Sunday 8 November 2015

voltage measurement - KVL not satisfied during simple diode series


A power diode in the reverse biased position is connected in series to a 2.5Mohm resistor. The voltage source is connected in series to these two components and is set at 10V dc. The voltage across the diode is 7.5V while the resistor is ~0V. Where did the "missing" voltage go?



Answer



It seems your voltmeter has an input resistance of 7.5M ohms, that would give the results you see.


Each time you connect to the circuit you are placing that resistance in parallel with the component.


So when measuring across the diode 1uA will flows and you see the voltage divider action.


But when measuring across the 2.5M resistor the diode stops all current, so that is why you see 0v.


Try modeling the equivalent circuit.


(Credit to Photon as he hinted to this in the first comment).



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