Leds on heatbed - only one lights

Hi all,
I soldered two leds with opposite polarity and resistor, but when I heat up heatbed only one led lights. Why there are two, what is the functionality difference of these two leds ?

That means you are feeding it DC rather than AC.

There are two leds so one or the other will light, no matter which direction you hook up the power.

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DaveX
That means you are feeding it DC rather than AC.

There are two leds so one or the other will light, no matter which direction you hook up the power.

What?

I would like to see just simply answer

So what is the right work of these leds ? When both light or when one lights. And if one should lights, what is the meaning of second one ? And what is the meaning of leds generally, just that heatbed is powering, no other signal ?

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Arnold
I would like to see just simply answer

So what is the right work of these leds ? When both light or when one lights. And if one should lights, what is the meaning of second one ? And what is the meaning of leds generally, just that heatbed is powering, no other signal ?

Your question was answered...

"There are two leds so one or the other will light, no matter which direction you hook up the power."

Ok, and meaning of led is just that heatbed is powering ?

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tjb1

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DaveX
That means you are feeding it DC rather than AC.

There are two leds so one or the other will light, no matter which direction you hook up the power.

What?

I've got an old plug-in 6.3VAC diode/continuity tester that uses a circuit like that, and the AC lighting both LEDs lighting up was the way that you tell that the Device UnderTest was shorted.

You could light up both LEDs if you use AC for the PCB heatbed, which you could do if you had a 12V, 120VA transformer and switched it through a SSR. But I don't think it is worth doing, because a 12V 120VA transformer and a SSR would likely cost more than a 12VDC switching supply.

On my PCB heatbed, I didn't put on the LEDs and resistor so I'd have a flatter/simpler top surface.

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Arnold
Ok, and meaning of led is just that heatbed is powering ?

Yes. If there is a light, there's power to the heatbed.

See [reprap.org] -- the pic is a little misleading though -- it lights both LEDs and has undersized wires.

Edited 1 time(s). Last edit at 04/02/2014 02:28PM by DaveX.

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Arnold
Ok, and meaning of led is just that heatbed is powering ?

A LED is like a check valve. It will only operate with current flowing in one direction. If you hook it up backwards, it won't light up. The idea with the heated beds is that when you (or whoever makes your board) solder the LEDs to the board, they don't know how the board will be wired up. So it's designed so that two LEDs are used, with one of them flipped from the other. This way, regardless of which wire is positive and which is ground, you'll still have an indication that the board is working.

If you are the one soldering the LED on and you know for sure how the wiring will be attached, then you can get away with only installing one in the proper orientation. It's only a few pennies for a SMD LED at any decent quantity though so that's why you get two. It's fool proof that way even if it does mean that you'll only ever use one LED at a time.

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tjb1

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DaveX
That means you are feeding it DC rather than AC.

There are two leds so one or the other will light, no matter which direction you hook up the power.

What?

Beat me to it...LOL

If you were able AC power to feed polarity opposed LEDs connected in parallel, from the MOSFET powering the PCB, both would actually light up. For 50% of the AC cycles, each LED is in the correct orientation, and with persistence of vision, would look like they are both lit up. Of course, each one is being alternately turned on/off between 50 and 60 Hz..the leds are being lit up between 25 and 30 times per second, and off for the rest of the cycles per second.

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ShaneH

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tjb1

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DaveX
That means you are feeding it DC rather than AC.

There are two leds so one or the other will light, no matter which direction you hook up the power.

What?

Beat me to it...LOL

If you were able AC power to feed polarity opposed LEDs connected in parallel, from the MOSFET powering the PCB, both would actually light up. For 50% of the AC cycles, each LED is in the correct orientation, and with persistence of vision, would look like they are both lit up. Of course, each one is being alternately turned on/off between 50 and 60 Hz..the leds are being lit up between 25 and 30 times per second, and off for the rest of the cycles per second.

Each cycle of the AC wave has one positive, and one negative, so you would get 50 or 60 pulses on each, or for the pair 100 or 120 pulses. There's a project that uses audio signals at ~20kHz into some opposite polarity IR LEDs to make an audio to remote-controller adapter: [www.instructables.com]

My apologies -- I didn't intend to suggest that Arnold should run AC through his system, merely that one or the other of the opposite polarity LEDs would shine with DC.

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Each cycle of the AC wave has one positive, and one negative, so you would get 50 or 60 pulses on each, or for the pair 100 or 120 pulses. There's a project that uses audio signals at ~20kHz into some opposite polarity IR LEDs to make an audio to remote-controller adapter: [www.instructables.com]

My apologies -- I didn't intend to suggest that Arnold should run AC through his system, merely that one or the other of the opposite polarity LEDs would shine with DC.

No need to apologise..and yup...I stand corrected 50/60hz does mean 50 or 60 "on" pulses. Is it beer-o-clock yet? :-)