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**Physics, Astronomy & Cosmology / Re: What is the current and pd?**

« **on:**12/02/2021 14:34:33 »

Firstly, the usual rule for exam questions is that if you aren’t given information, it’s because you aren’t expected to use it. Secondly, considering the trivial nature of the exercise, and the non-triviality of solving it analytically with a diode characteristic, I’d consider that to be a hint, too. Thirdly, my experience of a career in electronics is that you rarely if ever get all the information you want, but the boss won’t be very impressed if you just throw your hands in the air and tell him it’s all hopeless, and there’s no point in doing anything. Generally, you’re expected to do what’s reasonable and defensible with the information you have, and to state what assumptions you have made, if any.

No manufacturer’s data sheet I’ve ever seen specifies the diode characteristic numerically as a polynomial, they will normally specify voltage at one or two currents and temperatures, and include a graph or two. Since the voltage across a diode is similar between types, and remains relatively independent of current, it is usual to assume the voltage is a fairly constant standard for most purposes. Indeed in many applications it would be considered bad design if the diode characteristic were critical.

Assuming a constant voltage drop across the diode leaves a simple exercise that can be solved by the application of Ohm’s and Kirchoff’s laws. Alternatively, a typical diode characteristic in graphical form can be used to solve the exercise by drawing the load line on the characteristic to find the operating point at the intersection of the two lines.

No manufacturer’s data sheet I’ve ever seen specifies the diode characteristic numerically as a polynomial, they will normally specify voltage at one or two currents and temperatures, and include a graph or two. Since the voltage across a diode is similar between types, and remains relatively independent of current, it is usual to assume the voltage is a fairly constant standard for most purposes. Indeed in many applications it would be considered bad design if the diode characteristic were critical.

Assuming a constant voltage drop across the diode leaves a simple exercise that can be solved by the application of Ohm’s and Kirchoff’s laws. Alternatively, a typical diode characteristic in graphical form can be used to solve the exercise by drawing the load line on the characteristic to find the operating point at the intersection of the two lines.

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