How to make an LED light regardless of polarity?

Please excuse the newbie question:

How can I make an ordinary LED light up regardless of the VDC polarity?

I can imagine a way to do it involving four additional diodes, but
that's more than I have room for in this application. Is there some
clever way to do it with only two (or less)?

Thanks,
- Joe

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Just put two leds back to back so they share the same current limiting.

Works!

 

No way I know of. You do know you can get dual Leds in one package don't you?

 

Sorry, I wasn't clear: I need to light *one* LED regardless of polarity.
Two won't do; the spec allows for just one.

Thanks,
- Joe

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I think you mean in parallel (so they share the same current limiting

 

what is your source voltage?
use a bridge rectifier and connect the LED to
the + and - terminal.

 

You could use a two-colour LED--but then the colours would be different in
different polarities.

Cheers,

Phil Hobbs

 

9 VDC, though of course I use a current-limiting resistor.

Thanks, that's exactly what I was looking for!

Best,
- Joe

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And four diodes are usually smaller than a bridge rectifier. Though I
haven't looked at SMD models that may be out there.


Thomas

 

Try to find those two-color LEDs that have two different color
LEDs in one LED package, those two are installed on different
directions. Installing such LED would make the LED to light
on different colors on different polarity power, and third
(combination of those first two) when AC is feed to it.
This could be one option to consider if you specification
allows it.

 

But it's one component instead of four. Is it so amazing that I have
room for one more component (possibly even two), but not four? Or have
I missed something?

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An 1n914/1n4148 with leads is very small and 4 of them mounted behind
the LED will make it a bit longer but not wider.


Thomas

 

---
I think you've missed something.

You said, in essence, that you didn't have room for four motorcyles,
but that a Honda Civic would be fit in the same volume.

That is, not that there aren't any, but I haven't seen any full-wave
bridges in packages smaller that four tiny diodes, and they're both
four diodes. There is another possibility, though, and that's that
there are lots of tiny dual diodes out there, so you could make up
your bridge that way if you could fit two of them in there and the
full-wave bridge wouldn't.

 

Have a look at the size of the components mentioned. A diode bridge
requires (much) more space then four small diodes (1N4148 for instance.)

petrus bitbyter

 

The Siemens BAV99S has 4 doides which can be bridge connected in a 6 pin
SOT-363 package which is about 2mm square.

 

Thank you all for your input. I've already got some bridge rectifiers
on order from Mouser, but if none of them fit the task, I'll try four
separate diodes (or two dual diodes) instead.

Thanks,
- Joe

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`------------------------------------------------------------------'

 

Sent some gear for EMC testing.
The leds illuminated in one of the RF immunity tests.

Although I never tried it I guess they would illuminate whichever way round
they were (not) connected.