Then when the Vf of the LEDs is the lowest value (3.2V) they might burn out.Guys, the LEDs I have, have a Vf of 3.2-3.8. I have based my wiring schematic on 3.8. Is that the best thing to do?
Calculate the current when the Vf is the highest and the lowest. then select a resistance that does not overload the LEDs but allows enough brightness.
The Wizards are VERY STUPID!I also got a laptop power brick which is DC 19V 65W. If I divide 19v by 3.8 (Vf) it comes back with a straight 5. The LED wizards out there all say this is fine with a 1ohm resistor. Is the resistor even necessary?
You need a fairly high value resistor because the "19V" supply might actually be 18V or it might actually be 21V. Also the "3.8V" LEDs might actually all be only 3.2V.
Example:
21V supply, five 3.2V LEDs in series, 1 ohm current-limiting resistor.
The total voltage needed by the LEDs is 5 x 3.2V= 16V.
The 1 ohm resistor has a voltage of 21V - 16V= 5V across it.
Then the current in the LEDs is 5V/1 ohm= 5A!
If the "19V" supply is actually only 18V then five 3.8V LEDs will barely light up.
Are you going to measure the voltage of all your LEDs then throw away the hundreds that are not 3.8V? Maybe none will be 3.8V.
Design a circuit that works with ANY spec.
1) 21V supply, four 3.2V LEDs, 150 ohm current-limiting resistor.The circuit I've drawn up has a series of 4 LEDs with a 150 ohm resistor. Is this better?
The total voltage needed by the LEDs is 4 x 3.2V= 12.8V.
The 150 ohm resistor has a voltage of 21V - 12.8V= 8.2V across it.
Then the current in the LEDs is 8.2V/150 ohms= 55mA.
Aren't your LEDs supposed to use only 20mA and might burn out if the current is more than 30mA?
Then a resistor value of 8.2V/28mA= 300 ohms should be used. It heats with (8.2V squared)/150 ohms= 0.22W so use a 1/2W resistor.
Now what happens if the supply is actually 18V and the LEDs are actually 3.8V?
2) 18V supply, four 3.8V LEDs, 300 ohm current-limiting resistor.
The total voltage needed by the LEDs is 4 x 3.8V= 15.2V.
The 300 ohm resistor has a voltage of 18V - 15.2V= 2.8V across it.
Then the current in the LEDs is 2.8V/300 ohms= 9.3mA.
Will that be bright enough?