BrianGT LM3875 and power LED - how to?

[maurycy]

I have Rev C of BrianGT's LM3875 that I put together few years ago. I am now building a newer case for it and would like to have power LED. I have noticed that PSU board has space for LED and resistor. Now, since my electronic knowledge is very limited, how do I know what resistor value do I need to use? My trafo is 42VCT 21V-0- 21V 80VA which gives me about 29V after rectification.

I am also not sure if I can use the LED with PSU modified for CT transformer (I am only using 4 diodes and 1 PSU board).

Any help is appreciated.

[pikkujöpö]

Hi

This calculator will help: LED calculator for single LEDs

[Droidiphile]

I'm sure someone on these forums will say I'm full of it, but:

Method 1) Lift the existing resistor, either use it directly or measure it and use a resistor of similar value.

Method 2) Review the schematic and acquire a resistor of similar value.

Method 3) Assuming the LED drops 2V (which can vary a lot), and aiming for 0.020A (20mA) :
Resistance:
29V - 2V = 27V
27V / .020 = 1350 ohms
Power:
27V * 0.02A = ~0.5W

So : ~1350 ohm 1/2W resistor minimum (if you believe my assumptions). A 1W resistor would be mo-betta.

[maurycy]

Thanks for replies. Actually, the board just has space for resistor and LED. Nothing is mounted there yet.

Is LED voltage and current pretty much standard? I have a lot of LEDs but no specs for them. The one that I picked for case is green LED.

[godfrey]

Using standard resistor values:

1.5K resistor gives about 18mA current for the LED
2.2K gives about 12mA
3.3K gives about 8mA
4.7K gives about 6mA

More current makes it brighter.

It should work fine with your single PSU board and CT transformer.

Most common LEDs drop a little under 2V and will handle at least 20 mA. I'd aim lower than that though so it's not too bright.

Resistors only cost a couple of cents anyway so it's cheap to experiment.

[cjkpkg]

The manual for the chipamp LM3886 kit - which I have built - calls for the LED resistor to be 10K ohms. This is what I used and the LED seems plenty bright...

[maurycy]

Looks like 10K resistor it is. BrianGT just replied to me stating that 10K should work with pretty much any LED.

[Redshift187]

I use 50k multi-turn pots for LEDs, so I can adjust the brightness to suit my taste. Yes, that may mean mounting it off-board.

[maurycy]

That's a good idea Redshift. I will look into my parts bin to find out if I have one. Thanks for the tip.

[AndrewT]

Hi,
check you have DC voltage at the resistor/LED pads, not AC.
Use the 10k as suggested with your green LED.
Check it's brightness suits your taste both at night and during the day.
Adjust the resistance to get what you prefer.