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Old 26th June 2004, 01:26 PM   #11
jaycee is offline jaycee  United Kingdom
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LEDs do not "run" at a voltage as such. The only requirement is that the current flowing through them is kept to a low level, typically 15mA or so. LED's have a voltage drop, so you work out the resistor by ohm's law as follows - (Vsupply-Vdrop/current)

As for it running at 120VDC, it's probably more likely that it runs at +-60VDC and you're measuring across the rails. Is this some kind of high power amp ?
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Old 26th June 2004, 01:53 PM   #12
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Sorry, I don't design electronics. I just build the stuff. This particular beast is an old Studiomaster slave amp that is rated at 200W per chan into 4ohm.

The blue LED at maplin are listed as max foward voltage 5V. A silicon diode drops 0.6V so :
12-(0.6V X 2)/2 X 30mA = 170 ...say 200ohm?

I COULD just use a voltage divider to get to 12V or get close and use a voltage reg chip.
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Old 26th June 2004, 04:14 PM   #13
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you might try tracing the circuit. I drove myself nuts trying to find replacement lamps for an amp once that appeared to have 100V lamps (the front end regulated supply voltage.) There was a series resistor that drops it down to 12V when the lamp draws current. It won't show with a voltmeter's high impedance.

I just dropped in a 12 volt lamp since it was the only thing I could find in that "grain of wheat" style. But I don't recomment the "see if it blows up method."

Instead, you might also try putting a 1W 10K ohm resitor in place of the lamp and see if the voltage across it drops - a good sign that there is a resistor upstream. If there isn't a resistor upstream you'll see over a watt dissipation, so this should be a measure the voltage quickly and power down so you don't smoke the resitor test.

If it really is 120 Volts across the lamp, you'll need a lot more than 200 ohms in series to keep your LED alive. You want to keep the current below 30 mA, the max for the LED. The resistor(s) will need to drop 115 volts (120 - 5V dropped in the LED) V/I=R, you can do the math. A little more resitance can be used to dim the LED if desired. Used 20 K in my modified A75 (130 Volts cross the regulated rails), and a low output led (300 mcd) - it is still too bright. Don't forget to size your resitors properly - calculate the dissipated power, and add a safety factor.

good luck.
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Old 30th June 2004, 09:37 PM   #14
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I am now starting to doubt everything I ever knew.
*Do 2 LEDs in series double the forward voltage? I'm running off the 60V rail through 2 30mA LEDs. (60-5V)/0.60 = >1000R.
*LEDs light when you connect the annode to positive?
*Looking into an LED, the anode is the stick and the cathode is the flag?

Well I did that and nothing! I measure 40mA current flow. The 1K res is getting hot. I measure 1.3 V across both diodes.
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Old 30th June 2004, 11:45 PM   #15
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2-3k is a better number (don't add the currents for series connections)
try turning the leds around

edit
the two leds in series or parallel?
I think in practice a blue led drops about 3.6v 5V is the max figure
20ma ea is a better current
try turning the leds around
2-3k is still a better number
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Old 1st July 2004, 07:01 AM   #16
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HA!
Finally got the damm things to work. I found a short. Up they came. I used a 2K resistor. I'm a little worried that a 2W resistor gets almost too hot to touch. I'll back it off with a 3K, but I wonder what this is doing to the power supply. I would hate for this to be the cause of clipping at high power.
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Old 1st July 2004, 08:40 AM   #17
karma is offline karma  Canada
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should be fine led's take verry little to power up. nothing your power supply would notice at high volume

hopefully thay outlast the amp good luck

post a pic

p.s

if the resister run's a little warm make sure it's not touching the
clear plastic on the meter over time it may turn brown from the heat. if it's anywarez around it.
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Old 24th July 2004, 09:07 AM   #18
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Default VU LED conclusion

I was asked for a photograph. Sorry that there is a little camera shake. I could only take pix in the half light. They really do light the meters well........ with the bonus that they really stand out.
Well if you're going to make a show...

Thanks guys.
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Old 24th July 2004, 02:32 PM   #19
karma is offline karma  Canada
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nice
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Old 25th July 2004, 04:43 AM   #20
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sexy (as in cool)
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