I am currently building an LED array. I have a 12 volt switch-mode power supply which I would like to use to power my LEDs. My LEDs have a forward voltage of between 3.0 and 3.6 volts (according to the datasheet), and require 20mA. If I run my LEDs in series in strings of four, do I need a resistor since the combined forward voltage of four LEDs should be 12-14.4 volts?
I have previously used the "(Supply voltage- forward voltage)/ LED current" equation to calculate resistor values, but the "(Supply voltage- forward voltage)" part seems to suggest that if the supply voltage equals the forward voltage then no resistor is needed. Is this right?
I know that if the supply voltage equals the forward voltage and current is not limited, if the supply voltage increases only slightly then the current drawn by the LED increases very rapidly until the LED fails. But what happens if we go the other way? If the supply voltage is less than the forward voltage what happens to the LED and its current consumption?
Will I be OK using my 12 volt power supply and not having resistors to limit the current (since the usual equation for working out resistor values would suggest one is not needed), or should I get a higher-voltage supply (maybe a 15 volt laptop psu?) and add resistors to each string of four LEDs, or should I keep my 12 volt psu and add low-value resistors (maybe 1 ohm)??