Identifying polarity on SMDs

[Philip H]

Inspired by @DKS and @wm3798 i recently ordered some pre wired smds to do building lighting. I may have gone too small as I can see the polarity Mary on the back (!) but in for the proverbial penny I want to see what I can do. So, here’s an enlargement of the best photo my iPhone could get. Anyone got any idea which side is such?

[ Guests cannot view attachments ]

[NDave]

I just touch the wires to a button battery... if it lights, I remember which color is plus. If it doesn't, I swap the polarity and try again, then remember which color is plus.

[Philip H]

These were done with magnet wire so both wires are the same color - though I suppose I could sharpie it.

Thanks!

[jdcolombo]

In the photo, you will see a green line.  Note that you can just barely make out the fact that there is another green line intersecting it on the right - like a "T" lying on its side with the "stalk" of the T pointing to the right.  That's the Cathode (-). 

But these things are so small that seeing the "T" and which way the stalk is pointing is really difficult.  SO . . . using a 1.5v battery to see which way the leads make the LED light up is preferable (and you should test them, anyway, before using them in a project). 

John C.

[peteski]

Yes, like John stated, LEDs have their cathode identified in some way. The "T" he mentioned is one such example. Sometimes there is just a color bar (usually green) close to the cathode pad.  I have also seen a triangle with the point facing cathode. Sometimes there is also a cathode identification mark of some sort on the lens side of the LED.

A 1.5V battery might dimly turn on and illuminate the standard red, yellow, and green LEDs (which have forward voltage rating of 1.7-2.0V), but I doubt that any white, blue, or true green LEDs will light up from 1.5V.  Those have forward voltage around 3V.  A 3V lithium coin battery (like CR 2032) could be used for those, although I would recommend adding a 100 ohm resistor in series to limit the current passing through the LED.  I'm being overly cautious because  that battery cannot supply much current, but those really small SMD LEDs (like 0201 or 0402) have very small operating current (usually around 5mA).

[peteski]

These were done with magnet wire so both wires are the same color - though I suppose I could sharpie it.

Thanks!

That's exactly how I identify polarity of the magnet wires.  I usually mark the negative (cathode) lead with black Sharpie, but I have also used red Sharpie for positive (anode), leaving the other wire natural color.

[Point353]

These were done with magnet wire so both wires are the same color - though I suppose I could sharpie it.

Are both leads indeed the same color?
One of the vendor's pictures shows the anode lead having what seems to be clear insulation while the cathode lead has red insulation.

[misiek]

I just touch the wires to a button battery... if it lights, I remember which color is plus. If it doesn't, I swap the polarity and try again, then remember which color is plus.

Best method.

Maybe this picture will be helpful. If there are markings on the LED.

[DKS]

Are both leads indeed the same color?
One of the vendor's pictures shows the anode lead having what seems to be clear insulation while the cathode lead has red insulation.

I've got these same LEDs, although possibly not from the same vendor. After testing, I found that the lead with clear insulation was the cathode.

Sometimes polarity is indicated by lead length. If one lead is shorter, chances are very good it's the cathode.

Also, some LEDs have resistors already wired in. Most of the time I've found that the resistor is attached to the anode.

[jdcolombo]

Yes, like John stated, LEDs have their cathode identified in some way. The "T" he mentioned is one such example. Sometimes there is just a color bar (usually green) close to the cathode pad.  I have also seen a triangle with the point facing cathode. Sometimes there is also a cathode identification mark of some sort on the lens side of the LED.

A 1.5V battery might dimly turn on and illuminate the standard red, yellow, and green LEDs (which have forward voltage rating of 1.7-2.0V), but I doubt that any white, blue, or true green LEDs will light up from 1.5V.  Those have forward voltage around 3V.  A 3V lithium coin battery (like CR 2032) could be used for those, although I would recommend adding a 100 ohm resistor in series to limit the current passing through the LED.  I'm being overly cautious because  that battery cannot supply much current, but those really small SMD LEDs (like 0201 or 0402) have very small operating current (usually around 5mA).

Oops.  Yep, 3V.

John C.

[Philip H]

So, post coffee -

• These SMDs have different length leads.  SMDH 
• Even if these have differing color lacquers for insulation, I can't tell the difference.
• I have 2025 3V batteries for our car key fobs, and holding one vertically in a small all plastic spring clamp makes a decent test rig.

So far I only have one bad SMD.  But getting good contact is not easy owing to the insulation lacquer. What have we decided is the right removal protocol?

[MK]

What have we decided is the right removal protocol?

Lacquer thinner or just use a flame from a lighter.  Some use the tip of a solder iron with a glob of melted solder.  Just run the wire through it.

[DKS]

Some use the tip of a solder iron with a glob of melted solder.  Just run the wire through it.

This. Then you get a tinned wire at the same time you're stripping it. However (this may sound trivial, yet it's important), stick the very end of the wire into the glob of solder first, then slowly slide the wire in further.

[peteski]

The iron has to be hot enough to burn of the insulating enamel.  If you have non-temperature-controled iron, it will likely be hot enough.  If temperature-controlled, I would crank it to 800 degrees F. 

Dipping the end of the wire in paste flux before sticking it in the liquid solder also makes the stripping.tinning easier.

[DKS]

The iron has to be hot enough to burn of the insulating enamel.  If you have non-temperature-controled iron, it will likely be hot enough.  If temperature-controlled, I would crank it to 800 degrees F. 

Dipping the end of the wire in paste flux before sticking it in the liquid solder also makes the stripping.tinning easier.

I've found 600 F (315 C) is adequate. I just finished stripping 40 wires for Ed's Dirty Dog, and it went very smoothly. Starting with the very end of the wire was the only thing I found important, for reasons I don't yet understand--if you just stick the wire in sideways near the end, it won't strip.