Measuring Schottky Vf...

Thread Starter

Externet

Joined Nov 29, 2005
1,817
Hi.
Most Schottky diodes I have measure around 0.54 Vf. But a couple show 0.18 and 0.19 Vf . Why the difference ?
 
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Papabravo

Joined Feb 24, 2006
18,102
Hi.
Most Schottky diodes I have measure around 0.54 Vf. But a couple show 0.18 and 0.19 Vf . Why the difference ?
Every diode is different and Vf is usually a function of the diode current. Can you give us more details on your experimental setup and a summary of the results with specific part numbers.
 

ronsimpson

Joined Oct 7, 2019
2,007
There are different type of Schottky diodes. This one is low voltage. You said 0.18V and at room temp that will take 15mA of current to see this voltage.
1640138235954.png
Another diode to look at.
1640138491375.png
 

Thread Starter

Externet

Joined Nov 29, 2005
1,817
Thanks.
All diodes out of circuit, tested with the same vulgar multimeter at -|>|- (diode setting) at same room temperature with no current other than whatever the meter applies.
The ones reading near 0.2Vf are IR0135 31DQ04 and 1N5822
All two dozen more canibalized from equipment, in different flavors show around 0.54Vf. Non-Schottky read around 0.69Vf
 

Papabravo

Joined Feb 24, 2006
18,102
Thanks.
All diodes out of circuit, tested with the same vulgar multimeter at -|>|- (diode setting) at same room temperature with no current other than whatever the meter applies.
The ones reading near 0.2Vf are IR0135 31DQ04 and 1N5822
All two dozen more canibalized from equipment, in different flavors show around 0.54Vf. Non-Schottky read around 0.69Vf
So I would guess that using the diode setting on the meter provides a constant current and measures the voltage. The only real ambiguity is the value of the constant current source. It should be self evident that a single valued constant current source may have some limitations when it comes to measuring the set of all diodes. It gives you one point on a curve whose shape can vary unexpectedly.

I am familiar with 1N5822 and the values seems reasonable for a very low current. The 1N5822 is specified as a 3.0 A part and the datasheet value for Vf @ 1A is 0.38V±0.01V. Using 0.2V for a typical use case might be considered misleading. How does your measured value compare with the datasheet values for the IR0135 and 31DQ04?

https://www.onsemi.com/pdf/datasheet/1n5820-d.pdf
 
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DickCappels

Joined Aug 21, 2008
8,686
A diode with a large area will, all else being equal, have a lower forward voltage at a given test current than a diode with a smaller area. This the same for silicon PN, Shottky, germanium, etc.
 

Papabravo

Joined Feb 24, 2006
18,102
A diode with a large area will, all else being equal, have a lower forward voltage at a given test current than a diode with a smaller area. This the same for silicon PN, Shottky, germanium, etc.
A meter with an adjustable diode test current would be handy, but I don't think I have ever seen or heard of such a thing.
 

k1ng 1337

Joined Sep 11, 2020
431
I tested 3 1n5819 diodes on my meter with results of 159mV, 160mV and 162mV.

The test condition of my meter is 2.8V open circuit voltage at 1mA current and the prediction can be seen on this graph. Consult your meters manual for the test condition and then compare to datasheet for accuracy. Unfortunately the graph they gave me only goes down to 10mA which makes my 1mA test condition difficult to validate against the datasheet.
 

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