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<blockquote data-quote="thch" data-source="post: 2082176" data-attributes="member: 562032">a diode has a current - voltage characteristic that is very sharp.&nbsp; if you apply 4V you will have maybe 50mA of current.&nbsp; at 5V the current would be much higher.this is why 12V is dangerous to the LEDs.&nbsp; but also why resistors are VERY effective.the voltage across the resistor is V = I*R.&nbsp; thus as the current through the diode increases, the voltage across the resistor increases.&nbsp; because of the sharp V-I curve on the diode, and the lack of need for percision current to the diode, the math is pretty much:I_diode = (V_source - V_diode) / Rif you want 30mA on a 4V LED on a 12V source, its just:30mA = (12-4) / R, thus R = 8/30m = 267ohm.the series LEDs should be approximatly equally bright in series -- equal current must flow through each.for 4V LEDs on 12V, i&#039;d put 2 LEDs in series.&nbsp; 20mA may be ok for currents.&nbsp; this is 12V - 4V - 4V = 4V across&nbsp; the &quot;current limiting resistor&quot;.&nbsp; for 20mA, 4/20 = 200ohm.&nbsp; a 0.25W resistor is fine.find out the suggested forward current, and rated forward voltage @ this current for best resistor selection.</blockquote>

[QUOTE="thch, post: 2082176, member: 562032"] a diode has a current - voltage characteristic that is very sharp. if you apply 4V you will have maybe 50mA of current. at 5V the current would be much higher. this is why 12V is dangerous to the LEDs. but also why resistors are VERY effective. the voltage across the resistor is V = I*R. thus as the current through the diode increases, the voltage across the resistor increases. because of the sharp V-I curve on the diode, and the lack of need for percision current to the diode, the math is pretty much: I_diode = (V_source - V_diode) / R if you want 30mA on a 4V LED on a 12V source, its just: 30mA = (12-4) / R, thus R = 8/30m = 267ohm. the series LEDs should be approximatly equally bright in series -- equal current must flow through each. for 4V LEDs on 12V, i'd put 2 LEDs in series. 20mA may be ok for currents. this is 12V - 4V - 4V = 4V across the "current limiting resistor". for 20mA, 4/20 = 200ohm. a 0.25W resistor is fine. find out the suggested forward current, and rated forward voltage @ this current for best resistor selection. [/QUOTE]

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