about the potential for leakage current in electrolytic capacitors to keep a relay closed, along with suggestions for insulation to prevent unintended closure.
  • A later reply proposes using a NE555 timer in monostable mode to provide a time delay for switching the motor on and off, indicating a preference for a more electronic approach to timing.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of using capacitors for timing in this circuit design, with some advocating for a relay-based approach and others suggesting electronic timing solutions. The discussion remains unresolved regarding the best method for achieving the desired timing functionality.

Contextual Notes

There are uncertainties regarding the assumptions about the motor's resistance and the behavior of capacitors in the proposed circuit. The discussion also highlights potential issues with relay operation and the need for proper discharge paths for capacitors.

daniel_i_l
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Hi,
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
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?
Thanks
 
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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?
 
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?
Thanks
 
daniel_i_l said:
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?
Thanks

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.
 
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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