sol wrote:Irondukes' comments about LED's not turning on if below 15.4 volts ( for 7 of in series) - well I run LED's that are nominal 2.2 volts at about 1.5v & they work OK - current is down a bit as I use a higher value resistor - makes them last longer; it also protects them if the mains power spikes a bit & increases the LED voltage. The amount of light from the LED is still suitable for normal use.
this is a list of LED forward voltages:
White and warm white: 3.2 to 3.8 volts
Blue (up to 480nm or so): 3.2 to 3.8 volts
Green and aqua (up to 530nm): 3.0 to 3.8 volts
Green and yellow-green (up to 570nm or so): 2 to 2.4 volts
Red, amber and yellow (up to 660nm or so): 1.8 to 2.4 volts
Infrared: 1.4 to 2 volts
UV: 3.2 to 4 volts
(source: www.ledsales.com.au )
Although admittedly my contention is based on theory. a LED has a voltage drop the same as a diode and until you reach that voltage no current will flow. Where did you get the 1.5V?