Looking for an Odd Light Bulb: 8V .25A "Pigtail" for Meters

[bob p]

I've had reasonable success in making my own edge-lit plexiglass faceplates. I use 1/8" clear plexi with the legends I want laser-engraved into the back side, and a bunch of 3mm ultrabright LEDs firing into the edge of it.

That is the problem: they have to shine into the edge, and if you just stick a bunch together like that guy Bob mentioned, the light comes out in the wrong direction and gets wasted. The side emitting Luxeon was designed to solve that problem, but I'm sure you could achieve something just as good by tinkering with regular LEDs, 2-part epoxy and a Dremel.

IIRC the Carver plexi is a lot thicker, I'd say more than 3/8" but less than 1/2". The light enters the light-plate perpendicularly, and the front edge of the plate has a 45-degree bevel. that bevel sits flush atop the edge of a square-edged piece of vertical plexiglass in front of the meters. so although the incadescent bulb isn't oriented to provide edge-lighting of the light-plate that its mounted in, i guess there's enough sidelight coming off of the bulb to be the equivalent of edge-lighting the first light-plate. then the first light plate has that 45-degree bevel, to reflect the light down into the front plexi plate that sits in front of the meters. the reason that incandescents work well in this perpendicular mounting orientation is because they radiate light in all directions, unlike LEDs which tend to focus the light forward.

i have seen the 1W high output LEDs in the catalogs, but I never knew that they made side-light models specifically for radial illumination when the LED is mounted to a hole drilled into a plexiglass plate. that kind of design would simplify the edge-lighting problem.

of the three types that they list (hey use nomenclature like "side emitting," "batwing," and "lambertian"), I think that "side emitting" is what I'm after. according to the spec sheet, most of the illumination with that model is at 90-degrees. sounds perfect.

Yes, 1 watt, Bob will have to wear sunblock anywhere near his hi-fi!

that's the one thing that bothers me. will that be way too bright? i'm assuming that even a 1W LED will operate cold, am I right?

[bob p]

i'm looking at the spec sheets on that site (DS25.pdf), and the typical voltage requirements for the LEDs are about 3.0-3.5 VDC. current ratings are 100-350mA.
(for example, I'm looking at the LXHL-DD01, side-emitting LED that's red, but blue or aqua or white would be just as good).

my power supply is switchable between 7.5 VAC and 9.5 VAC, and the two 8V 0.25A bulbs are wired in parallel for a 500mA load.

i know that some people run LEDs off of AC supplies, but IIRC the right way to do it involves DC supplies that are current limited (for whatever reason). can you give me advice about the best type of PSU adaptation for the LEDs? IIRC LEDs require DC supplies with current limiting. i'm thinking that depending on how the power supply works out, it might be worthwhile to consider stacking the LEDs in series, vs. running them in parallel. i don't know if voltage regulation is really necessary, or exactly what the current limiting design objectives are, or how elaborate the LED power supply really needs to be.

[Enzo]

LEDs are not just wired across a supply, but the current limit resistor is easy to figure. Take supply voltage, you know the LED voltage, and the current you want. Ohm's Law will tell you the series resistor needed to drop the excess voltage at the LED current.

SO if you had a 5v rail, and a 3v LED, you'd need to drop the remaining 2v across a series resistor. Selecting oh maybe 200ma as a target current, gives you all teh numbers you need for Ohm's Law.

[Marc]

I've got what you're looking for,,,

Bob,
I have what you need.
Back in my scavanging days I must have deconstructed some Japanese stereo.
How many do you need?
Shoot me an email with your address.

Marc

[bob p]

LEDs are not just wired across a supply, but the current limit resistor is easy to figure. Take supply voltage, you know the LED voltage, and the current you want. Ohm's Law will tell you the series resistor needed to drop the excess voltage at the LED current.

SO if you had a 5v rail, and a 3v LED, you'd need to drop the remaining 2v across a series resistor. Selecting oh maybe 200ma as a target current, gives you all teh numbers you need for Ohm's Law.

thanks. one of the reasons i've been wondering about better PSU designs for the LEDs is because these amps have lamps that tend to flicker and dim under extreme loads. when the amp is running hard, its common for the incandescent meter lights to dim. that makes the amp look pretty cheesy, IMO. this happens because the lamp circuit is on its own transformer that is hooked up to the AC line, and the amps have been known to cause brownouts on some AC circuits. i've seen them dim the room lights, for example, even on 20A circuits wired with 10 ga wire.

so i'm wondering if i'd run into the same problem if i chose the simple LED solution like a series resistor for current limiting. i think that if the incandescent bulb is likely to dim under load, then a colored LED would probably dim under load, or a white LED would shift to blue or yellow under load. maybe it would make sense to use a LM317 voltage regulator to drive the LEDs. maybe it would make sense just to use a DC circuit with big caps. maybe it would make more sense to use a current regulator.

here's are some tech papers i've found that seem helpful:

http://www.maxim-ic.com/appnotes.cfm/an_pk/3256

[bob p]

Marc, you have email.

[JC@]

I'm looking for a light bulb that has been VERY difficult to find:

Stanley 8V, 0.25A bulb. Its a "pigtail" type bulb with two wires coming out of the back.

I've also seen them rated as 8V, 0.3A or 0.35A.

This bulb was very common in Japanese built HiFi gear in the 1980s. It was often used to illuminate power meters. The bulb mounted in a blue rubber boot that was pressed into a cutout in the plastic meter assembly. The two wires came out of the back of the boot.

Does anyone know where to source a part like this? I've been looking high and low without any luck. I think that the bulb just might be unobtainable.

If all else fails, I guess I could convert the electronics over to use a different bulb, but finding the right part would be a lot simpler than modding a bunch of gear.

TIA.

I know it's late but just my two cents:
There are lots of auctions at ebay selling all type of bulbs, search for marantz lamp kit or better yet, for japanese receivers like sansui lamp kit, ie

"My" source for imposible to find bulbs is: http://www.donsbulbs.com/cgi-bin/r/t.pl
sometimes expensive but if you want it...

[bob p]

thanks for the idea. i had previously checked with don's bulbs, and they didn't seem to have it. i couldn't find it anyway. :/

[Enzo]

Well, if the whole mains is bouncing up and down, count on your light supply joining in. It would be a simple matter to add some crude regulation. Even a zener in there somewhere would establish a steady supply.

A three leg regulator would also be simple. I wouldn't bother with a 317, it doesn't have to be adjustable. Just establish a 5v with a 7805 or whatever and then configure the LED/resistor from that instead of from whatever raw supply you started with. Since the LEDs don't draw a lot of current, and since you can also series them to use the same current through a number of them, the supply/regulator don't have to be large. You could likely even use a little 78L05 or similar - that is the lower current version in a little TO92. They come in standard voltages, and I think the 317s come in that version too.

LEDs won't change color, the color is determined by the physics of the materials. They would just get dimmer or brighter. Slap an LED and resistor with a pot in series across your bench supply and vary it. See how LEDs react. Mindful not to exceed the LED current limit.

[bob p]

BTW, the blue lit bridge they show on that page is actually in my home town

Interesting reading, Steve:

http://www.lumileds.com/pdfs/glasgow_case_study.PDF

[bob p]

thanks for your help enzo. you may be right -- if the mains supply is boucing around, maybe the meter lights are the least of my problems. but in that case where you have the amp on a dedicated circuit so the house lights aren't dimming objectionably, the meter lights will still fluctuate. maybe that's good, as its an indicator of the line voltage swaying. but its annoying to look at.

regarding my thoughts on using an LM317 -- that's because its what i'm familiar with. thanks for the other recommendations.

regarding color variation, when i was reading those spec sheets about white LEDs i ran into that chromatic shift thing. i thought that color shift didn't occur with LEDs, but i learned something new when doing some reading. it turns out that color shifts do occur, but only with the white LEDs. because there is no such thing as white light (its a collection of light of various frequencies), to make the white LEDs they actually use a combination yellow + blue LED. when the voltage varies on the white LEDs, they do have chromatic shifts between yellow and blue. who would have guessed?

[Enzo]

Ah. I have no experience with white LEDs. Monocolor LEDs don't do that as far as I know.

The 317 is fine, I suppose a couple extra resistors is not big deal.

Come to think of it, you could wire the regulator as a constant current source for the LED instead of regulating the voltage. Although either would work. That would facilitate adjusting the brightness though with a 317.

[bob p]

i've seen examples of the 317 used as a voltage regulator, but never as a constant current regulator. how do you do that with a 317?

[Enzo]

Look up the LM317 data sheet. Better yet, look in the Nat Semi analog manual under Vregs, and there are usually application notes. I don't know if those notes are included in the data sheet itself. The 78xx series can all be used as current regulators.

[bob p]

When the going gets tough, the tough get... Side emitting Luxeon LEDs
http://www.lumileds.com/products/line.cfm?lineId=1

The side emitting models are designed for firing into holes in light guides and plexiglass signs, and they are rated at a whopping 1 Watt

Steve, it looks like somebody else read about your idea and beat me to the punch in executing it. Personally, I would have used the LXHL side-emitting LEDs that you had mentioned. This fellow chose to use the LXK2 lambertian-emitting LEDs (which I thought would not be as good), but he got a very nice result.

Put on your sunglasses and sunscreen! Here it is:

http://www.carveraudio.com/phpBB2/viewtopic.php?t=3676





I probably would have mounted the regulator directly to the metal chassis crossbar instead of on a board mounted to the chassis.