diode polarity

[Elenappli]

Hi, I am using a surface mount diode on a pcb I am designing. I have created the footprint in Altium but I can't figure out which pad is the cathode and anode. Which one is the larger pad?
Here is the datasheet and the digikey link:

http://www.diodes.com/datasheets/ds30516.pdf

http://www.digikey.com/product-detail/en/DFLS240L-7/DFLS240LDICT-ND/700152

Thanks!

 

[btrueblood]

Wow, what a crappy datasheet.

There is a hint in note 1, which seems to indicate that the larger pad is the cathode.

The packaging datasheet mentioned in note 6 is just as useless as the main datasheet - no polarity information given.

 

[benta]

As with all diodes, the printed/painted line at one end is the cathode.
See the drawing on p.3 of the datasheet.

Benta.

 

[btrueblood]

"As with all diodes, the printed/painted line at one end is the cathode."

Except for those diodes where it's a dot, or just two different colors, or it's the package shape (flat side or a notch), or its a 3-pin device, and you are supposed to know that the middle pin is the cathode...etc. I've seen an awful lot of variation in diode packages out there, and I'm just a mechy type.

Every decent data sheet has the polarity/pinout for whatever device specifically called out, to avoid having to determine the polarity by trial-and-error.

 

[Skogsgurra]

FWIW: Diodes usually get hotter at the cathode side and that is why the cathode pad usually is a little larger, for cooling.
Not a fire-proof rule, but something that can be used to verify or falsify a preliminary guess.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[benta]

btrueblood, it took me around 5 seconds to identify the cathode.
My statement was a little broad, I agree.

Benta.

 

[mcgyvr]

looks very clear to me on page 3...

 

[analogkid2digitalman]

Procure a sample and get out the DMM?

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

[IRstuff]

Can't you just call up the manufacturer?

http://www.diodes.com/contacts/sales/

TTFN
faq731-376

 

[btrueblood]

mcgyvr,

Is there something on pg. 3 that clearly identifies anode or cathode and tells which pin/pad is which? Besides the stripe that benta noted, which I think even he will admit, is not clearly specified as marking the cathode. Yeah, ok, "convention says" the cathode is marked, but those kind of assumptions make guys like me fry little electronic bits. And not just me, either - my EE compatriot had a whole slew of boards built with the diodes reversed, because the packaging drawing was not clear, and he made an ASSumption. Stupid, incomplete or poorly written data sheets should be called such. We nearly missed a shipment because of the screwup.

Sorry, had to vent. I will go back to sleep now.

 

[OperaHouse]

I actually find the notation of anode and cathode quite quaint. Each time I get that reference I have to think of how an electron tube would be connected to form a diode to get the + side.

I remember testing a batch of failed boards that looked like open diodes in a bias circuit. Pulled out one of the 1N4002 diodes and did a quick test with and ohm meter. Conducted one way and not the other. It was then I noticed that the band was painted on the wrong side.

 

[mcgyvr]

mcgyvr,

Is there something on pg. 3 that clearly identifies anode or cathode and tells which pin/pad is which?

Yes the very bold... clearly visible... black cathode mark on the top view in the "Package Outline Dimensions" section. I wouldn't even think twice about it. That's CLEARLY a cathode marking.

 

[MikeHalloran]

I too have found occasional lots of 1N4xxx diodes with the bar on the wrong end. ... not many, but enough to justify incoming surveillance.

I don't know enough about surface mount diode manufacture to guess whether to expect a similar mis-mark rate or not.


Mike Halloran
Pembroke Pines, FL, USA

 

[btrueblood]

"That's CLEARLY a cathode marking. "

Unless it isn't.

 

[btrueblood]

Wups. Mumbling in my sleep again....zzzz.

 

[zappedagain]

Side note on the footprint: IPC-7351 states that the cathode should be on the left when the footprint has 0 degrees of rotation. It is amazing how many stupid errors I've seen pop over the years when that rule wasn't followed.

Z