Determining polarity (with multimeter or otherwise?)

by vputz on Sun Feb 24, 2013 7:27 am

Recently put together a project, a 32u4 interface board to a RS232 device. For a first time out, a number of things went wrong (badly burnt my board, melted an LED, etc)

But at the end of the day, the damn thing basically worked (except it was undervolted; I think the Max3232 was a poor choice and I need the 10v off a 232a, but that's neither here nor there).

Here's the problem: I looked at my 10uf tantalum cap and convinced myself that the big stripe with what sure looked to me like a "-" on it must be the negative side...

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the caps--It sure LOOKS like a big - sign!
130224122027448333687.jpg (265.81 KiB) Viewed 2589 times


so you can imagine my surprise when it let all its magic smoke out and remained a glowing cinder for some time. I tried replacing it, checking connections, checking voltages to be sure the + side was + and the GND side was GND, and the second cinder lasted even longer (the miracle is that the ATMega actually survived all this happily). I eventually patched over the smoking crater with a wire...

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The horror... the horror...
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and just dropped both the 10uf cap and the power LED and the circuit seems to be working fine (aside from the undervolt from the 3232). I still don't know if the cap was marked strangely or if something was badly wrong with the circuit.

So, the point of all this:

Is there a standard marking for SMD polarized caps? A standard way they are loaded into the reel tape? Or a way to use my trusty EX330 multimeter to figure out which end is which? Same with diodes; I can't find any useful marking on the LEDs at all:


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My faceless LEDs...
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HALP!

(incidentally... if you do any SMD work, I LOVE my USB microscope, basically the same one sold here. Highly, highly recommended)
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Re: Determining polarity (with multimeter or otherwise?)

by BruceF on Sun Feb 24, 2013 2:42 pm

That's a 'polarity band' on your capacitor, not a minus sign. The polarity band goes on the positive side of the capacitor in my experience, but you should really check the datasheet for any parts you're unsure of.

As for the LEDs, if you look at the bottom side do you see any markings? A small green mark, for instance? Again, the datasheet should help.
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Re: Determining polarity (with multimeter or otherwise?)

by vputz on Sun Feb 24, 2013 3:30 pm

Well! Naturally, since most through-hole electrolytics highlight the negative terminal, it makes sense that for SMT they highlight the positive. The nice thing about standards is there are so many to choose from, I suppose.

You're right, though--I should have looked harder at the datasheet. Since my caps didn't bear much resemblance to the picture
caps.PNG
caps.PNG (18.1 KiB) Viewed 2549 times

...I confess I just punted. But sure enough, later on they did indeed say
cap.PNG
cap.PNG (11.28 KiB) Viewed 2549 times

...lesson learned. Same with the diode,
cathode_mark.PNG
cathode_mark.PNG (15.53 KiB) Viewed 2549 times

although again I couldn't see the mark so I got flustered (it didn't occur to me it would be on the bottom, though in retrospect I don't know where else they'd put it so as not to obstruct the light...). And I'm still not clear; does the "cathode mark" T point in the direction of the cathode?
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Re: Determining polarity (with multimeter or otherwise?)

by BruceF on Sun Feb 24, 2013 10:03 pm

I assume you're right, but the SMT LEDs I worked with had a green dot right close to the cathode on the bottom so the interpretation was more obvious. Maybe you could wire up a power supply and resistor and just touch the ends of the LED to confirm?

If you think about it, it makes sense that SMT parts would often have bottom markings for bottom-vision pick and place machines - these things aren't really designed with home tinkerers in mind.
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Re: Determining polarity (with multimeter or otherwise?)

by vputz on Mon Feb 25, 2013 3:35 am

If you think about it, it makes sense that SMT parts would often have bottom markings for bottom-vision pick and place machines


But sir, that involves thinking, and we'll have none of that when I can produce magic smoke in quantity like this! But yes, now that I know the marks are there, I'll know what to look for. I learned a LOT from this first time out (I'm getting/making a vacuum pen after squirting some of these LEDs across the room with tweezers never to be seen again, I'm not trying a hot-plate reflow again until I have my sous vide controller modded to avoid the big jumps I was getting toggling the control by hand with a digital thermometer, I now know about the markings, I also know that daubing solder paste onto pads with a wire actually works but you need a lot less than I even thought...). Very very informative.

Oddly I wasn't able to get my DMM to register anything with the LEDs; I assume they just didn't have enough contact surface to register. I'll play more.
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Re: Determining polarity (with multimeter or otherwise?)

by adafruit_support_mike on Tue Feb 26, 2013 4:07 am

vputz wrote:But sir, that involves thinking, and we'll have none of that when I can produce magic smoke in quantity like this!

Okay, you just scored a Coke-to-nose event with that one. :lol:
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.
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