LED question, Festoon VU Meter bulb replacement

[nielsk]

I am working on a LED retrofit for the SSL console VU meter that will utilize the existing festoon type bulb holder. This runs off 36 volts (+18 and -18 volts). I am considering this:
http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTA-1000Y.pdf
it is 10 LEDs in a bargraph arrangement. Can anyone advise on the advantages / disadvantages of running 10 LEDs in series or parrallel? It would be most simple to connect all the anodes together and all the cathodes together, and use one current limiting resistor (or one on each end)

 

[PRR]

Two LEDs is usually plenty-much light.

LEDs are low-voltage, much lower than usual audio power rails. You have 36V available and need <2V per LED. One or two LEDs will wind up wasting over 90% of the power you point into them. LEDs are not high power, but not low-power, so you need to think what your system can spare and if you need to be careful about it.

The 10-LED array may be interesting. I'd run 2 LEDs at maybe 10mA each (in series, with 3K 1W resistor across the 36V). But you could run 10 LEDs at 2mA each and get similar light. And now the LED voltage is nearer 17V total. You can't just tie that to an 18V rail because current is uncontrolled and small tolerance change will give zero to blow-up current. But you could resistor to the other rail at 19V 2mA which is 10K 1/8W. And instead of 10mA (generally negligible in larger systems) it is just 2mA (hardly any wall-power system will notice 2mA extra.)

I'm neglecting the fact that you are removing the incandescents, which may free-up a relatively huge lump of power. But incandescents usually sucked unregulated power, not regulated power. So while heat in box goes down, lighting from the regulated power raises regulator heat. 10mA is probably fine, 2mA is more-probably fine.

 

[PRR]

> current limiting resistor (or one on each end)

If you plan to poke a screwdriver at the back of the meter, two resistors makes it much harder to short a supply rail and blow-up your supply. Put them at the rail, then all the wiring and LED stuff may be shorted recklessly. If you expect to do this a lot, you should check resistor dissipation for all possible shorts; for most DIY, these resistors and the LEDs are frills, the show goes on without them until you can get back to the bench, so a rigorous failure analysis is not necessary.

 

[Slenderchap]

If you have the inclination and time you can always make something like this;

It is a long time since I did it ... but if I remember correctly it has a couple of resistors/diodes on the back of it.... runs on 9-13v... and actually looks better than a standard festoon as it gives a more even light spread..... also a bit yellower which give a "vintage" look.

Commercially it was not worth making.... so it was shelved.

Colin
www.audiomaintenance.com

 

[sahib]

I agree with PRR that two LEDs will give you more than what you need. Particularly now LEDS at 16,000 mcd are easily available. Also the surface mount types as Colin suggested occupy less space. These will even blind you if you look at them straight, very powerful.

A small tip. If you place a self adhesive silver tape around the VU's front glass, then the light will bounce back and spread inside the VU much more evenly. It may even give a strip of light all around the edge of the glass as an additional feature.

My advice is to get some samples and experiment first.

 

[nielsk]

Part of why the bargraph is nice is because it spreads the light around and simulates the bulb illumination. Using 2 LEDs looks like the corners are lit only.... the unit Colin posted looks wonderful, and there are all kinds of LED cluster bulb replacements available for 12 volt festoon lamps:
http://www.superbrightleds.com/festoon.html
for as low as $1 each. I used the blue ones on a Neve 3228 with 12v bulbs and they look great.
The SSL uses regulated power fed from each module to each meter with the meter feed cable, how about a resistor voltage drop on each terminal so I can use one of these 12 volt units? Is that possible / practical?
Of course, there is the option of changing the wire loom to the meter bulbs to give them a 12 volt feed, then it's just a case of popping in the LED replacement bulbs, but that would be a considerable amount of work.

 

[MikeHolmes]

I am working on a LED retrofit for the SSL console VU meter that will utilize the existing festoon type bulb holder. This runs off 36 volts (+18 and -18 volts). I am considering this:
http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTA-1000Y.pdf
it is 10 LEDs in a bargraph arrangement. Can anyone advise on the advantages / disadvantages of running 10 led lights in series or parrallel? It would be most simple to connect all the anodes together and all the cathodes together, and use one current limiting resistor (or one on each end)

Were you able to complete the project? SO is it better to apply leds in series or parrallel?

 

[pinebox]

Sorry to dig this up a decade later, but can someone confirm for me the size of festoon used in the Sifam SSL VU meters?

 

[sodderboy]

That has two bi-pin lamps in it, not a festoon. The SSL meter in my junk pile has no bulbs or sockets, but my guess is T1 18V, two bulbs in series.
Amazing how the lamps timeline has progressed since 2008! LED replacements I was using back then are obsolete at least a generation.

 

[pinebox]

That has two bi-pin lamps in it, not a festoon. The SSL meter in my junk pile has no bulbs or sockets, but my guess is T1 18V, two bulbs in series.
Amazing how the lamps timeline has progressed since 2008! LED replacements I was using back then are obsolete at least a generation.

Thank you for the reply, there are actually two different meter illumination styles, one is the pair with bayonet bases like you are saying but there is another more common festoon bulb from above that I cannot find any size reference for.