Powering red LED diode as my on/off indicator in my amp build

[hesamadman]

Anyone have any way to achieve the amount of voltage required to power a red LED. My build has a 5v winding that I wont be using since I am using SS rectifier. Is it unheard of or any reason why I should not use that, rectify, and filter to achieve the 1.7v I believe I need?


Edit:
My goal was to use a DPDT switch. one side have the main power and the other pole LED source.

[eleventeen]

No reason at all, except you really don't need to filter LED power. If this is strictly an on/off pliot lamp, you don't need to filter it at all. If it is wiring that is going to travel alongside other wires, say out to a pedal, you may want to filter the DC you create with the rectifier. If it were me, I would run it from the 6.3 volt winding only because that would allow the complete and total non-use of the 5 volt winding. Tape the 5 v wires up, tuck them out of the way. But it's not a big deal either way. Be sure to include the LED current limiting resistor!

[hesamadman]

No reason at all, except you really don't need to filter LED power. If this is strictly an on/off pliot lamp, you don't need to filter it at all. If it is wiring that is going to travel alongside other wires, say out to a pedal, you may want to filter the DC you create with the rectifier. If it were me, I would run it from the 6.3 volt winding only because that would allow the complete and total non-use of the 5 volt winding. Tape the 5 v wires up, tuck them out of the way. But it's not a big deal either way. Be sure to include the LED current limiting resistor!

Excellent. That makes sense about the filtering. This would only be for indicator use so I wouldnt have to filter. Is there anything else I should do besides the current limiting resistor? I guess I wouldnt need any diodes since the LED its self is a diode.

[sluckey]

You actually need MORE than 1.7v for a LED. The LED will have a constant 1.7v (depending on particular LED) dropped across it. Use a series resistor to drop the excess voltage applied.

I just used a 4.7K resistor in series with the LED and connected to the 6.3V filament string. Would work fine on your unused 5V winding too. See this schematic...

     http://home.comcast.net/~seluckey/amps/PeeWee/PeeWee.pdf

Works fine, however... The maximum reverse voltage for most LEDs is only 5V, so to protect the LED it would be wise to put a 1N4007 in series or back to back across the LED. Doing so will eliminate any reverse voltage across the LED. My AC powered LED has been running for a few years now, but if it dies, I'll know why.

[eleventeen]

"My goal was to use a DPDT switch. one side have the main power and the other pole LED source."


No need to do that. If you have a DPDT switch stout enough for powering on/off the amp, then most, I think, would say as safety matter, switch *both* sides of the AC line. You don't need to switch the LED at all if it runs off a heater winding.

[MakerDP]

Have you seen this document from Merlin? Look at the end of it...

http://www.valvewizard.co.uk/OtherStuff.html

[Tom_Hull]

hi


for a power led light ..
.and to know if the there is power to or at  the on off switch .


what i did was buy a dollar store led nightlight that plugs in the outlet  .....took it apart and use that to the 115volt power switch installed in the amp.
but i change the light and the diode  and resistor later using the same curcuit design.cheap and just a normal switch required .

[hesamadman]

"My goal was to use a DPDT switch. one side have the main power and the other pole LED source."


No need to do that. If you have a DPDT switch stout enough for powering on/off the amp, then most, I think, would say as safety matter, switch *both* sides of the AC line. You don't need to switch the LED at all if it runs off a heater winding.

   im really not as dumb as i sound on here. I guess I get in a hurry and dont think things through before posting.

[hesamadman]

I just used a 4.7K resistor in series with the LED and connected to the 6.3V filament string. Would work fine on your unused 5V winding too. See this schematic...

     http://home.comcast.net/~seluckey/amps/PeeWee/PeeWee.pdf

Thanks Sluckey this is exactly what I was looking for

[PRR]

> if it dies

It won't die.

Reverse breakdown is not fatal if the power rating is not exceeded, if the current is limited. And you have the resistor.

Say the LED reverse breakdown really is 5V. (It is probably more like 7V; 5V is a CYA spec.)

The 5Vrms winding peaks at 7V.

The excess, 7V-5V = 2V, appears across the 5K resistor.

2V/5K = 0.4mA.

P = V*I. The LED feels 5V at 0.4mA which is 2mW.

What is the power rating of an LED?

We "know" that any little semiconductor is good for 50mW. Mostly these-days, 300mW. There is a spec that the forward power can be 1.7V*20mA or 34mW.

The 2mW of the reverse breakdown is far-far below this.

You "can" get in trouble at high forward current and much higher supply voltage. Hmmmm.... lets try forward-bias half-wave 40mA (to get 20mA average) and a 120V supply. Use a 3K resistor. The reverse-breakdown is 5V at 38mA, 191mW. ~~200mW is "probably" safe, but nothing on the data-sheet assures us of that. (Also note that this condition puts 5 Watts in the resistor-- getting 1.7V from high volts is very inefficient.)

If you want to be truly paranoid, put a 1N4007 backward across the LED. Now the reverse voltage won't be over 0.6V.

Filtering *may* be nice. The half-wave action gives a 60Hz flicker, worse than the 120V flicker of a Neon or Fluorescent. If you move your head fast you can see it. If your youth involved mind-altering drugs, it may bring-back odd illusions. I didn't mind giving a half-wave LED indicator to students, but if I had to work around it much longer it wudda got a proper rectifier and a capacitor.

[DummyLoad]

220R works 100R put me on target @ ~20mA on my breadboard. i'm using older 1980's production monsanto LED's.  from memory, for best results: If =< 20mA; typ. 15-17mA.

fired it up on the breadboard with a 6.3VAC source. 4.7K R was so dim had to turn off lights to see a faint glow.

actual experiment telemetry:

Vsource = 7.15VAC
Rlimit = 120R
VeseR = 2.8VAC
VeseLED = 4.8VAC
Ice of LED= 17.6mA

Rlimit 100R
VeseR = 2.75VAC
VeseLED = 4.82VAC
Ice of LED = 20.5mA

i'd use 120-150R to be on the safe side of 20mA.

--pete
 

[PRR]

> using older 1980's production monsanto LED's.

Haitz's Law

[shooter]

"Haitz's Law"
So will my local R.S. store start giving me money soon?

[John]

No, but they'll give you a good deal on batteries!