Need help with LED spec

[Thalis]

I am assembling a 47 headphones amp and have sourced better capacitors for it but I also want to change the included blue LED to amber. I am not sure what to get so I hope to get some help here. What voltage and current should I look for in an LED?

Specs say the amp can be powered from 12V to 18V but i am going to supply it with 12V. Does it matter if one day I decide to increase the voltage to 18V for the LED installed?

Thank you in advance.

PS. The link on the paper does not work somehow and i cannot read Chinese (and I am Chinese!!! )

 

[solderdude]

An LED requires a current. Not a voltage. The current determines the brightness (along with its sensitivity).
There is always a resistor or current source feeding the LED.
In most cases it is simply a resistor.
On 12V the resistor value will be between 1k to 50k depending on the type of LED and required brightness.

There are a few LED'lamps' for usage on 12V (car mostly) but these are ordinary LEDs that have a resistor built-in the enclosure.

 

[Thalis]

An LED requires a current. Not a voltage. The current determines the brightness (along with its sensitivity).
There is always a resistor or current source feeding the LED.
In most cases it is simply a resistor.
On 12V the resistor value will be between 1k to 50k depending on the type of LED and required brightness.

There are a few LED'lamps' for usage on 12V (car mostly) but these are ordinary LEDs that have a resistor built-in the enclosure.

Hi solderdude... thanks for replying. I am looking at the traces on the board and it would seem that the positive terminal of the LED (the longer lead) is going to a 10k.

PS... was it you who wrote the guide to mod the Bravo tune hybrid amp? I followed most of it for the Little Bear P-1

 

[solderdude]

PS... was it you who wrote the guide to mod the Bravo tune hybrid amp?

I wrote a guide for the Indeed G2. The Bravo v2 was written by Florian Gerard.

When you have selected/bought a LED you can change the value of the 10k resistor (or use a small 50k pot ?) to adjust the brightness if needed/desired.

 

[Thalis]

Yep.

When you have selected/bought a LED you can change the value of the 10k resistor (or use a small 50k pot ?) to adjust the brightness if needed/desired.

So it doesn't matter what LED to get even if I keep to the 10k resistor?

The Little Bear P1 now actually behaves more like a SS amp after the MOSFET replacement

 

[solderdude]

So it doesn't matter what LED to get even if I keep to the 10k resistor?

When you leave the 10k I would recommend a 'high efficiency' LED. Usually these are specified at 2mA instead of 20mA.

 

[Thalis]

When you leave the 10k I would recommend a 'high efficiency' LED. Usually these are specified at 2mA instead of 20mA.

ok I see 3mA is available under the heading "Forward Current If". "Mounting Hole Diameter" refers to the diameter of the LED?

 

[March Audio]

So it doesn't matter what LED to get even if I keep to the 10k resistor?

The Little Bear P1 now actually behaves more like a SS amp after the MOSFET replacement

Are you sure that its 10k? That would be very low current.

Vsupply - Vdrop (of LED) / Resistance = current through LED.

Voltage drop (forward voltage) of the LED might be 1.8 volts.

so 12V - 1.8V = 10.2V

10.2V/10000ohms = 1.02mA

I would be highly surprised if it lit up.

 

[Thalis]

Are you sure that its 10k? That would be very low current.

Vsupply - Vdrop (of LED) / Resistance = current through LED.

Voltage drop (forward voltage) of the LED might be 1.8 volts.

so 12V - 1.8V = 10.2V

10.2V/10000ohms = 1.02mA

I would be highly surprised if it lit up.

yes it is a 10k.... there is a 4.7k next to it but no trace to the LED

 

[March Audio]

random led picked out of RS

Forward voltage is 2 volts.

current V luminosity

 

[March Audio]

yes it is a 10k.... there is a 4.7k next to it but no trace to the LED

As you can see from the graph above it will produce hardly any light a 1mA. 8 to 10 mA might be more realistic numbers.

Any way you should look at the specs for the specific LED you want to use and change the resistor to achieve the brightness you like.

 

[solderdude]

High efficiency LEDs run fine on 10k and 12V when used as an indicator/pilot light in indoor circumstances.
For backlit or to illuminate something it is another story. It is highly dependent on the LED though.
Whenever I build something and grab a random LED I use a 1k in series with a 50k log type potmeter and adjust the intensity and then fit a fixed resistor.
Blue LEDs are often particularly efficient and if you don't want it to light up de room 1mA is usally enough.

The shown plot shows relative illumination is almost linear to current.
Our eyes, just like the ears also work in a log. type of way.

 

[March Audio]

performed a filter for 1mA on RS and found only 5 leds.

Over 5mA came up with thousands.

 

[Thalis]

ok i found some 5mm LEDs and this is the only spec shown... this is for amber:

- Wavelength : 605nm - Luminous intensity (@20mA) : 3000-4000 mcd - Forward voltage : 2.0-2.1V

 

[Thalis]

sorry I did not clarify I need a 5mm x 8mm LED as shown in my pic

 

[March Audio]

ok i found some 5mm LEDs and this is the only spec shown... this is for amber:

- Wavelength : 605nm - Luminous intensity (@20mA) : 3000-4000 mcd - Forward voltage : 2.0-2.1V

Thats very bright.

Have a look on Mouser or Digikey or RS Components, you will have thousands to choose from.

 

[solderdude]

The current rating is where they emit the mentioned light level.
No one needs an indicator to have 3 or 4 cd. This would be blinding for indicator.
Just fit it and adjust brightness when needed.
Chances are it will still be bright enough at 1mA or even less.

 

[Thalis]

Thats very bright.

Have a look on Mouser or Digikey or RS Components, you will have thousands to choose from.

ah ok... so thats where the mA comes in? Forward voltage is fine yes?

 

[Thalis]

The current rating is where they emit the mentioned light level.
No one needs an indicator to have 3 or 4 cd. This would be blinding for indicator.
Just fit it and adjust brightness when needed.
Chances are it will still be bright enough at 1mA or even less.

ok I think i now understand... 20mA too bright.... 1mA or less dimmer... forward voltage leave it at 2V or so

 

[solderdude]

Yes, A LED is a current driven device and the voltage drop only changes a little.
Voltage drop is colour also dependent by the way given the same current is used.

Intensity is almost linear with current. Because our eyes are close to logarithmic a substantially lower current results in less perceived brightness.
Roughly, there is a knee in the lowest currents and above the nominal current the relative increase in light is also smaller.