Diode limiter output - Expression for output voltage and comparision with Multisim

[siliconsurfer]

Hello,

This is a question about calculation of output voltage from a diode limiter.

I built the diode limiter shown below and simulated the output voltage in Multisim. The output voltage, Vout, is connected to channel A on the oscilloscope and shown with an orange trace. The input voltage is shown in blue.

To calculate the output voltage we use the expression

**broken link removed**

This is pretty close to the simulated value 9.014 V as seen in the image, i.e. oscilloscope channel A.

Does anyone know how the expression above is derived?




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[MikeMl]

It is the standard voltage divider equation. Only holds when the diode is not conducting.

 

[siliconsurfer]

Off course, voltage divider when the diode is not conducting, i.e.

**broken link removed**

1N914 is a silicon diode and according to the book the forward voltage is 0.7 V. But from Multisim it looks like to be 0.594 V, is this what you would expect?

 

[AnalogKid]

The expression is correct for the circuit *without* the diode, but not for the circuit as shown in the schematic. It can be derived using nothing but Ohm's Law.

ak

 

[AnalogKid]

It is within the normally expected range of values for this kind of diode. The instantaneous forward voltage drop in a diode depends on the specific diode junction construction, temperature, current, etc. The sim program opinion of the diode's Vf is just that, an opinion. Always remember that test equipment, simulation software, and other analysis/diagnosis tools *never* show you reality; they show you the tool designer's opinion of reality.

ak

 

[ronsimpson]

forward voltage is 0.7 V. But from Multisim it looks like to be 0.594

The forward voltage is dependent on current and temperature. (and what type of diode)
You have 10 volts and 10k so about 1mA.
Look at the graph. 1mA=0.6 to 0.65 (hard to see but some thing like that)

 

[crutschow]

....................
Always remember that test equipment, simulation software, and other analysis/diagnosis tools *never* show you reality; they show you the tool designer's opinion of reality.

It may be a "designer's opinion" but the models for most discrete semiconductor devices are generally quite elaborate and give a simulated value close enough to "reality" for most design work.
Of course the circuit always needs to be bread-boarded to verify how close the two really are before commiting the circuit to production.

 

[Ratchit]

Hello,

This is a question about calculation of output voltage from a diode limiter.

I built the diode limiter shown below and simulated the output voltage in Multisim. The output voltage, Vout, is connected to channel A on the oscilloscope and shown with an orange trace. The input voltage is shown in blue.

To calculate the output voltage we use the expression


**broken link removed**

This is pretty close to the simulated value 9.014 V as seen in the image, i.e. oscilloscope channel A.

Does anyone know how the expression above is derived?




**broken link removed**

In the above equation, you have RL equal to 10. It should be 100.

Ratch

 

[siliconsurfer]

In the above equation, you have RL equal to 10. It should be 100.

Ratch

Yes, I corrected this.

 

[siliconsurfer]

The forward voltage is dependent on current and temperature. (and what type of diode)
You have 10 volts and 10k so about 1mA.
Look at the graph. 1mA=0.6 to 0.65 (hard to see but some thing like that)
View attachment 93622

It's a bit difficult to read the x-axis, I used Plotdigitizer (http://sourceforge.net/projects/plotdigitizer/files/plotdigitizer/2.6.6/) and constructed the graph below. Why are there two curves, one hits x-axis at 0.5 (blue curve) and the other (red curve) at 0.54?


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[ronsimpson]

Why are there two curves

Min and Max value. The actual voltage will be in between the two curves. Each part is tested and must between the two lines to pass.

 

[siliconsurfer]

so, the way I understand this is that Multisim has some fairly complex model for diodes that contain the I-V data as discussed above.

Actually, clicking on the diode in Multisim there is a button "Edit model" that opens a window with a long list of various parameters, see image below. I have seen people on this board talking about SPICE models, is this the model that they are referring to?



**broken link removed**

 

[ronsimpson]

SPICE models, is this the model that they are referring to?

Yes.
There should be a different table for each diode.
Don't change things!
Some of the descriptions are easy to understand. Some not so.
Spice diodes do not break with over voltage or over current. I wish they did. I often see people using the wrong part and because the part does not break they think their circuit is fine. (not real life) Much like video games. LOL

If you use the parts correctly spice is pretty good.
It is possible to make your own parts but........not today!
If you have a spice Zener diode for 10 volts and another for 4.7 volts then it is easy to make a 6.8V diode by looking at the two you have and making one that is in between.