# Voltage drop across diode



## pagosarr (Jan 5, 2008)

I'm working on some experimental circuitry which will include diodes. I thought the convention wisdom was that one would experience a voltage drop of 0.7v across a silicon diode. So I ordered some diodes from AllElectronics ( cat# 1N5406S) but I am getting a voltage drop of approx 0.25-0.25v. Can anyone explain to me why I didn't get a 0.7v drop? Perhaps the diodes I ordered are not silicon. They are considered rectifier diodes so I assumed they would be silicon. Thanks for any help.

Roger Bush


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## toddalin (Jan 4, 2008)

Are you testing the voltage drop with some load on the system? Can make a difference.


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## TonyWalsham (Jan 2, 2008)

They would be Schottky diodes. Usually a bit more expensive than regular silicone diodes.


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## kormsen (Oct 27, 2009)

Posted By TonyWalsham on 03 Jan 2011 05:59 PM 
They would be Schottky diodes. Usually a bit more expensive than regular silicone diodes. 

and which ones have the 0.7 drop? the regular ones?


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## George Schreyer (Jan 16, 2009)

A silicon PN junction diode will have about 0.7 volts forward voltage drop. This would be typical of the 1N4000 family. This depends a little on the current, but 0.7 volts is close enough. A silicon Schottky diode uses a different junction structure. Their drop is closer to 0.25 volts. They are usually used for high efficiency power supply rectifiers. The downside is that a Schottky diode tends to leak more and they aren't available at really high inverse standoff voltages.


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## kormsen (Oct 27, 2009)

thanks.


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## pagosarr (Jan 5, 2008)

Many thanks for the informative replies. 

Would it be correct to assume that any diode in the 1N4XXX family would produce a 0.7v drop across the diode? 

I think I detect from the replies that a silicon diode could well be a: 

1) rectifier diode 
2) a Germanium diode 
3) a Schottky diode 
4) or a Zener diode 

As an additional question - while I have not as yet constructed a bridge rectifier on my own - if I were to do so, which diode would I select? 

Again, thanks for the informative replies. 

Roger Bush


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## Semper Vaporo (Jan 2, 2008)

Posted By pagosarr on 03 Jan 2011 07:16 PM 
Many thanks for the informative replies. 

Would it be correct to assume that any diode in the 1N4XXX family would produce a 0.7v drop across the diode? 

I think I detect from the replies that a silicon diode could well be a: 

1) rectifier diode 
2) a Germanium diode 
3) a Schottky diode 
4) or a Zener diode 

As an additional question - while I have not as yet constructed a bridge rectifier on my own - if I were to do so, which diode would I select? 

Again, thanks for the informative replies. 

Roger Bush 

Um... there is a Schottky diode. invented by German Physicist Walter Schottky; characterized as having a very low voltage drop across it.

A Shockley diode is the semi-conductor equivalent of a Thyristor.


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## Greg Elmassian (Jan 3, 2008)

1 yes 
2 no 
3 yes 
4 maybe, but anyone that would call a zener just a diode, get a different person to give you advice. 

buy a bridge as a single unit... 

reg


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## krs (Feb 29, 2008)

Posted By TonyWalsham on 03 Jan 2011 05:59 PM 
They would be Schottky diodes. Usually a bit more expensive than regular silicone diodes.


Tony -

What makes you think the 1N5406 is a Schottky diode?
The "S" suffix?

I can't even find a 1N5406S diode with google, only the 1N5406 diode and that is a standard silicon rectifier diode rated at 3 amps and 600 volts.
All Electronics also lists tis diode as a 1N5406 on their website, no "S" suffix. 


To me it sounds as if the 0.25 voltage drop measured is a measurement issue - if there is very little current flowing through the diode one will measure a lower drop than expected.

To get a 0.7 volt drop for this particular diode tere needs to be a current flow of around 500ma.

I would think another test is in order if the voltage drop is important.
Otherwise this is a perfectly good rectifier diode if that is the intended purpose.

Knut


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## Greg Elmassian (Jan 3, 2008)

I have not found the part with the S on the end, but there are a LOT of variations.

Could be a schottky.

Have you verified the accuracy of your meter? 


Greg


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## krs (Feb 29, 2008)

Different suffixes typically indicate different packaging or SMD devices or reel-packaged for automatic insertion machines. 

I can't remember ever coming across a JEDEC code where the suffix indicates a different type of semi-conductor, one finds that only with manufacuring codes like the Motorola series of CMOS devices using the standard TTL logic device designation with an MC in front of them to indicate CMOS instead of TTL. 

Question to Roger - Does it actually say 1N5406S on the diode itself? 
And who is the manufacturer? 

That should allow us to identify the device. 

I would also like to know at what current the voltage drop was mrasured.


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## George Schreyer (Jan 16, 2009)

1) rectifier diode... no, any diode can be used as a "rectifier" in the right application 
2) a Germanium diode.. no, the diode is made from Germanium, a different element. BTW, they will also have a voltage drop of around 0.3 volts. You won't find many of these around anymore, they were mostly used for detectors for AM radios. 
3) a Schottky diode... is silicon, but the diode is formed by a connection to various metals. 
4) or a Zener diode ... is a variation of a silicon diode that has a very well defined REVERSE breakdown voltage that doesn't vary much with current. One side of the PN junction is very heavily doped. These usually have low reverse voltage, typically 5 to 15 volts. They are used as voltage references. They tend to be a little noisy so the more stable and less noisy band gap reference (at 1.25 volts) is typically used instead. 

The iN5406 is a garden variety silicon rectifier rated at 3 amps and 600V PIV. This part is a beefier version of the 1N4000 family which is rated at 1 amp, otherwise very similar. 

If you were measuring 0.25 volts, then you were either measuring at too low a current or your measurement was incorrect. Try increasing the forward current to 10 mA or more.


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## pagosarr (Jan 5, 2008)

Boy am I feeling very humble and small. My "problem" has been resolved. All that was necessary was to measure the voltage drop with a load in place as some of you suggested. I now have measured 0.7-0.75v drop across the diodes.

To all of you legendary forum participants who did their best to help me out, please forgive my ignorance. At age 77 I guess it is not too late to learn! Hope other visitors to this thread learned the same lesson I did.


Again, thanks to y'all.

Roger Bush


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## Greg Elmassian (Jan 3, 2008)

Measurement can be a fine art. Not everything can be measured in an unpowered state. 

Luckily we DO have a rocket scientist on the forum! 

I did notice that all the charts of voltage drop were done with a minimum of 10 milliamps of current flow. 

Regards, Greg


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