LED Ring Light

I am a programmer getting into vision programming and ran into an issue with our ring light. Somehow, the solder came undone when someone was driving the robot and I do not know which pads to solder positive and negative to. Same problem last year and it may have went on fire.

A picture would help. Without one, we don’t even know the model, let alone enough details to guide you in the correct direction.


Since LEDs are diodes, and have at least some level of reverse breakdown voltage (although maybe not full voltage), one approach is to test it. Hook it up to a power supply and gradually increase the voltage in each polarity (e.g. test it at 3V, then swap terminals, then go to 4V, swap terminals, etc) and stop when you see it light up (which tells you you have the right polarity).

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In addition to its “Light Emitting” function, an LED is also a “Diode”, which means that electricity passes through it in one direction easily, much harder in the other.

  • Do you have a multimeter? (If no, skip my post or get one. They’re not that expensive, and rather useful for robotics. Free, if you’re near a Harbor Freight and bring the right coupon.)

  • If your meter’s wires are removable, make sure black is in COM (common) or - (negative) and red in +/positive. If these aren’t there, post what your plug-in’s labels look like, and we can provide some guidance.

  • Does it have a diode function (symbol like this, but without spaces or quotes):" ─►▌─" ? (if no, skip a bullet, but I haven’t seen one without this in a couple of decades, so look again before skipping)

  • Ok, you have a diode function, put the meter in that diode function. Touch the probes together. It will likely beep; if not, the display will give you a distinctive message or a number that is rather small. Now, touch one probe to one pad of your LED ring and the other probe to the other. Does it beep/give the message? If not, try crossing them over. When it beeps/gives the message/displays a small number, the one touching the red probe is positive and the one touching the black probe is negative. (skip the next bullet; you’re done)

  • OK, you don’t have a diode function. Put your meter in resistance (ohms, usually symbolized with a capital greek letter omega: Ω). If you have multiple resistance settings, pick the smallest, though it usually doesn’t matter. Try touching the probes to the pads each way. Where you get the (analog) larger deflection of the meter, or (digitally) the smaller number (after taking into account the k and M labels meaning thousands and millions) is the “preferred” direction of the diodes: the red probe was on your positive pad, black was on negative.

(OBTW, the “somehow” and “may have” are rather disturbing; care to elaborate?)

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