# How to estimate the diode forward voltage drop by forward current?

Discussion in 'General Electronics Chat' started by shenhome, Oct 10, 2009.

1. ### shenhome Thread Starter New Member

Oct 10, 2009
3
0
I got the diode forward voltage drop in the datasheet, which is 3.5V@If=30A. I didn't get any profile about the current vs voltage drop in the datasheet. Now I would like to know how to estimate the drop voltage if If=10A is used in my design.

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Can you provide the part number of the LED?

hgmjr

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
3.5v @ 30A is a pretty hefty power dissipation; 105 Watts.

It would help a great deal knowing first the part number of the rectifier.
It would also help to know the voltage range you're dealing with. You may be better off using a different rectifier.

4. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
You can very roughly estimate it as being linear, starting at 0.7V for low current.

3.5 - 0.7 = 2.8V
/3 (for 10A) = approx 0.9
add back the 0.7 = 1.6V

So roughly 1.6V @ 10A for about 16W dissipation.

5. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
The specs can either be "typical" or "maximum". The data sheet usually has a typical curve showing VF versus current.

The formula for calculating the change in diode drop at two currents is:

delta VBE = .026 x natural log (I1 / I2)

In your case, going from 30A down to 10A only changes the VBE by about 30 mV. Not much. In reality, it may be more due to other effects.

6. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
I don't think this method will yield accurate values. Here is a VF curve for a high current silicon rectifier:

http://pdf1.alldatasheet.com/datasheet-pdf/view/81693/DIOTEC/1N3671.html

The VF @ 30A is 1.1V and at 10A it's about 0.92V. The curve is not close to being linear.

Do you have a data sheet for the part in question?

• ###### VF curve.jpg
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Last edited: Oct 11, 2009
7. ### Wendy Moderator

Mar 24, 2008
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Could be wrong, but I don't think he's talking about an LED.

8. ### bountyhunter Well-Known Member

Sep 7, 2009
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I'm not sure either. Based on the OP, it could go either way...

9. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Yes, it could very well be that the OP meant 3.5v @ 30mA instead of Amperes; and they want to try running it at 10mA.

That would make a lot more sense than a Vf of 3.5v for a rectifier diode.

10. ### shenhome Thread Starter New Member

Oct 10, 2009
3
0
I am talking about TVS diode from Onsemi, whose part number is P6SMB47AT3G. Maybe you would ask why I need Vf for TVS, but I do need it for some reason. I am asking data from supplier, but right now I don't get it. I would like to know if there is any method to estimate the Vf vs If.

Thanks a lot for your great help!

11. ### bertus Administrator

Apr 5, 2008
15,521
2,299
Hello,

Did you get hold of the datasheet?
Here it is attached.

Greetings,
Bertus

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