Not homework but self study. I found this circuit online and it is the circuit for an LED lightbulb. I built it in Spice so I can see the voltages for myself and I'm surprised by what I found. V(n001) is the mains electricity and V(n002) is after R1 and C1 in parallel. I'm surprised that Spice says that there would be 240V between these two points and I'm wondering if this is actually correct. Furthermore, the voltages are the same at V(n005) (between D1 and D3). This would mean that one diode is taking 240V from the mains. If this was how the circuit was supposed to work then I would expect the mains to be connected directly in between D1 and D2. Since it is not, I'm assuming there is a mistake. Can anyone tell me if this is indeed incorrect or explain how these components take such voltages?

 

Recheck your circuit. I think it should be as below.

 

Using circuit from post #2:
Find the impedance of 1uF cap at 50hz. This is what limits the current into the LEDs.
This current gets rectified by 4-diodes and charges up C2.

 

Recheck your circuit. I think it should be as below.
View attachment 207011

Thanks. That works better now. Should R2 be in the circuit? Now at V(n004) there is a 50Hz voltage going between 2.1V and -3.5V. Between R2 and C2 there is a voltage between -0.7V and 5V. Wouldn't it be better to have the -0.7V and 5V going through the LEDs?

 

Thanks. That works better now. Should R2 be in the circuit? Now at V(n004) there is a 50Hz voltage going between 2.1V and -3.5V. Between R2 and C2 there is a voltage between -0.7V and 5V. Wouldn't it be better to have the -0.7V and 5V going through the LEDs?

Is this LTSpice? If so please post the .asc file.

 

Thanks. Here is the file

 

LTspice displays voltages relative to the ground node. Connect the ground to C2 negative.
Also you have normal diodes rather than LEDs on the output.

 

Moved ground over.
Picked some LEDs.
Used two resistors to force the current to share through the LEDs. In spice all the leds are the same. In real life they are not. What happens when you parallel LEDs, the current will not be equal. One side some time will take much more current.

This circuit works with out C2. When marketing wants the price down …. you learn what parts are not needed. Yes it works better with C2.

 

Your mains is shown as 266V peak which is only 188V RMS.

 

Moved ground over.
Picked some LEDs.
Used two resistors to force the current to share through the LEDs. In spice all the leds are the same. In real life they are not. What happens when you parallel LEDs, the current will not be equal. One side some time will take much more current.
View attachment 207038
This circuit works with out C2. When marketing wants the price down …. you learn what parts are not needed. Yes it works better with C2.

Just to note, without some DC filter cap you would get "turn your head blink syndrome", where when you turn you head or move the LED(s) across your field of view the light will appear as dots or dashes. It illustrates the principle by which the moving LED displays work which often rotate to create a 2d surface display.
Most people cant see 100Hz or 120Hz stationary blinking, but if the light sweeps across their field of view then virtually all peple can.
This is not to say that the values shown in any schematic here are the correct values to prevent this though.

Also of general interest, the ratio of C1 to C2 may affect the output voltage level.

 

The only reason to parallel LEDs is when you have a low voltage supply. Here the supply is 220vac. I would series the diodes up to half supply. (100V) I have made these with out R2. C2 is a little small, the ripple current is large but during the valleys the light is 50%. Just something to think about.

 

I have just dismantled one of these. It has a long series string of pairs of LEDs in parallel. A total of 72 LEDS, but basically that same circuit.
One pair of LEDs have a brown mark in the middle and they are open circuit and another pair read short, perhaps just one of the pair is short. Some replacement LEDs arrived today so that's a job for tomorrow.