AcousticBruce
Member
lets say I have a 120V source, capacitor, 2 LED's rated at 40mA and a resister (or no resister) and I want to run the LEDs on with a low power loss. Questions at bottom.
Circuit #1
+120VAC
+3K Ohm
+2 LEDs ran in parallel.
This runs fine, but power loss on resister is 4.8 Watts!
Circuit #2
+120VAC
+Capacitor 884nF
+2 LEDs ran in parallel.
I used [tex]1/(2pi * f * Xc) = 884nF[/tex]
This just seems like it is not ok.
circuit #3
+120VAC
+300 Ohm resistor
+and I did a 982.6nF cap
The reason I chose that cap is because I needed 3K of impedance in order to get it at 40mA. I took 1/(2pi*(60Hz)(2700 Xc)), well it ended up being over 40mA. I imagine because I needed to do the impedance equation.
Circuit #4
+120VAC
+300 Ohm resistor
+888.6nF cap
I did sqrt(R^2 + Xc^2) = Z. I made Z = to 3k Ohm and derived an equation to solve for Xc. I ended up getting 2985Xc needed.
So for all this I have a few questions.
1) In AC circuits, is it good or bad practice to use a capacitor for Xc instead of using a resistor?
2) Why did the Xc work without the resistor (circuit 2), but when I added a resistor in Circuit 3, it seemed to be of by 5mA?
3) Did I use the formula, Z = sqrt(R^2 + Xc^2) or Z^2 + R^2 = Xc^2, correctly? Is that formula what you need to use a resistor and capacitor at the same time in a circuit?
4) Why is this stuff so damn interesting?
Circuit #1
+120VAC
+3K Ohm
+2 LEDs ran in parallel.
This runs fine, but power loss on resister is 4.8 Watts!
Circuit #2
+120VAC
+Capacitor 884nF
+2 LEDs ran in parallel.
I used [tex]1/(2pi * f * Xc) = 884nF[/tex]
This just seems like it is not ok.
circuit #3
+120VAC
+300 Ohm resistor
+and I did a 982.6nF cap
The reason I chose that cap is because I needed 3K of impedance in order to get it at 40mA. I took 1/(2pi*(60Hz)(2700 Xc)), well it ended up being over 40mA. I imagine because I needed to do the impedance equation.
Circuit #4
+120VAC
+300 Ohm resistor
+888.6nF cap
I did sqrt(R^2 + Xc^2) = Z. I made Z = to 3k Ohm and derived an equation to solve for Xc. I ended up getting 2985Xc needed.
So for all this I have a few questions.
1) In AC circuits, is it good or bad practice to use a capacitor for Xc instead of using a resistor?
2) Why did the Xc work without the resistor (circuit 2), but when I added a resistor in Circuit 3, it seemed to be of by 5mA?
3) Did I use the formula, Z = sqrt(R^2 + Xc^2) or Z^2 + R^2 = Xc^2, correctly? Is that formula what you need to use a resistor and capacitor at the same time in a circuit?
4) Why is this stuff so damn interesting?