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Old 27th March 2005, 02:10 AM   #1
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Default How to use LED with +- 25v tranny?

Building brian gt lm3875 rev 3 stereo gainclone, with Avel 330va +-25volt tranny, expecting +-35vdc after power supply board rectification and filtering.

Have very small enclosure and want to use this power switch/LED "12V LED" combo, but not sure if I can drive this LED.

http://www.mouser.com/catalog/specsh...SH-R13-523.pdf

This is the Mouser part, and it only says pushbutton switch, 3A 125Vac, 12V LED illumination. 3 amp should be just enough, but how to drive this LED - is it possible?
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Old 27th March 2005, 03:03 AM   #2
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Attached Images
File Type: jpg led.jpg (8.3 KB, 512 views)
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Old 27th March 2005, 03:19 AM   #3
jcayer is offline jcayer  Canada
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6K8 1 Watts (at least )
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Old 27th March 2005, 06:46 AM   #4
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Old 27th March 2005, 12:34 PM   #5
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LEDs don't like reverse voltages over 5 volts.. Put in an antiparallel diode or a diode in series...
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Old 27th March 2005, 12:44 PM   #6
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megajocke wrote:
Quote:
LEDs don't like reverse voltages over 5 volts.. Put in an antiparallel diode or a diode in series...
True....but therez no reverse voltage here....LED is connected to DC...
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Old 27th March 2005, 06:17 PM   #7
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Default Still puzzling LED circuit - not always so simple!

Thanks so much for helping. Still a puzzler though...

Is that a 6K ohm resistor? I dont understand why 1 watt is required, LEDs draw around 20 ma... Please clarify...

Since the switch includes the "12 volt LED", wouldnt that mean there is already a series resistor included in the device, chosen so that the led fires correctly with 12 vdc?

If so, wouldnt then choose a resistor that, together with the inclued resistor (series) would total 6k, or better yet try increasingly lower values until something around 20 ma is drawn...

Seems that datasheet is not detailed enough...
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Old 27th March 2005, 06:27 PM   #8
GregGC is offline GregGC  Canada
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Default Re: Still puzzling LED circuit - not always so simple!

Quote:
Originally posted by ransom peek
Thanks so much for helping. Still a puzzler though...

Is that a 6K ohm resistor? I dont understand why 1 watt is required, LEDs draw around 20 ma... Please clarify...

Since the switch includes the "12 volt LED", wouldnt that mean there is already a series resistor included in the device, chosen so that the led fires correctly with 12 vdc?

If so, wouldnt then choose a resistor that, together with the inclued resistor (series) would total 6k, or better yet try increasingly lower values until something around 20 ma is drawn...

Seems that datasheet is not detailed enough...
According to the picture above: 35+35=70V 70V/6.8k=10mA (LED current)
10mA x 70V = 700mW (the power dissipated in the resistor)

Hope it makes sense

Greg
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Old 27th March 2005, 06:36 PM   #9
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Default Getting Closer but not there yet!

Thank Greggc (is that gc for gainclone?)! Now it's clear. It just didn't seem intuitive that a 1 watt resistor would be needed for an LED.

But, I still think there is likely to be a resistor inside the switch/led assembly that will be in series with the added resistor. So, I'll check it. If there is a resistor there, I think it wont work, because the spec sheet says 12 volt led, so at +- 35 vdc = 70 vdc, the current rating of the internal resistor will be exceeded. Now, an added series resistor will drop some of the voltage.

So I need to figure out the math on this one...
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Old 27th March 2005, 06:50 PM   #10
indoubt is offline indoubt  Netherlands
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Another very simple, efficient method is to take a thin isolated wire and make 5 windings on your transformator. To be shure just measure the voltage that this 5 windings will supply. Works like a charm and does not have any adverse effects on any of the circuits (if it would at all, in some cases it is mentioned that leds coud have an audible effect if you place them on the supply rails, I have never been able to hear it though)
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