# Confused on CFAH0802A operating voltage for Backlight

#### Appel

##### New member
Confrused on CFAH0802A operating voltage for Backlight

OK a real newbie here -

I think I understand VO is power for the backlight - I'm lighting up the backlight with 5V through a 5ohm resistor.
That part seems to be OK.

What I'm lost on is the pads RA and RK on the back of the LCD module itself. Power and ground has to go across the pads to light up the backlight - Are these for:

A) Resistors to limit the current?

C) Something else I'm just not seeing here?

Thanks!

-Jerry
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#### CF Tech

Vo does not drive the backlight.

Vo is typically connected to the wiper of a 10K pot. The ends of the pot are connected to +5v and ground. Adjust the pot so Vo is about 1v for starters.

In general, see this thread on backlights:

For the CFAH0802A-YYB-JP the backlight should be driven through A & K.

To calculate the backlight curent limiting resistor see page 20 of the data sheet:

Vled = 4.2v
Iled = 70mA = 0.07A

Assuming Vsupply is 5v, the Rlimit would be:

(5v - 4.2v)/0.07A = 11.43 ohms

Power is:

(5v - 4.2v) * 0.07A = 0.056 watts

So use a 1/4 (0.250) or 1/2 (0.500) watt 12 ohm resistor (or the next larger standard size).

#### Appel

##### New member
OK - I was getting a little mixed up there on the inputs -

So what I should do is:
1) Install a zero-ohm across RK.
2) Install a 12-ohm across RA.

Use the 10K-Ohm pot as described.

Thanks again!

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