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Step 2Design - electrical
There are three possibilities when designing the Christmas tree circuit, depending on the number of LEDs, their forward voltage drop, and voltage of the power supply.
1. The LEDs drop LESS voltage than the power supply supplies.
(That is for example, if you have a 12V supply, and you have 5 LEDs - each with a forward voltage of 1.8V - then the LEDs drop only 9V)
If you connect the LEDs in series directly to the power supply, too much current will flow, and at least one of the LEDs will destroy itself (hopefully breaking the circuit, and protecting the rest)
You need a resistor to restrict the amount of current to a safe level. To calculate the total resistance needed:
R = (Vs - Vf*N)/Is
Vs: Supply voltage
Vf: Foward voltage drop across 1 LED.
N: Number of LEDs
Is: Safe current for a LED.
My initial design looked like schematic A: R1 and R2 are each HALF of R_total (for symmetry) because resistors in series are added to calculate the total resistance.
2. The LEDs drop exactly the same voltage that the power supply ... supplies. Great! No resistors are needed, just connect every LED up in series, and wire to the power supply terminals.
Be careful you have calculated correctly though, or a LED may fry.
3. The LEDs drop MORE voltage than the power supply can supply. Bad news - you can't connect every LED in series. You CAN however split the LEDs into parallel chains. If you look at schematic B, you can see that there are two paths from Vcc (+) to Gnd (-). The path on the left only has 2 LEDs, so it needs a resistor to keep the current at a safe level. (Scenario 1) The path on the right has 15 LEDs, which because each LED drops 2.0V, and the power supply is 30V, gives me exactly the right amount of voltage drop - no resistor needed. (Scenario 2)
Once you have your power supply and desired number of LEDs, you can see which scenario you have, and design your tree!
1. The LEDs drop LESS voltage than the power supply supplies.
(That is for example, if you have a 12V supply, and you have 5 LEDs - each with a forward voltage of 1.8V - then the LEDs drop only 9V)
If you connect the LEDs in series directly to the power supply, too much current will flow, and at least one of the LEDs will destroy itself (hopefully breaking the circuit, and protecting the rest)
You need a resistor to restrict the amount of current to a safe level. To calculate the total resistance needed:
R = (Vs - Vf*N)/Is
Vs: Supply voltage
Vf: Foward voltage drop across 1 LED.
N: Number of LEDs
Is: Safe current for a LED.
My initial design looked like schematic A: R1 and R2 are each HALF of R_total (for symmetry) because resistors in series are added to calculate the total resistance.
2. The LEDs drop exactly the same voltage that the power supply ... supplies. Great! No resistors are needed, just connect every LED up in series, and wire to the power supply terminals.
Be careful you have calculated correctly though, or a LED may fry.
3. The LEDs drop MORE voltage than the power supply can supply. Bad news - you can't connect every LED in series. You CAN however split the LEDs into parallel chains. If you look at schematic B, you can see that there are two paths from Vcc (+) to Gnd (-). The path on the left only has 2 LEDs, so it needs a resistor to keep the current at a safe level. (Scenario 1) The path on the right has 15 LEDs, which because each LED drops 2.0V, and the power supply is 30V, gives me exactly the right amount of voltage drop - no resistor needed. (Scenario 2)
Once you have your power supply and desired number of LEDs, you can see which scenario you have, and design your tree!
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