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My question surrounds a simple scenario.

I have a 12V DC power source connected to 2.2kΩ/10kΩ/22kΩ resistors in series.

For an assignment, I have theoretically calculated Voltage drops across all resistors, as well as respective current draw for each resistor. I am aware that the current entering a resistor should be the same as coming out.

I have simulated the circuit in Circuit Wizard (Uni assigned software package), and found that after each resistor the current drops fractionally. For example - The Total Circuit resistance is detailed as 350.97microA. Then: Through 2.2kΩ - drops to 350.92MicroA Through 10kΩ - drops to 350.88MicroA Through 22kΩ - drops to 350.86MicroA

Obviously - the difference between my calculated values and the simulated values is only around 0.02%.. However I wouldnt expect the current to change.

When I increase voltage on the power supply - to signifcantly larger values (50V+) - current measurements remain as expected.

Am I right in presuming that this is due to the impedance thats been calibrated into this digital Ammeter? Im drawing conclusions and this is the only thing I can think of.

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    \$\begingroup\$ In a series circuit there is only one current. There is no 'current going in' vs current going out', and it doesn't drop as at passes through a resistance. In your simple circuit, no matter where you 'measure' you should see 350.877uA. \$\endgroup\$
    – brhans
    Commented Sep 30, 2018 at 16:56
  • \$\begingroup\$ Yep - I just meant that 350.877microA through each resistive element wouldn't change the current. \$\endgroup\$
    – James
    Commented Sep 30, 2018 at 17:01
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    \$\begingroup\$ Simulators usually add a very high resistance between any node and GND to avoid (decrease) convergence issue. This could the cause of what you see. Gmin conductance is what is called this parameter in LT-SPICE, you might try to add it in your calculations and see if it makes sense. \$\endgroup\$
    – carloc
    Commented Sep 30, 2018 at 17:05
  • \$\begingroup\$ If you chose a single 34.2 kohm resistor what current does your sim tell you? \$\endgroup\$
    – Andy aka
    Commented Sep 30, 2018 at 17:26
  • \$\begingroup\$ @brhans It's perfectly fine to talk about the current at different points in a circuit, including the current entering an element and the current leaving the same element. You could insert an ammeter at any of these points and make a measurement, just as the OP did. Of course, for a given element those currents must have the same value and direction, and in a series connection the current in all elements must have the same value and direction, but there was nothing wrong with the language that the OP used. \$\endgroup\$
    – Elliot Alderson
    Commented Sep 30, 2018 at 17:28

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If the discrepancy was caused by some high resistance to ground that the simulator adds to aid convergence or because of the ammeter burden resistance then the same errors (as a percentage of the current) should occur even at higher voltages. Since the discrepancy did not occur at higher voltages I am suspecting some kind of numerical problem in the simulation calculations, perhaps as simple as roundoff error for very small values of current.

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  • \$\begingroup\$ This is more or less what I have come to conclude Elliot - I believe its discrepancies within the calculations. I would upvote your answer, however I am too new to be able to do so. But i appreciate your input \$\endgroup\$
    – James
    Commented Sep 30, 2018 at 17:48
  • \$\begingroup\$ I'd rule out ammeter burden, even if there were one it would be series connected, it should change the three readings by the same amount \$\endgroup\$
    – carloc
    Commented Sep 30, 2018 at 18:55

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