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### Topic: how to read LCD datasheet to figure out required resistor? (Read 988 times)previous topic - next topic

##### Jan 11, 2013, 02:19 am
I bought this LCD screen to use with my Uno
http://www.hacktronics.com/datasheets/20x4_white-blue-lcd.jpg

and i'm trying to follow this tutorial

I'm stuck on calculating the resistor i need for the backlight. It says that most LCD displays come with a series resistor. I'm not sure if they mean to check if there's a resistor on the board itself or that it came with a separate resistor because mine did not if that's the case.

It then says the following to calculate the required resistor:
Quote
To calculate the value of the series resistor, look up the maximum backlight current and the typical backlight voltage drop from the data sheet. Subtract the voltage drop from 5 volts, then divide by the maximum current, then round up to the next standard resistor value. For example, if the backlight voltage drop is 3.5v typical and the rated current is 16mA, then the resistor should be (5 - 3.5)/0.016 = 93.75 ohms, or 100 ohms when rounded up to a standard value.

I've never had to read a datasheet before so i'm struggling with figuring out the datasheet. Which value is the maximum backlight current and the backlight voltage drop?

I know the tutorial says to use a 220 ohm resistor if i can't find the datasheet but i'd like to learn how to do this when i don't have a tutorial.

Should i be looking at the "Type" value of LED Voltage and LED Current? or should i also be adding these values to their respective 'Operating' values when doing the calculation?

Any clarification would be awesome.

#### floresta

#1
##### Jan 11, 2013, 06:19 amLast Edit: Jan 11, 2013, 06:23 am by floresta Reason: 1
Quote
Should i be looking at the "Type" value of LED Voltage and LED Current?

Yes.  The term 'Type' should be 'Typ', short for 'typical'.

Quote
or should i also be adding these values to their respective 'Operating' values when doing the calculation?

No.  VDD is for the LCD not the LED.

It would appear that your particular display does not require an external current limiting resistor.

I say that because in the "Absolute Maximum Ratings" section it lists the typical LED Voltage as 5.0v and in the "Pin Connections" section it shows +5V for the Backlight Power.

Don

#### Krodal

#2
##### Jan 11, 2013, 02:28 pm
I checked my board, and I used 220 Ohm. It looks okay to me.

#3
##### Jan 12, 2013, 12:24 am
awesome. thank you!

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