The manufacturer spec sheet gives all of the pertinent information about a device. The spec sheet is important to the designer so he/she can make the wisest decision regarding a part. A part as simple as a two terminal diode that needs 9 pages to describe it's characteristics should be a well understood device. When a run of parts are made (grown) they are tested and sorted according to min/max electrical characteristics. As a case in point, a batch of 10,000 Ohm resistors. If one tests at 10,043 Ohms it would fall into a 1% category. 10,922 Ohms goes into a 10% category. 11,012 Ohms goes into the scrap bin.
The diode 1N4448 is the target part (1%) the 1N4148 is the grade out part. As the spec sheet shows, the 1N4448 has a Ir (reverse leakage) of 3uA while the 1N4148 has a Ir (reverse leakage) of 50uA.
Some devices use an A,B,C grade designator such as 1N753A or 1N753B to designate the tighter tolerances.
Thanks for the info. How do you find out about the 1% part and grade out part?
The 50uA is at a different tested voltage than the 3uA. Do you think they tested the 1N4148 at an extra Vf, 50uA? All of the other Vf figures show both diodes have the same characteristics, no?
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