hello guys,
I am new to this forum and I am interested in electrical & electronic.
I would like to know

4 led (4v) are connected to DC 4v parallel. Therefore, each led got DC 4v equally or got less than 4v? How can I know each led get how much exact voltage? is there a way to calculate?

Thank

 

I would never connect LED's to a power supply without current sharing resistors.

 

Devices in parallel have the same voltage across them. I suggest reading through the educational info on this site to begin.

 

LED's have a typical (and range) of forward voltages Vf from their datasheet. The value changes based on color. In some cases you might have to select LEDS based on Vf, if you want less variation in intensity.

The operating voltage must exceed this number, but they don;t have an operating voltage per see.

They operate with current, again, a typical value is in the datasheet).

4V is not a lot of room.

Sample calculation of each series resistor: R = (4 V - Vf(max))/(typical operating current)

You can plug in 2.1 V for Vf and 10E-3 for operating current. The resistor wattage also needs to be checked. The resistor you chose has to be greater than the calculated wattage. Any of the power formulas can be used.
Add slop to the calculated wattage.

 

Thank for reply
it helped me

 

Good.

Then I can add some stuff. You can add LEDS in series as long as it;s below the supply voltage. You just have to leave enough headroom for the in the voltage remainder.

Voltages and currents are all over the map now. There are 5V and 12 v LEDS and they MAY have resistors built in. High intensity, high current LEDS are designed to be run from a constant current power supply.

 

Most leds drop 2.8 to 3.5v, so i would use 330 to 560 ohms resistors.