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# Electronics and Electrical Engineering Design Forum

## What is voltage across A and B?

Capacitor is fully charged to given volt in figure. Then v2 is applied. What can be Vab?

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1 year ago

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The voltage Vab will be (V + V1) + V2

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Neglect diode voltage. Assume its an ideal one. Now wat will be the p d across A and B

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I believe the most likely answer is V1 – Vdiode, as colin says – assuming 0.6V then
Vab = V1 – 0.6v as the capacitor blocks the DC voltage from. The meter will draw a small current therefore the diode will forward conduct – but not sure what the actual voltage drop will be across the diode.

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assuming v2 is initially off and the capacitor is at voltage (V+V1), then the voltage Vab = V2 + (V+V1) = V2 + Vdiode

if we assume V > 0, V1 > 0 and V2 > 0, then Vab = V2 + (V + V1) which is always greater than V1, so the diode never turns on.

at least this is what i’m thinking. seems like the problem has a few different cases of operation depending on the values of these voltages.

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Vab= (V+V1)+V2=V1-Vdiode

If; (V+V1)+V2 = 9V and

V1-Vdiode= 6V

then; Vab= 9V . Both are connected in parallel.

If; (V+V1)+V2=6V and

V1-Vdidoe=9V

then; Vab=9V

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