# Homework Help: Finding voltage change for current change for diode

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1. Jul 14, 2016

### bl965

1. The problem statement, all variables and given/known data
Find the change in diode voltage if the current changes from 0.1 mA to 10 mA. Ans. 120 mV

2. Relevant equations
V2-V1=VTln(I1/I2)

3. The attempt at a solution
That is all the information given. The only equation I can think to use is the diode voltage/current relationship, but a value for initial voltage is not given. The calculation assuming V1=0.7 V V2=0.025ln(10/0.1)+0.7 = 0.8151.

Thanks for the help.

Last edited by a moderator: Jul 14, 2016
2. Jul 14, 2016

### Staff: Mentor

But the problem statement only asks for the change in voltage, not the final voltage...

3. Jul 14, 2016

### bl965

The only other equation in the section is i = IseV/VT. How would I find change in voltage?

4. Jul 14, 2016

### Staff: Mentor

What is the diode equation?

5. Jul 14, 2016

### bl965

i = IseV/VT is the only equation and Is is not given.

6. Jul 14, 2016

### Staff: Mentor

I_s is a constant...

7. Jul 14, 2016

### bl965

From what i researched it depends on the diodes physical characteristics. The information above is all that is provided. I could not find a constant value for Is.

8. Jul 14, 2016

### Staff: Mentor

Just assume it is the same for the test diode for the two test currents. Can you write the two equations and solve them to get the delta-V?

9. Jul 14, 2016

### bl965

V2-V1=VTln(I1/I2) = 0.025 ln (10/0.1) = 115.1 mV ?

10. Jul 14, 2016

### Staff: Mentor

Looks good. The only thing I would change would be to use 25.85mV for Vt (at room temperature).