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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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 Posted: Mon Nov 30, 2009 9:28 am Post subject: Lamp polarity and unpluggability |
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Hello!
Got a couple questions about lamp wiring. I bought a couple 12 volt LED lamps from Radioshack, #272-345 the other day. The pic below is from their site. My terminals have a couple tiny solder tabs instead of the wires shown.
The lamp needs to be inserted from the front of the panel, with the lone locknut coming in from behind. So if I solder the tabs to the wires, I'll have to cut them to take the lamp out. I understand it would be good to place a rectifying diode across the leads in spike catcher fashion. $.85 for the lamp and $.60 for the diode. Think I might be able to afford a new one if I need to replace it.
I soldered on and heat shrunk a couple 3" leads to these and figure I'll solder or butt splice them along with the diode leads to the supply and ground leads of my panel. Unless there's a niftier way, I figure putting in male and femal fastons is overkill, adding another 10 minutes of construction time, 0.1 oz of weight, and $.40 of my hard earned cash. But I'm curious if there's an easier way for something like this where it really would be nice to have unpluggability.
After I soldered leads to both little LED lamps it occurred to me that polarity might be an issue. Oh, yeah, the 'D' in LED! I read that hooked up one way there would be no light. The lamp solder tabs are covered up with solder and heat shrink now, but as far as I recall, they looked identical to each other. I hooked up the leads to my 12V battery and the light worked. Also worked with the leads reversed, which confuses me.
I also flipped my Flukometer to diode mode. It measured 0.016V positive and zero with the leads reversed. Just for kicks, I did the same with the incandescent lamp and it measured 0.015V in both directions.
I had wired that little yellow incandescent lamp provided with the B&C alternator disconnect relay kit the same way, soldering the tabs to short leads and putting on heat shrink. This lamp also worked regardless of lead orientation, which was what I thought should happen.
So it looks like I don't need to pay any attention to the polarity of either of these types of lamps. Is that right?
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Dan |
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user9253
Joined: 28 Mar 2008
Posts: 1938
Location: Riley TWP Michigan
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| Posted: Wed Dec 02, 2009 7:52 am Post subject: Re: Lamp polarity and unpluggability |
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LEDs should not work with opposite polarity. So I looked up the Radio Shack part number that you gave: 272-345. At first I thought that it was an LED too. But then I realized that it is a RED lamp, not an LED. So actually you are using incandescent lamps.
| Quote: | | I understand it would be good to place a rectifying diode across the leads in spike catcher fashion. |
I do not think so. A reverse-biased diode would have no affect. A forward-biased diode would blow the fuse.
Joe
http://www.radioshack.com/product/index.jsp?productId=2103779
[/quote]
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Joe Gores |
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nuckolls.bob(at)aeroelect
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| Posted: Wed Dec 02, 2009 8:52 am Post subject: Lamp polarity and unpluggability |
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At 09:52 AM 12/2/2009, you wrote:
| Quote: |
LEDs should not work with opposite polarity. So I looked up the
Radio Shack part number that you gave: 272-345. At first I thought
that it was an LED too. But then I realized that it is a RED lamp,
not an LED. So actually you are using incandescent lamps.
|
Good catch Joe!
| Quote: |
> I understand it would be good to place a rectifying diode across
the leads in spike catcher fashion.
I do not think so. A reverse-biased diode would have no affect. A
forward-biased diode would blow the fuse.
|
WAaayyyy back when, LEDs were considered to be
fragile devices when it came to reverse voltage
stresses. Indeed, they ARE less tolerant of reverse
voltage than their power rectifying cousins. It was
not uncommon for a designer to place a reversed polarity
diode across an LED to clamp off POTENTIALLY damaging
reverse voltage "spikes". Forward voltage "spikes"
were less worrisome because the current limiting
resistor in series with most LEDs would mitigate stress
on the device.
Nowadays, we KNOW that are airplanes are not plagued
with such spikes. Hence, the folks who make drop in
incandescent replacement LEDs don't include such
diodes in their product.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
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messydeer
Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA
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| Posted: Wed Dec 02, 2009 10:17 am Post subject: Re: Lamp polarity and unpluggability |
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Thanks for clearing these things up, guys  I was about to call Radio Shack and ask them to explain this. I would have really confused them.
It's only a matter of a few bucks for these lights, but any reason to switch to LED warning lights? I know they last much longer, but I'd think as infrequently as these standard lights would come on (starter engaged, low voltage, and visual alarm for EFIS) they'd never burn out anyway.
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Dan |
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MauleDriver(at)nc.rr.com
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| Posted: Wed Dec 02, 2009 11:09 am Post subject: Lamp polarity and unpluggability |
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One might use LEDs for reliability (do you check the operation of all
warning lights before each flight?). An incandescent filament is
subject to failing due to cycling and vibration I'd guess. A solid
state LED is forever (?).
I'm using them just because they're neat.
messydeer wrote:
| Quote: |
Thanks for clearing these things up, guys I was about to call Radio Shack and ask them to explain this. I would have really confused them.
It's only a matter of a few bucks for these lights, but any reason to switch to LED warning lights? I know they last much longer, but I'd think as infrequently as these standard lights would come on (starter engaged, low voltage, and visual alarm for EFIS) they'd never burn out anyway.
--------
Dan
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