Which LED/resistor combo to use?

I just purchased a MK2B dual-power heated bed to use on my Mendel90 project, using a 12-volt power supply (hacked ATX 600 watt PSU). What value/type/voltage/etc. LED and resistor should I use on it? Anything that Radio Shack would have on the shelf, since we have one in town? Yeah, I know they're optional, but it would be nice to have a visual indication that the bed is turned on.

There are many LED resistor calculators on the web. Just google it. Get whatever LED radio shack has and plug the spec numbers in to calculate the resistance. There're only 2 or 3 numbers involved.

Try this! [led.linear1.org]

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Turns out Radio Shack has 12-volt LED's with integral resistors, so problem should be solved.

Not having dealt with LED's very often, I am wondering - will they burn out if you try to run current through them backwards? Or will they just not light up? I know my MK2B heated bed has a place where 2 LED's can be soldered, and from what I understand that is so that it doesn't matter what polarity you have the bed hooked up, one LED will always be lit.

backwards they are just off.

It depends on the voltage. LEDs are typically rated at 5V maximum reverse voltage, so with a 5V supply the LED will just be off if it is connected the wrong way round. But at 12V, the LED might suffer reverse breakdown and could be damaged if the situation persists.

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Instead of putting one of those RadioShack LEDs with integrated resistor onto you MK2B bed, I think it's better to put one resistor and two normal LEDs (I use a red and a green one) in opposite direction, as foreseen on the MK2B. This way, the active LED protects the other one from reverse breakdown, while the resistor limits the current through the active LED. As the LED doesn't need to shine brightly, I usually put a 2k or similar resistor, which gives about 5mA current through the active LED with a 12V supply.

Okay, I tested this out, and found that, at least with the power supply I have (a hacked 600 watt ATX power supply, per the Mendel90 build manual), the Radio Shack LED's do NOT burn out if power is applied in reverse, at least for a short time (long enough to figure out the polarity is wrong and correct the connection). I soldered it to my MK2b heated bed, and bridged the resistor soldering pads for the LED on the MK2b with a piece of wire and solder, then applied power. The bed heats up fine, and the LED comes on and lights steadily. No worries!

Edited 1 time(s). Last edit at 10/18/2014 12:25PM by Rich K..

You could do just what I do - figure out the polarity of your wiring and just install one LED. I put a small section of red heat shrink tubing over the positive wire and I've never had an issue.

Except for the red heat-shrink tubing, that is exactly what I did. Since I didn't want solder on the other side interfering with my glass plate, I "surface mounted" the LED leads across both sets of LED terminals (makes a more rigid set-up), with the "flange" of the LED against the edge of the heated bed, so that the LED is poking out at right angles to the edge of the bed. And yes, it DOES work properly when power is applied.