# Determining LED polarity

Super ModeratorPosts: 5,070Super Moderators
Quick question,,

I bought a couple of 12 vdc LED puck lights to experiment with for under counter lighting.

My question is, how do I determine the correct polarity? The come with modular plugs and are designed to use a polarized power supply. One wire on each light (they are different) has a stripe on it. My intuition is that this is the positive side, but I don't wish to burn them up.

Is there a simple way I can put a meter on them to determine which is positive?

Tony

• New User Posts: 1,571Solar Expert
Re: Determining LED polarity

No danger of burning them up because of polarity. Wrong polarity just won't work.
• Fizzycist Posts: 3,102Solar Expert ✭✭✭
Re: Determining LED polarity
icarus wrote: »
Quick question,,

I bought a couple of 12 vdc LED puck lights to experiment with for under counter lighting.

My question is, how do I determine the correct polarity? The come with modular plugs and are designed to use a polarized power supply. One wire on each light (they are different) has a stripe on it. My intuition is that this is the positive side, but I don't wish to burn them up.

Is there a simple way I can put a meter on them to determine which is positive?

Tony

You cannot use the simple diode test function of most meters because the forward voltage of the LED (which is the condition under which it lights up) may be 3 or more volts and the fixture may contain two or more LEDs in series. Usually the reverse breakdown voltage of the LED is high enough that connecting it backwards will just cause it to not light up or draw any current. But if you wish to be completely safe, you can apply a limited voltage (like 3 or 4 volts) to the normal terminals through a current limiting resistor and see what happens.
SMA SB 3000, old BP panels.
• Super Moderator Posts: 5,070Super Moderators
Re: Determining LED polarity

Thanks,

I was under the impression that reverse polarity would toast them,, they are multiple LED fixtures.

Tony
• Super Moderator Posts: 5,070Super Moderators
Re: Determining LED polarity

Works like a charm. As a side note, the striped wire is the negative.

These are little puck lights, 3 watts from HERO LED in China, ~\$15. They are really quite bright, with a warm white color. They are set to replace a 15 watt halogen, and are equally bright. Couple that with the switch over of my CFLs to 4 watt LEDs and my lighting load is dropping by about 75%! Six lights go from 90 watts to 24.

Pretty slick,

Tony
• Fizzycist Posts: 3,102Solar Expert ✭✭✭
Re: Determining LED polarity
icarus wrote: »
Works like a charm. As a side note, the striped wire is the negative.
Tony

That is consistent with the convention that the "identified" wire is the grounded or neutral wire and that the neutral in DC situations (other than telecom) is usually the negative.
SMA SB 3000, old BP panels.
• New User Posts: 814Solar Expert
Re: Determining LED polarity

simple check is use a 1k 1w resistor in series with the LED .it will show then correct polarity .just connect one way if it lights a little then thats it if it doesent then reverse the connection. No possibility of damage,but its usually ok to connect them "wrong" and no damage.
• New User Posts: 348Solar Expert
Re: Determining LED polarity

I'm running a bunch of 4W and 5W MR16 LEDs in my cabin. They are very bright and make for great reading and task lamps.

At \$3.50 ea from ebay, they're a great deal. I also picked up 10 MR16 ceramic mounts with pigtails on them for \$0.15 ea. There are no + - markings on the lamps or on the mounts. I've found that it doesn't matter how I connect them.
• Just some guy Posts: 23,887Super Moderators admin
Re: Determining LED polarity

Those MR16 LED lamps are probably designed for a "12 VAC" transformer and have a rectifier built in.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• New User Posts: 814Solar Expert
Re: Determining LED polarity

I have found all the MR16 LED lamps work equally well on AC or DC and because of that the pins have no polarity.
I think they are one of the best of the recessed light sources available. Sure beat the terrible power wasting halogen bulbs.
• Ken Posts: 114Solar Expert ✭✭✭
Re: Determining LED polarity

Isn't it amazing. Right now on ebay 10 watt MR16s less than \$2.00/ea.
Or if you want 120 Vac the 9 watt GU10s for about \$3.00/ea.
Actually they rate them to run on either 120 or 240 Vac.