Electronic – Infrared spotlight project, need x50 resistors. How many ohms what type to use

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Infrared spotlight project.

I have a PCB BOARD set up for 150x 5mm IR LED
1.5-1.6 forward voltage.
Forward current is 60 mA

150 LED’s are divided into 50 groups (3 LEDs each + 1 resistor)

What ohm resistors are needed for the 50 groups of three LEDs each?
I was thinking of using x2 18350 Ultrafire 3.7v x2= 7.4 volts.

Suggestions on more or less voltage using increments of 3.7.

Using the online resistor LED calculator: for 150 LEDs grouped 3 LEDs + 1 resistor, the answer given was x50 1/2 W 56 Ohm resistor needed.
Being a broke novice I don't want to order and wait for fifty wrong resistors.
Do you think this is a safe bet? To better understand my confusion please refer to item #251216249922 on eBay or look up PCB board for 150 x LED 3 mm or 5 mm. The third photo completed tells a lot.

Best Answer

This is an application of Ohm's law.

Assuming each set of three LED's are connected in series with a current limiting resistor:

Using a \$V_f\$ of 1.5 means the total \$V_f\$ for each diode group is 4.5 V.

The resistor value is \$R = \frac{E}{I}\$, or \$R = \frac{E}{0.060}\$

  • at 6V: \$R = \frac{(6 - 4.5)}{ 0.060} = \$ 25 ohm (90 mW, use 1/4 W resistor)
  • at 12V: \$R = \frac{(12 - 4.5)}{0.060} = \$ 125 ohm (450 mW, use 1/2 W resistor)

Be sure to use a resistor rated for the power dissipation (\$P = IE\$). Assuming these 50 groups are in parallel, that works out to be a total of 3 amperes. You'll lose less power by having smaller resistor from a lower supply voltage, so of these examples, 6 volts is preferable to 12 volts. To be more efficient you may want to research LED driver circuits.

Make sure your power supply/batteries can supply at least 3A!

Edit:

Per your edit, using 14.8 V would require larger current limiting resistors and waste more power:

14.8V: \$R = \frac{(14.8 - 4.5)}{0.060} = \$ 172 ohm (618mW, use 1W resistor)

At this voltage, the power dissipation of the resistor is now 618 mW and you would therefore have to use 1W power resistors.

Edit 2:

Some of the math and resistor values were off. It was late at night apparently, and I was trying to get the TeX right. I've adjusted the math and values to be accurate.