# Cal­cu­lat­ing re­sis­tor val­ues for LEDS

Model Rail (UK) - - Workbench -

In or­der to work out the re­quired value of a re­sis­tor, it’s nec­es­sary to know the LED’S spec­i­fi­ca­tions. With these fig­ures at hand, along with the rat­ing of the power source, we can cal­cu­late the de­sired re­sis­tance us­ing Ohm’s Law. All LEDS have a max­i­mum cur­rent limit and a max­i­mum voltage. As an ex­am­ple, let’s take an LED with a max­i­mum voltage of 4V and max­i­mum cur­rent of 20ma (0.020 amps). We’ll be con­nect­ing the LED into a cir­cuit pow­ered by a 12V sup­ply. Ac­cord­ing to Ohm’s Law, the re­sis­tance is calculated thus: OHM’S LAW dic­tates that: R =( VS – VL) di­vided by I

Where… R = re­sis­tance VS = Sup­ply voltage (12V) VL = LED max. voltage (4V) I = LED max. cur­rent in amps (0.020A) In our case: R = 12 – 4 = 8. We then di­vide 8 by 0.020 to get 400. The to­tal of 400 is the amount of re­sis­tance re­quired, ex­pressed in ohms. As re­sis­tors come in var­i­ous stan­dard val­ues, we have to choose the near­est value (al­ways round it up­wards), which is 430 ohms.