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I have a LCD Display with white LED backlight. I want to connect it to a MOSFET which will be controlled by my ESP-12E Microcontroller.

I'm wondering which resistor to use. The data sheet says:

Absolute Maximum Ratings: Back Light Forward Current ILED -- 25 mA For each LED

Electrical Characteristics: Forward Current IF -- 20 -- mA Forward Voltage VF -- 3.2 -- V

(4 LEDs parallel)

Question 1:

Is the value per LED?

My power supply will either be 5V or 12V

The other data sheet page says:

LED Backlight Power Supply: max. 3.2V (by 3.3V Power Supply 2-5 Ohm Resistance, or by 5V 20-25 Ohm Resistance)

But if I calculate for 5V power supply I get:

25 Ohms * 0.02 Amps = 0.5 V

Question 2: So does that mean a voltage drop of the resistor of only 0.5 which would result in a voltage of 4.5 V for the LED which seems to be too much?

Or do I have to calculate with 0.08 Amps because of the 4 LEDs?

25 Ohms * 0.08 Amps = 2 V

Question 3:

Would this mean the overall voltage drop is 2 V but every LED still gets 4.5 V?

For 12 V that would mean a 150 Ohms resistor?

I'm not that good at physics, I just want to make sure everything is correct so I don't damage anything.

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    \$\begingroup\$ This question needs a big tidy-up on formatting. It is too difficult to read. Please also provide a link to the display datasheet. \$\endgroup\$
    – Transistor
    Commented Nov 12, 2017 at 14:45
  • \$\begingroup\$ Can you share the datasheet of the LCD? Also what is the purpose of using mosfet? \$\endgroup\$
    – Sanjeev Kumar
    Commented Nov 12, 2017 at 15:26
  • \$\begingroup\$ There are two documents: ecksteinimg.de/Datasheet/CP11005.zip Interface Definition.pdf LCD Panel\TM028HDH01.pdf \$\endgroup\$
    – dfsg76
    Commented Nov 12, 2017 at 15:59
  • \$\begingroup\$ I need the MOSFET because I don't think that the ESP12 can handle that much current. I also want to dim multiple LEDs (15 pieces) along with the LCD backlight at the same time. \$\endgroup\$
    – dfsg76
    Commented Nov 12, 2017 at 16:03
  • \$\begingroup\$ Draw a schematic. \$\endgroup\$
    – winny
    Commented Nov 12, 2017 at 16:45

1 Answer 1

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  1. The Vf and If would be the same for each led. Leds in parallel will share the same voltage while the current adds up.

  2. 25 Ohms * 0.08 Amps = 2 V

This. Each led uses 20 mA, so you need to calculate using the combined current.

They give you the recommended resistance to use too.

  1. The voltage drop across the resistor will apply to the entire series circuit. Each led in series with the resistor will only see their part of the voltage, which in this case would be 5V source - 2V resistor voltage = 3V led voltage.
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