basic question Leds and Diodes Resistor hook-up

[filefox]

??? I am trying to find out if say for a 4.5 volt circuit if you can use 2 diodes(0.7v) and a white led(3.2v) in series without problems. Is this good practice or a bad idea? that is, to use diodes instead of a resistor to drop the voltage.

:P what are advantages/disadvantages. If i plug 2 white leds in parallel on one resistor? Is this a good idea?

:o third what if on say 5 volts I put a red led(1.7) and a white(3.2) one in series what happens? how many mA would go through?

;D I am trying to get back into the hobby doing little projects but got stumped cause most everyone seems to put resistors etc.
thanks for any clarifications  ;)

[audioguru]

LEDs are not made accurately. They have a range of forward voltage that is 2.8V for one then 3.6V for the one beside it. Their average is 3.2V.
So you must limit the current instead of feeding a regulated voltage.

The spec's for most little LEDs show a max current of 30mA so use 20mA.
With a 4.5V battery, an LED that might be 3.2V needs a current-limiting resistor that is 
(4.5V - 3.2V)/20mA= 65 ohms. Use a 68 ohms resistor which is a standard value.
If the LED has a forward voltage that is actually only 2.8V then its current will be (4.5V - 2.8V)/68 ohms= 26mA which is fine.
If the LED has a forward voltage that is actually 3.6V then its current is (4.5V - 3.6V)/68 ohms= 13ma which is a little dimmer than the brighter one.
When the battery quickly runs down to 3.5V and less then the 3.6V LED will be turned off or very dim and the 2.8V LED will still be pretty bright.

If your battery had more voltage then a current regulating circuit can be used so that both LEDs will have the same brightness.