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Simple circuits -- flowchart that will help solve a series of unknown simple circuits

  1. Dec 4, 2014 #1
    I am trying to create a flowchart that will help solve a series of unknown simple circuit setups hidden within a box. It is powered by a 9v battery. There are 12 different probes that connect to 1 common ground and I want to have a method to discover what is contained. Options include 3 different resistors, a short(negligible resistance), a diode, 2 capacitors, an open circuit. 2 DC voltage sources and 2 ac voltage source. Using basic VOM meter and an oscilloscope how could identify each one?
     
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  3. Dec 4, 2014 #2

    sophiecentaur

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    Hi and welcome.
    That could be an interest one to solve (for you I mean!) It looks like a good exercise to help students in their first steps in problem solving and fault finding.
    The order of tests would not be absolutely critical but there will be optimal routes through the process. You could do without the Scope for all of those measurements - as long as you were to use appropriate values for the Capacitors.
    I guess you ought to start with Health and Safety (of the test meter at least) in mind - i.e. check whether the component is a source or sink of power (with the V setting) and use the AC/DC switch to determine which it is . So V or No V would be your first decision followed by the choice of AC and DC with the values found (on one branch).
    Then for the other branch, with the Meter on Ω, there would be a choice based on a delay in the meter reaching its final value (i.e. Capacitors present pr not?). The chains of decisions after that decision outcome could be in any order - selecting the measured value.
    To resolve the values of Capacitors, you would obviously choose capacitance values to make a significant difference in settling time.
    Just one comment on the 'flow chart' approach. I would encourage them to throw away the paper work after they have done the exercise, lest they rely on it, rather than learning how to approach such problems in principle.
     
  4. Dec 4, 2014 #3
    Thank you for your thoughts. Yes this is for students to help them learn the provided equipment and understanding of simple circuits. My only questions that im not certain on are would the ac/dc volts readings all be equal since they are all powered by a single 9 v battery and assuming wired similarly??

    Also, the capacitors are very small and the internal resistance of the VOM are large so damage is not likely but definitely an issue to be discussed. However, my concern in trying to notice the time delay in the charging or discharging of the capacitors is to make sure they start uncharged so we notice a time delay in charging them or vice versa. Is simply the current established through a voltmeter going to discharge them properly according to their capacitance w respect to time?
    Thank you so much for your help
     
  5. Dec 4, 2014 #4

    sophiecentaur

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    Won't you need an oscillator inside one of the boxes, if you want AC (???) Then the meter will be able to discriminate between AC and DC as the 'wrong' switch position will show (nearly )zero.
    If you want to be sure of starting with your capacitors discharged, it may be a good idea to provide a reset button, which students are told to press before each measurement. This could discharge the Capacitors via a 1k (say) resistor. When the Ω meter is connected to a discharged capacitor, it will start at near zero and then increase to infinite (O/L). The bigger the Capacitor the longer this will take. I imagine your "small" Capacitors are in the order of a fraction of 1uF. pF capacitors would not be suitable for such crude measurements as the leads and internal wiring could account for a few tens of pF. A good rule of thumb is that the time constant of 1mΩ and 1uF is 1 second. You could roughly deduce the resistance of your Ω meter by seeing how long a known Capacitor takes to 'nearly' charge - that would correspond to two or three time constant intervals..
    You should make them aware that connecting an Ω meter to a Voltage source can damage it (they may use Mains Volts one day!) so they should always measure Volts first, to check it is a passive device. (Part of the Flow chart - along with pressing the reset switch)

    Note- the meter should be switched off the Auto Range position if that is available.
     
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