Diode polarity

[Kalee20]

I've just had an interesting discussion!

I work at a company which, amongst other things, manufactures build-to-print designs to customer drawings. And one of our test people was having trouble, appealed for help...

The assembly in question is an old PCB assembly, pre-CAD days. The fault turned out to be a 1N4148 diode fitted backwards (found using a DVM by logical tracing, Smithy-style - if I'd looked at the board legend like Dick would have done, I'd have saved myself 5 minutes).

Anyway! The diode is indicated on the screen-printed legend as two square brackets, each with a short white line emerging, and a " + " sign against one. The assembly people had fitted the diode so that the end with the bar is against the unidentified hole in the PCB, and the end without the bar against the + symbol. The inspectors, and the tester, having been to college, accepted this as correct, because "The anode you make positive; the cathode you make negative. And the end with the bar is the cathode, innit."

So I explained, "Well, if you mark a diode at all, the cathode you mark with a +, the anode you would leave as being fed with AC, because in a rectifier circuit the cathode becomes the positive terminal."

But, are our colleges really instructing people to fit diodes with anode to + symbols? It seems they are!! And every diode on every board has been fitted backwards.

(Incidentally, my own circuit board diode legends are rectangular boxes with a bar on one end, to exactly match the component).

[peter scott]

I think a dot on the board is a better indicator and should be on the cathode end.

Peter

[ppppenguin]

At least the electrolytic is the right way round.

[Kalee20]

Yes, especially as it is a tantalum type with a 125V rating - very expensive!

But the diode - is there anyone here who would be confused by the polarity marking (or worse, quite confident - and wrong!!)

[radioman]

When I worked as a technician at a local college, one of the science lecturers was demonstrating an AC to DC half-wave power supply circuit - but they kept blowing up the electrolytic because they'd connected up the diode with it's anode going to the positive end of the capacitor...
After this had happened twice I got involved and weren't too pleased when I told them (in front of the class) that they'd connected up the diode the wrong way 'round...
Your board would be better marked with 'a' & 'k'.
'+' is too ambiguous IMHO.

Andy

[ppppenguin]

The other alternative is a bar across at one end. The cathode end I hope. Don't do the same with electrolytics where the marked end has varied. -ve with modern radials but not sure about others.

I can never remember which is the +ve side of the symbol for a battery. Is it the long bar or the short one?

[radioman]

The long one.

[Kalee20]

Your board would be better marked with 'a' & 'k'.
'+' is too ambiguous IMHO.

It is. I agree. Unfortunately, we can't do anything about it, only our customer can - and they won't want to.

But, the polarity of diodes - I've been looking online to see, the vast majority of basic electronics tutorials do seem to mark the anode as +, the cathode as -, yet look at a bridge rectifier and you'll see two terminals marked AC, the twin cathodes as +, and the twin anodes as -.

[ken]

I can see why all of this might be confusing. I don't have a background in electronics but instead mine is in chemistry. There's a related area to the above in chemistry and that's in the electrolysis of molten or aqueous salts. As you might remember from school the positive electrode is the anode and the negative the cathode. However, the ion that migrates to the anode is negatively charged but is called the anion. Likewise the cation is positively charged. This caused confusion for some. Also what happened at each electrode in terms of ion electron half equations caused problems. The chemists were supposed to stick to a convention which helped. I'm not sure that there is a convention, in terms of the marking of components, to which everybody sticks but I'm open to correction here.

Regards,

Ken.

[Ed_Dinning]

Hi Peter, I'm glad I'm now retired as this sort of thing has been coming for some time. Too much concentration on paper qualifications and a lack of practical experience

Ed