Hi,

i want my LED to light up when the power supply is on, so question is i can design the circuit( image attached)
supply voltage is 24V, and is there any problem doing this?


thanks

 

It should be fine if the current, or more directly, power is within the LED's specs. Also make sure you use a resistor rated at 1.3 times its power dissipation or more.

 

If the led is rating at 2V/20mA, and considering the brightness and usage life, I will recommended that you should use it about 16 mA is better, so the resistor will be as 1K+360Ω or 1K+390Ω and the current will be as 16.2 mA or 15.8 mA.

 

For battery-powered equipment, where the additional battery drain of the LED indicator subtracts from the useful battery life, you might consider a self-flashing LED, which has lower total current demands. Also, I have operated Red "ultra-bright", high-efficiency LEDs on as little as 500uA (not near 20mA as Scott suggests), and they are still bright enough to be seen in direct sunlight as a power ON indicator.

Do an experiment; the LED current only needs to be whatever it takes for you to see the indicator...

 

I often set my indicator LEDs to just 5mA or so. It's plenty. Flashing would be even better.

 

Get a blinker LED (LED Indication) and pick a resistor which is going to limit it to low current like 10mA.

 

Get a blinker LED (LED Indication) and pick a resistor which is going to limit it to low current like 10mA.

You can also get ultra efficient LEDs that give a useable indication at only 2mA.

 

I often set my indicator LEDs to just 5mA or so. It's plenty. Flashing would be even better.

What's your suggestion for the circuit and current?

 

What's your suggestion for the circuit and current?

Me? I would likely use the resistor current limiter the TS showed in #1, just using a higher ohms resistor to get the current in the 1-5mA range. If I had a flashing LED and I cared a LOT about current draw, I'd use that.

If I really needed to limit power, I'd look at a 555 flasher with a duty cycle of, say, just 1%. (I assume a normal flashing LED has a higher duty cycle.) A 1ms flash every one second or so, that sort of thing. Of course it would have to be a low power 555.

 

If I really needed to limit power, I'd look at a 555 flasher with a duty cycle of, say, just 1%. (I assume a normal flashing LED has a higher duty cycle.) A 1ms flash every one second or so, that sort of thing. Of course it would have to be a low power 555.

As you suggested that as 555 and 1% duty cycle, how is the current (mA)?

 

This feels like a hijack of the TS's thread, but see here.

 

This feels like a hijack of the TS's thread, but see here.

I don't think that is hijack, you just given another idea and I'm trying to make it clearly.