# Diode 1N4002: voltage drop

Discussion in 'General Electronics Chat' started by Jan Luthe, Oct 18, 2015.

1. ### Jan Luthe Thread Starter Member

Jan 10, 2015
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Generally what I see on 'spec' sheets for voltage drop is: "max 1 volt@1 amp".
In my circuit running at 1.2vdc, 50ma, I measure a votage drop of about 0.2v. Is this correct? Does voltage drop change with current?

2. ### GopherT AAC Fanatic!

Nov 23, 2012
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Yes, there is a V/I curve for diodes. Look at the datasheet. it is not linear.

3. ### Jan Luthe Thread Starter Member

Jan 10, 2015
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Thanks GopherT. I'll try to find a data sheet. (when I said 'spec sheets' I meant the data that is listed on a site like digikey)
I looked at some data sheets and voltage drop seems to be about 0.6v @ 100ma. Would a measured voltage drop of 0.2v be reasonable for say 10ma?

Last edited: Oct 18, 2015
4. ### dl324 Distinguished Member

Mar 30, 2015
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According to this from the attached datasheet; forward voltage would typically be 0.6V @ 10mA and 0.75V @ 100mA.

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6. ### WBahn Moderator

Mar 31, 2012
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No, that is not reasonable at all.

For the Fairchild 1N4002 (https://www.fairchildsemi.com/datasheets/1N/1N4002.pdf), you see that the typical (as opposed to max) voltage at 100 mA is about 750 mV.

For diodes, a good rule of thumb is that the current changes by a factor of ten for a change in forward voltage of about 120 mV (this corresponds to an ideality factor of 2 near room temperature). As you can see from Figure 2, the voltage at 10 mA is about 600 mV, which is in pretty decent agreement with this rule of thumb.

As you can see, the current-voltage characteristic (on a semi-log plot) is roughly linear for lower currents. At higher currents other factors, such as parasitic resistances, start playing a more pronounced role.

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7. ### Jan Luthe Thread Starter Member

Jan 10, 2015
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Thanks WBahn. I'll double check in what I am doing/measuring.