# Voltage drop across diode



## wvgca (Jan 21, 2013)

for some reason i thought the voltage drop was 0.7 volt across a diode, [standard silicon, not tunnel or germanium]... in some cases it is, in some , notably a 1n4004 at over 3 amp current, it went to over a volt ??
how come ??


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## gunrunnerjohn (Nov 10, 2010)

.7 is just a nominal value, and the drop will go up with increased current. In addition, each diode type will have a slightly different voltage drop curve. It's not chiseled in stone that it'll be .7 volts.


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## Dennis461 (Jan 5, 2018)

All diodes are non-linear devices
Forward voltage drop is a function of current.
Here is a chart for the 1N4004, and should be similar for many manufacturers.


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## wvgca (Jan 21, 2013)

okay, good to know how the variation applies


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## daschnoz (Dec 12, 2016)

How are you pulling 3+ amps through a diode rated for 1A max - without letting the smoke out of the diode?


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## wvgca (Jan 21, 2013)

daschnoz said:


> How are you pulling 3+ amps through a diode rated for 1A max - without letting the smoke out of the diode?



carefully ...


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## Dennis461 (Jan 5, 2018)

daschnoz said:


> How are you pulling 3+ amps through a diode rated for 1A max - without letting the smoke out of the diode?


Heat shrink tape over the diode.
The hotter it gets, the more it shrinks.
Pushes smoke back in.

And, since I am a Professional Engineer, where do I send my bill?


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## wvgca (Jan 21, 2013)

^^^^^^ I like that !


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## gregc (Apr 25, 2015)

daschnoz said:


> How are you pulling 3+ amps through a diode rated for 1A max - without letting the smoke out of the diode?


don't auto reversers and frog juicers require high currents to work?

if a diode is rated at 1A and an auto reverser can switch in 1msec, can't the diode effectively pass 1000A in the 1 msec?


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## Lemonhawk (Sep 24, 2013)

Great idea about using shrink tubing, I'm sure it will work And send the bill to wvgca, but wait till the harvest is in!


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## wvgca (Jan 21, 2013)

The diode is rated 1 amp continuous ... higher for short periods of time .... in the example that dennis461 gave, a 1N4004 diode passed 10 amps, but with only a 300us pulse width..


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## jimben (Jun 27, 2018)

Schottky dfiodes rated up to 40 volts PIV drop as low as ,45 volts. You did not say half wave or bridge rectifier and this makes all the difference. So, what do you have?

In a bridge rectifier voltage goes thru two diodes at same time, thus 1.5 volt drop is normal. 1 amp diodes carry 2 amps in a bridge with your rectifier. 3 amps is not good in a bridge. So, change out to 2 or 3 amp schottky in a bridge rectifier and get under a volt drop.

If half wave rectifier, use at least a 3 amp schottky diode for lower voltage drop.


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## wvgca (Jan 21, 2013)

in the first post it was mentioned a [single] diode, just an ordinary silicon one, in this case a 1N4004 ..
no schottky, no bridge,


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## jimben (Jun 27, 2018)

wvgca said:


> in the first post it was mentioned a [single] diode, just an ordinary silicon one, in this case a 1N4004 ..
> no schottky, no bridge,


Poster was talking about one diode voltage drop, does not refer to what circuit being half wave (one diode) or bridge (four diodes total), I know what a 1N004 is, schottky was an option I mentioned and 1N4004 are not a schottky obviously.


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