help me quickly: diode curves question

First of all you need to understand that a diode doesn't have "resistance" as such - it's a diode, not a resistor. All you can meaningfully say about it in terms of resistance is that it has a characteristic called "small-signal resistance", also called "incremental resistance", "dynamic resistance" and "AC resistance".

If your teacher is asking this question, you should have been taught about this. If you haven't, you will need to Google it.

That graph plots voltage against current, right? If you do that for a resistor, what is the shape of the graph? And how does it compare to the diode graph?

Can you match parts of the diode's graph to parts of a resistor graph?

 

For each part having a non-linear current-voltage characteristics you must discriminate between STATIC and DYNAMIC resistance.
The static resistance is simply the ratio of DC values (Rst=V/I for a certain operating point) and the dynamic (differential) resistance is r,dyn=v/i.
As mentioned above (post KrisBlueNZ) the latter is the incremental resistance (applicable for for small signals only) which is identical to the inverse slope of the given curve.
Similarly, the static resistance is the inverse slope of the line between the origin and the corresponding operational point on the current-voltage characteristic. For ideal resistors both resistances are indentical.

 

LvW, this is the homework forum. That means we don't give full answers; we encourage users to do their due Google diligence and only ask about specific details if they get stuck.

 

Have you Googled diode incremental resistance? Or diode dynamic resistance?

The curve shows you the relationship between two quantities, for a diode. What are those quantities?

How do you calculate resistance from those two quantities?

To calculate the dynamic or incremental resistance at a certain area of the line on the graph, you approximate that part of the line to a straight line, and use the difference in distance on the two axes to calculate the slope of the line at that point. From these two figures, dV and dI, you can calculate the incremental or dynamic resistance. What formula would you use for this?

What happens if you take two different sections of that curve, approximate them to straight lines, and calculate the equivalent resistance for those sections? Do you get the same resistance each time, or different resistances?

Is it meaningful to say that a diode has a certain AC resistance, without giving any other information? What other information do you need to give?

 

Because there are two types of diodes: a 1N4001 and a 1N5819. The 1N5819 is a special type of diode. What type of diode is it?

 

Kris - sorry, if I have explained too much. On the other hand, was it really remarkable more detailed than your answer? (.. it has a characteristic called "small-signal resistance", also called "incremental resistance", "dynamic resistance" and "AC resistance".)

 

LvW, you're right in this case. Hints didn't seem to be working. I don't like spoon-feeding users in the homework forum but sometimes they just don't seem to be able to think for themselves. And in such cases, even spoon-feeding them doesn't always enable them to answer their question!

But in general, the policy for the Electronics Homework Help forum is spelled out in the heading of the forum list: "Get assistance with your electronics homework and assignments in this section. No answers will be given, only guidance."

 

In the future, I will do my very best to follow the guide lines.

 

Thanks LvW. You are a valued member here.

 

If the curve is to be treated as "piecewise linear" then for each piece of the curve the diode can be replaced by a resistor and a voltage source. But how does one know which piece of the curve to actually use? All I can imagine is to make an assumption on where to start, find the dynamic values for each diode at that operating point, replace each diode in the circuit with its dynamic equivalent, and analyze the circuit to find the current through each diode. Now you have a new operating point on each diode curve. Repeat as many times as necessary. Hopefully the solution will converge.

By way of example, the instructions suggest assuming that the current will divide equally through each diode. So I found the dynamic resistance and voltage from the 0.4A point on each diode curve. See attached image. I wonder how many iterations will be necessary?