Well... when I mentioned using a resistor on each LED, we did control currentwhat happens if the LED's doesn't have same Current consume for example if first is 15 second is 20 and third is 25? will it be like the voltage in parallel i mean they wont work ?
so i have to control the current,did we do that in parallel?why we have to do that now?
and can i give every string 12 v since they are serie?
and since the 2 strings are going to be parallel does they have to have same Voltage consume?
You *can* connect a 15mA, 20mA, and 25mA LED all in series. You need to make a decision here though. The current through a series string like this *must* be the same through all components... It's a *law* not a guide-line. So if you size your resistor to allow 25mA through, you can damage the first, and possibly the second.
If you size the resistor to allow only 15mA through, then all 3 LEDs will run at, or under their limit. The 20mA and 25mA will not be as bright *as they could be*, but will still light. You can understand that if you have too much variation, bad things, or no things can happen xD ... ie, a 15mA LED and a 300mA LED . You either immediately kill the little one, or the big one hardly does anything.
As far as the strings are concerned, *you* dictate what voltage they will run at with the resistor you choose.
If you base your calculations on a 12V supply that will allow the resistor to provide 20mA to the LEDs, the increasing the voltage will increase the current as well.
If you have designed each string to run on 12V with the appropriate resistors and number of LEDs, then you can connect each string in parallel and by a happy camper
When you connect something in parallel, the voltage on *each* parallel piece will be the same.
So... You had initially (incorrectly) wired the LEDs directly in parallel... this resulted in the same Voltage being applied to them all and the lower rated LEDs would get damaged.
When you connect something in series, the current through *each* piece will be the same.
So... when you connect 3 LEDs and a resistor in series, you should properly size the resistor to allow 20mA (example) to flow through... this also means that 20mA is going through each LED in the same string.
You *can* and often will have a mix of series and parallel components... much like what you are building now.
You will end up having multiple groups in parallel, each group is a string of LEDs and a resistor that is in series.
Each string will be provided with the same voltage from one end to the other. (The voltage may not, and does not need to be the same part-way through all the strings! They can vary as long as the *total* voltage for the string is 12V) All of the components in a single string will have the same current flowing through them. *BUT* you could easily have one string made with 30mA LEDs, and another string made with 10mA LEDs.... because they are separate strings you don't have to worry about forcing 30mA through the poor little LEDs