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# Millett "Starving Student" hybrid amp - Page 48

yeah, i was just looking at it closer because it made no sense at all to have it rated for 12v! i feel silly now!

so time for the second silly question:

how do i wire up the led so it gets 12v delivered to it? if the power supply is 48v then i would need to put a few resistors in series right? what values?

use the current rating for the led, then work it out with ohms law.
"Rated Voltage* 12v
Lamp Life Approx. 40,000 hours
* Lamp contains a current limiting resistor to achieve the specified voltage."

It didn't really rate the mA specification for the LED. Does the above statement mean I don't need to add anything?

I was also wondering at what point in the wiring schematic it would be best to add the wires to power the LED.
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I got a little too excited yesterday and dove into this without any plan at all..... It wasn't pretty; however it did work.

Then I took it apart and built this.

What was wrong with the first attempt?

### Power LED

Where's the best place in the curcuit to insert an LED power indicator?
Quote:
 Where's the best place in the curcuit to insert an LED power indicator?
Off the 48V power supply.
Quote:
 Originally Posted by rds Off the 48V power supply.
Is this OK? When I used the LED resistor calculator at LED center to calculate the current-limiting resistor it warned me that a supply voltage >24v would dissipate excessive heat through the resistor (LED forward Voltage = 2.5v, Forward Current = 20mA, Supply Voltage = 48v. Resistor value = 2k7 2W)
Quote:
 Originally Posted by keiths Is this OK? When I used the LED resistor calculator at LED center to calculate the current-limiting resistor it warned me that a supply voltage >24v would dissipate excessive heat through the resistor (LED forward Voltage = 2.5v, Forward Current = 20mA, Supply Voltage = 48v. Resistor value = 2k7 2W)
It's OK as far as it goes, but you don't need 20ma - especially just for a power indicator. The chances are much higher that the LED will burn out over a short time. Plus, as you note - the power rating of the resistor becomes an issue.

You could pick a common-size resistor such as a 5K, then plug it into the Ohm's Law equations:

I = 48V/5000ohms = 9.6ma. P= (0.0096^2) * 5000 = 0.46W,
You'd want to have a 2X safety factor for heat, so a 1W resistor would do fine.

Or, you could go all the way to 10K:
I = 48V/10000ohms = 4.8ma. P= (0.0048^2) * 10000 = 0.23W,
a fairly typical 1/2W resistor would do fine in that case.

4.8ma for a panel-mounted power indicator LED is not at all unusual. For instance, amps such as the PIMETA or Mini3 use power indicator LED's that are down in the ~1-3ma range.
Quote:
 Originally Posted by tomb Or, you could go all the way to 10K: I = 48V/10000ohms = 4.8ma. P= (0.0048^2) * 10000 = 0.23W, a fairly typical 1/2W resistor would do fine in that case.
Thanks TomB. I don't have a 10K 0.5W resistor, but I have two 22K 0.25W ones - I'll be OK using those in parallel, won't I?
At the risk of sounding like a wise guy, The tubes on mine light up pretty bright...I think you could almost leave out the LED.
Quote:
 Originally Posted by vixr At the risk of sounding like a wise guy, The tubes on mine light up pretty bright...I think you could almost leave out the LED.
I was going to post something like that, so it's not just you. Even if they don't glow brightly it's pretty quickly obvious if you've got power or not.
Yeah, I guess I'd not have too much trouble telling if it's on or not without the LED

I'm using a knob on the volume pot that's numbered (not up to 11, unfortunately) and want something colourful to use as the index marker. A led seems to fit the bill nicely.
Quote:
 Originally Posted by keiths Thanks TomB. I don't have a 10K 0.5W resistor, but I have two 22K 0.25W ones - I'll be OK using those in parallel, won't I?
Well, I'm not a wise guy, so I would suggest just using the single 22K resistor, period. That would work out to 2.2ma and 0.1W, so a 1/4W resistor would be fine. If that doesn't work to suit you, you can always replace the resistor later.

The wise guys make a good point , but an LED taken close to the power switch can provide an indication of a hot circuit when the tubes aren't in there. They take awhile to light up, too - which can cause some uncertainty for a few seconds, anyway.
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