Finding voltage change for current change for diode

AI Thread Summary
To find the change in diode voltage when the current shifts from 0.1 mA to 10 mA, the equation V2-V1=VTln(I1/I2) is used, where VT is the thermal voltage. The calculated change in voltage is approximately 115.1 mV, assuming VT is 25.85 mV at room temperature. The initial voltage is often assumed to be around 0.7 V for calculations, but the problem only requires the change in voltage, not the final value. The discussion emphasizes that the saturation current (Is) is a constant dependent on the diode's physical characteristics, which is not provided in the problem. The conclusion is that the change in voltage can be accurately determined using the given parameters.
bl965
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Homework Statement


Find the change in diode voltage if the current changes from 0.1 mA to 10 mA. Ans. 120 mV

Homework Equations


V2-V1=VTln(I1/I2)

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.
 
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bl965 said:

Homework Statement


Find the change in diode voltage if the current changes from 0.1 mA to 10 mA. Ans. 120 mV

Homework Equations


V2-V1=VTln(I1/I2)

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.
But the problem statement only asks for the change in voltage, not the final voltage...
 
berkeman said:
But the problem statement only asks for the change in voltage, not the final voltage...
The only other equation in the section is i = IseV/VT. How would I find change in voltage?
 
What is the diode equation?
 
berkeman said:
What is the diode equation?
i = IseV/VT is the only equation and Is is not given.
 
bl965 said:
i = IseV/VT is the only equation and Is is not given.
I_s is a constant...
 
berkeman said:
I_s is a constant...
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.
 
bl965 said:
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.
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?
 
V2-V1=VTln(I1/I2) = 0.025 ln (10/0.1) = 115.1 mV ?
 
  • #10
bl965 said:
V2-V1=VTln(I1/I2) = 0.025 ln (10/0.1) = 115.1 mV ?
Looks good. The only thing I would change would be to use 25.85mV for Vt (at room temperature). :smile:
 

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