LED circuit board

[michaeltrego]

I have been thinking about building a mini LED circuit board, rather than having individual large indicator lights. The reason is two-fold, to learn how to build one, and to have a different style of status indicators. The goal is to have 4mm light pipes (salvaged from an old cable modem) embedded in the panel door with a legend next to it.

Based on some research, I think I have two options for circuit design. The first is a half-wave, which may have noticeable flicker, and the second is a full-wave rectifier for flicker-free.

Are there any electrical engineers out there that can validate my approach and the component values? Has anyone done something similar? I have seen a lot of warnings about building LED connections to mains power, so I don't want to get hazardous, but with all of the 120V LED indicators on the market I figured it would be achievable if done carefully.

 

[passedpawn]

I'd go with the half wave for simplicity. I doubt you'll see flicker. I'd also increase the current by doubling the capacitor value (i.e., 1uF). The way you have it now, you'll get about 18mA rms / 2 = 9mA rms through the LED since you're running the LED half cycle. It might be dim as it is now. If you do that, I think you're still OK with the 1W resistor.

 

[michaeltrego]

I'd go with the half wave for simplicity. I doubt you'll see flicker. I'd also increase the current by doubling the capacitor value (i.e., 1uF). The way you have it now, you'll get about 18mA rms / 2 = 9mA rms through the LED since you're running the LED half cycle. It might be dim as it is now. If you do that, I think you're still OK with the 1W resistor.

Thanks passedpawn - I was leaning towards simplicity as well. I understand that the LED wants about 20mA. Can you explain the calculations you used for the resulting current? I'm still learning...

 

[passedpawn]

Thanks passedpawn - I was leaning towards simplicity as well. I understand that the LED wants about 20mA. Can you explain the calculations you used for the resulting current? I'm still learning...

cap reactance is 1/(2*pi*f*C). that's going to be 5647 ohms as you have the design now. Add the 1k resistor and using ohms law (I = V/R) so I = 120V rms /(5647+1000) = 20mA rms. (not sure where I got 18 above - fat fingers on calculator I suppose)

But the LED is off during half the cycle, so really your average continuous current in the LED is only about 10mA. Double the cap value and you (nearly) double the current.

 

[michaeltrego]

cap reactance is 1/(2*pi*f*C). that's going to be 5647 ohms as you have the design now. Add the 1k resistor and using ohms law (I = V/R) so I = 120V rms /(5647+1000) = 20mA rms. (not sure where I got 18 above - fat fingers on calculator I suppose)

But the LED is off during half the cycle, so really your average continuous current in the LED is only about 10mA. Double the cap value and you (nearly) double the current.

Got it - the cap reactance was my missing piece - thanks! I think you were actually right with 18mA originally. So with 1uF cap and 1K resistor, I'll see 32.8ma peak current every half cycle. Is that going to be OK with the LED?

CapReact = 1/(2*pi*60Hz*1uF) = 2653
mA = 120V/(2653+1000) = 32.8

 

[passedpawn]

Got it - the cap reactance was my missing piece - thanks! I think you were actually right with 18mA originally. So with 1uF cap and 1K resistor, I'll see 32.8ma peak current every half cycle. Is that going to be OK with the LED?

CapReact = 1/(2*pi*60Hz*1uF) = 2653
mA = 120V/(2653+1000) = 32.8

Yes, I think it will be OK.

 

[Brumateur]

michaeltrego,

Keep it simple. For 120V just use 1Watt 15K resister in line with LED. And nothing else. I found it bright enough. Just try. If you want to increase brightness you can add another 1Watt 15K resistor in parallel with first one.

 

[passedpawn]

michaeltrego,

Keep it simple. For 120V just use 1Watt 15K resister in line with LED. And nothing else. I found it bright enough. Just try. If you want to increase brightness you can add another 1Watt 15K resistor in parallel with first one.

I would be concerned with the reverse voltage breaking down the LED. The diode there keeps that from happening.

 

[Brumateur]

OK, tnan add a one diode in line with LED or parallel (in reverse direction) to LED. Anyway you don't need a capacitor.

 

[huntb]

The cap might help with the flickering some (which would be the reason for increasing the capacitor instead of the resistor). Although as long as you are not shaking your head while looking at the panel you won't notice the flickering

Edit: Also, make sure the LED you choose can handle the current. I didn't see anywhere what the current rating is (maybe I skipped over it)

 

[michaeltrego]

The cap might help with the flickering some (which would be the reason for increasing the capacitor instead of the resistor). Although as long as you are not shaking your head while looking at the panel you won't notice the flickering

Edit: Also, make sure the LED you choose can handle the current. I didn't see anywhere what the current rating is (maybe I skipped over it)

These are the LED's that I'm using:
http://www.taydaelectronics.com/leds/round-leds/5mm-leds/red/led-5mm-red.html

I haven't been able to find the actual Kento datasheet for these, but most 20mA LED's that I have seen have a peak forward current of 30mA.

 

[kenh]

I have been thinking about building a mini LED circuit board, rather than having individual large indicator lights. The reason is two-fold, to learn how to build one, and to have a different style of status indicators. The goal is to have 4mm light pipes (salvaged from an old cable modem) embedded in the panel door with a legend next to it.

Based on some research, I think I have two options for circuit design. The first is a half-wave, which may have noticeable flicker, and the second is a full-wave rectifier for flicker-free.

Are there any electrical engineers out there that can validate my approach and the component values? Has anyone done something similar? I have seen a lot of warnings about building LED connections to mains power, so I don't want to get hazardous, but with all of the 120V LED indicators on the market I figured it would be achievable if done carefully.

I don't know where you got the circuit from but the cap is there to block DC and it's a complete waste of money across AC power because there is no DC in AC power. It looks like a telephone ring detect circuit.

The diode in parallel with the LED will work fine.