Bad Diode?

I have a diode Identified by manufacturer as 1N5819 SHOTTKY, 1A, 40V, 10NS, DO-41 made by Motorola.

As per Digikey the Vf should be between 550 and 600mv.

However, DCA75 as well as multimeter diode tester says it’s 220mv.

Can I assume this is bad?

Hate to order a .10 part for 5$ shipping but can’t get em locally.
I guess I thought bad diodes would be open or shorted most of the time.

At the maximum current, the voltage across the diode will be 550mV - 600mV, but at lower currents, it will be less.

Have a look at this graph from the 1N5819 datasheet : Figure 7.

At a current of 0.02A (20mA), the voltage drop is only just over 200mV. The current supplied by your multimeter on the diode test is likely to be less than this.

DCA75 Testor says it is .24 Vf @ 5ma

Sounds good to you? Fig 7 doesn't go that low but perhaps it's about right. I guess I'm just used to all my diodes showing around the .7v neighborhood.

The whole point - indeed the only reason - to use a Schottky diode - is that it has a low voltage drop - 200 mV would be about the desired value.

Paul__B: indeed the only reason

Plus, of course, their high speed switching.


I knew someone would pipe in along those lines!

I was just waiting for it. :grinning:

I have six of them in my hand. Using the DCA75 I get a VF of:


So, working as intended, I would suggest.

I knew someone would pipe in along those lines!

It was a test. :wink:

Those figures for 600 mV are quoted at 1 A (see the chart on the datasheet). At 5 mA (which the DCA75 measures at) it is more like 240 mV. In fact that chart on the datasheet only goes down to 100 mA.

With my other test equipment, I measure a VF of 200 mV, with the measurement at 1 mA.

Excellent diode in fact.

Actually there is a caveat for schottky diodes, which is why they are not
always the best choice, and that’s reverse leakage current.

The leakage is large to start with (compared to standard pn diodes) and rises
dramatically with temperature. This generally means they are unsuitable for
high voltage use as reverse leakage causes heating which increases
reverse leakage, leading to thermal runaway… You won’t see a general purpose
1000V schottky rectifier!

Normally the low forward voltage means they run cool and a few uA
leakage isn’t significant (standard silicon diodes have << 1nA leakage
in similar circumstances though).

Also when it comes to speed the 1N4148 will give most schottkys a run
for their money.

Thank You everyone for you knowledge and willingness to go to the drawer and test something independently. I feel very fortunate to have so many people around me to help. Some day when I'm at that level i will return the favor for some new person. Assuming the parts are not to small for us to work on in 30 years :)