Help with Diode Circuit with Current/Voltage through Diode

In summary, the circuit shown in Fig. 4.26 has two identical diodes and a resistor, with a current source of 5mA going to each diode. The goal is to find the value of R that results in a voltage of 80mV. This can be solved using the diode equation, Ohm's law, and Kirchoff's current and voltage laws.
  • #1
ischmob
3
0

Homework Statement



For the circuit shown in Fig. 4.26 both diodes are identical. Find the value of R for which V=80mV

Homework Equations



I'm not quite sure what equation to use here. If you look at the wiki page on diodes http://en.wikipedia.org/wiki/Diode
it might be Shockleys current equation

The Attempt at a Solution



My thought on how to approach this problem is find the current that comes out of the diode and then use that current in a voltage divider using 80mV to find a value of R. Though I have no clue what is coming out of the diode. Though I know 5mA is going to each diode.[URL]http://img218.imageshack.us/i/fig426.png/[/URL]
 
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  • #2
Hi,
The figure is not visible. Try uploading again.
 
  • #3
Can you upload Fig. 4.26?

Without seeing the circuit I can't say for certain, but you may be able to use the diode equation if you know the leakage current, temperature etc. Or you may be able to solve graphically with a 'load line' if you have a plot of the diode characteristic.
 
  • #4
thanks for your guys prompt responses. here's another attempt at the figure upload

http://img218.imageshack.us/i/fig426.png/" [Broken]

[URL]http://img218.imageshack.us/i/fig426.png/[/URL]
 

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    Fig 4.26.png
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  • #5
Just a wild guess. Might be violating the laws of electricity with this answer. :) Is it 8 ohm?

I assumed the diodes to be ideal.
No current flows through the diode D2 as there is no voltage drop across it.
10mA flows through D1.
So R= 80mV/10mA = 8 ohm
 
  • #6
so your saying that no current will flow through D2. And as far as your answer. I think its a little more complicated than that. I'm sure there's some sort of formula you need to plug in
 
  • #7
asp_physics said:
Just a wild guess. Might be violating the laws of electricity with this answer. :) Is it 8 ohm?

I assumed the diodes to be ideal.
No current flows through the diode D2 as there is no voltage drop across it.
10mA flows through D1.
So R= 80mV/10mA = 8 ohm

Sorry, but that's just not true. There is current through D2 and D1, since the 10mA current must be shared between D1 and D2, and thus there is a voltage drop across both D1 and D2.

It's not an easy problem, actually, since there are two nonlinear elements. You can write down a few different equations though:

The diode equation (Shockley's) for D1 and D2.
Ohm's law across the resistor.
Kirchoff's current law at the node between the diodes and the current source.
Kirchoff's voltage law around the loop at bottom (that goes from ground, through the diodes and resistor back to ground).

That's 5 equations, though 3 of them are quite trivial, and after some substitution you can solve for R given V = 80mV (I believe).
 

1. How does a diode circuit work?

A diode is a semiconductor device that allows current to flow in only one direction. When the voltage across the diode is positive, it conducts electricity and allows current to flow through. However, when the voltage is negative, the diode blocks the flow of current, acting as an insulator. This characteristic makes diodes useful in many electronic circuits, such as rectifiers, voltage regulators, and signal limiters.

2. What is the purpose of a diode in a circuit?

A diode is used in a circuit to control the direction of current flow. It can be used to convert alternating current (AC) to direct current (DC) by only allowing current to flow in one direction. Diodes are also commonly used to protect electronic components from damage caused by reverse current flow.

3. How do you calculate the current through a diode?

The current through a diode can be calculated using Ohm's Law, which states that the current (I) is equal to the voltage (V) divided by the resistance (R). In the case of a diode, the resistance is not constant and varies with the voltage. Therefore, the diode current can be calculated using the diode equation: I = Is(e^(Vd/nVt)-1), where Is is the reverse saturation current, Vd is the voltage across the diode, n is the ideality factor, and Vt is the thermal voltage (approximately 25 mV at room temperature).

4. What is the difference between current through a diode and voltage across a diode?

The current through a diode is the amount of electrical charge flowing through the diode per unit time. It is measured in amperes (A). On the other hand, the voltage across a diode is the potential difference between the anode and the cathode of the diode. It is measured in volts (V). The current through a diode is dependent on the voltage across it, but the voltage across a diode is not dependent on the current through it.

5. How do I choose the right diode for my circuit?

The right diode for your circuit will depend on the specific requirements of your circuit. Some factors to consider when choosing a diode include the maximum forward current and voltage ratings, the switching speed, and the reverse breakdown voltage. You should also consider the type of diode, such as a standard rectifier diode, Zener diode, or Schottky diode, depending on the function you need it to perform in your circuit.

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