# Questions on P-N junction circuits

Hi

I need circuit diagrams for studying P-N junction & Zener Diode under forward and reverse bias. I know they are available on Google, but I had a few questions, and if I understand the answers, I won't need circuit diagrams.

1. a) Some circuit diagrams have a rheostat connected in series with the diode and some have it in parallel. What is the difference ? (b) In parallel type circuits, is the logic behind placing a rheostat same as in series (In case you short-circuit some components, current would be limited) ?

2. If you want to study a diode under reverse bias, what changes would be necessary ? Can you just flip the diode, and the ammeter, and the voltmeter ?

3. Will the same diagram work for a zener diode, both under forward and reverse bias, or do I need a different diagram for that ?

4. In our physics lab, we have a box type of thing. Everything is located inside the box, and we're provided with sockets at the end of each component. We are just supposed to join the wires correctly.

What confuses me is that there are two different diodes for forward and reverse bias in that box. So if you need to study forward bias, you use diode 1, and for reverse bias, diode 2. What is the logic behind this ? Why can't one just reverse the diode 1 to use it in reverse bias ?

gneill
Mentor
Are you looking for equivalent circuit models for the two different types of diode, or test setups for them? Do you know, in general, what the current versus voltage characteristics should look like for both types of diode? (I presume that you're being given a standard diode and a zener diode to play with).

Hi

1. a) Some circuit diagrams have a rheostat connected in series with the diode and some have it in parallel. What is the difference ? (b) In parallel type circuits, is the logic behind placing a rheostat same as in series (In case you short-circuit some components, current would be limited) ?

There is no real difference in the way the diode function when the rheostat/resistor is in parallel or series with it. Usually, the placement of the resistor is circuit dependent and is based purely on the what the circuit intends to do.

In LED's, you place a resistor in series to limit the current going into the diode to within safe limits. I think, by and large, diodes are cheap, so not a lot of money is spent on protecting them. This is more important where thyristors are concerned (this of them as souped up diodes).

Usually, if there are current and voltage limits to be observed, we'll use resistors etc to limit them to within safe values.

2. If you want to study a diode under reverse bias, what changes would be necessary ? Can you just flip the diode, and the ammeter, and the voltmeter ?

Yeah, you can do that, just keep the applied voltage to below reverse breakdown values. If you want to study the diode under reverse breakdown conditions, then you need to use a zener diode. A normal diode will be destroyed after reverse breakdown. They're not designed to work under those conditions.

3. Will the same diagram work for a zener diode, both under forward and reverse bias, or do I need a different diagram for that ?

Yeah, it should work for the zener. Again, make sure all the current and voltage values are within safe limits.

4. In our physics lab, we have a box type of thing. Everything is located inside the box, and we're provided with sockets at the end of each component. We are just supposed to join the wires correctly.
What confuses me is that there are two different diodes for forward and reverse bias in that box. So if you need to study forward bias, you use diode 1, and for reverse bias, diode 2. What is the logic behind this ? Why can't one just reverse the diode 1 to use it in reverse bias ?