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Replacing meter lamps with Leds?
Old 8th May 2006
Lives for gear
Led's Avatar
Replacing meter lamps with Leds?

HI all, I've got VU meters that the lamps are blown in. They are fed 12 volts for the lamps and have 2 solder in lamps in series on each meter. To replace the globes with leds would I need to use enough leds to add up to 12 volts? For instance, 6 x 2 volt leds? Thanks for any advice.
Old 8th May 2006
Gear Maniac

I recently modified a Soundtracs which had "bipin" type filament lamps soldered to a long PCB fitted to the rear of the meters.

I found a suitable bright white bipin LED - with built in resistor for direct use on 12 volts - exactly the same as the filament lamp voltage.

So this was a very simple replacement job.....

I have had a quick look in my Farnell catalogue - they only seem to have 12 or 24 volt types - but perhaps you can easily modify your board for 12 volt types? These LED's are made by CML - perhaps have a search to see what is available?
Old 8th May 2006
Lives for gear
brianroth's Avatar

Several issues here...

First: LED's are "current driven", and typically need anywhere from 5 mA to 20 mA depending on the desired brightness. For discussion, use 10 mA as a starting point. Next, LEDs have a voltage drop that varies depending upon the color, and again, I'll use a ballpark of 2V per LED.

So, we would require an 8 Volt drop across a resistor in series with the two series-connected LEDs (12V power supply source minus two drops at 2V each). Ohms Law: R = E/I or R = 8/0.01 R = 800 Ohms. The power dissipated across the series resistor is quite low, around 0.08W, so a 1/4 W resistor would suffice.

Second: LEDs tend to "spew" their light out the end of the package vs. in an "omni" pattern like an incandescent bulb. That may be a problem in some situations.

Old 8th May 2006
Lives for gear
Led's Avatar
Hi, it's a Tascam. I haven't seen those type of leds here in Australia, just normal 2 volt and 3.4 volts. Two of the 3,4 volts would be bright enough but wouldn't match the voltage. The leds have a min voltage of 3.1 and max of 3.4, so is it best to use 3.2 as a typical voltage? Using 3.2 and 20 milliamps I get a 280 ohm resistor as needed.

Old 8th May 2006
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Led's Avatar
Thanks Brian and Mixermend.
Old 8th May 2006
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Led's Avatar
Actually, I think these are in parallel. Does anyone know if Tascam meters are usually wired in parallel? It seems if I touch a globe to either of the 2 globes contacts it lights up without a globe in the other side.
Old 13th May 2006
Gear Maniac
DaveH's Avatar
If they are in parallel and from brianroth example you
would have :

connect-------- LED --------- 1200 resistor -----------connect (replaces 1 lamp)

connect---------LED ----------1200 resistor -----------connect (replaces 1 lamp)

(1 led in series with one resistor to replace one lamp)

It may take some experimenting to get it to look right.
You can choose angle of light coming from LED
Wider tends to cover more area, with dimmer light.
Narrower tends to cover less area, with brighter light

You can also choose:
Brightness range of leds, usually expressed in MCD. (More is brighter).
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