Diode Silk Screen Markings

It still happens. In fact, it just happened last night. We had a PCB with plus (+) mark to indicate the polarity of diode. Unfortunately, that doesn’t tell us which way to put the diode. (Read why here.)

You just can’t always tell. If it’s a barrier diode or a zener, the cathode might very well be the positive side. Or, it could be the negative side. An LED will usually have the anode positive, but again, there may be a few scenarios where it’s not. The bottom line is that a plus (+) or minus (-) sign doesn’t give us enough information to orient the diode.

We prefer that you use the actual diode symbol, or an industry standard anode or cathode indicator. “A” orGood markation “C” for anode or cathode can also work. Just make sure you also put the reference designator (D1, D2…) so we know it’s not a capacitor.

In the job last night, the build instructions were conflicting so we called and with the help of the designer, figured it all out, but it’s always best to do it right the first time. So be clear with your silk screen, the PCB you save may be your own.

Duane Benson

Spider or worm?

http://blog.screamingcircuits.com

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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3 Responses to Diode Silk Screen Markings

  1. Jack Olson says:

    Good point, and not addressed often enough!
    Here’s some additional information that may help someone interested in the subject:
    http://frontdoor.biz/PCBportal/ResolvingComponentPolarityProblems.pdf

  2. It is not just diodes. How many have been caught out by Tantalum capacitors. The first time I got it wrong I rang the tech support of one of the largest component suppliers here in the UK (Farnell) and even he got it wrong. In fact of the 25 pages in their catalogue only 2 out of the 28 photos show the “bar” at one end with a plus sign.

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