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Topic: Resistor for LCD-backlighting (Read 8798 times)previous topic - next topic

gurka

Mar 15, 2010, 03:57 pm
Hello

I have GDM1602K (HD44780) LCD-display. The Data Sheet says:

LED Forward voltage: Min(-) Typ(4,2) Max(4,6)
LED Forward current: Min(-) Typ(120) Max(160)

I'm weren't really sure how to calculate which kind of resistor I needed to use, so I just used some online LED resistor calculator I found on google. I used the values 4,2V and 120mA and got the result 1/4W 6.8 Ohm resistor.

The only resistor I found were a 10 Ohm and I thought that "well, with a higher-valued resistor the LED should just lit darker" (correct?).

So I tried it and it worked. But, when I measured the voltage between ground and just after the resistor the result were 4.84V. That's higher then the LEDs max voltage.. Shouldn't the voltage be lower with an higher-valued resistor? If I add another 10 ohm resistor (in series) the voltage only drops to 4.70V which is still higher than max.

My questions is: Am I measuring wrong? If I'm not, which kind of resistor should I use to get the voltage to 4.2 - 4.6? What is the best way to make the LED lit darker (it's pretty bright atm). Just add another higher-valued reistor or can I damage the LED that way?

btw, I've hooked the display up with an Arduino Duemilanove, so the output voltage is 5V.

Grumpy_Mike

#1
Mar 15, 2010, 04:07 pm
Quote
I've hooked the display up with an Arduino Duemilanove, so the output voltage is 5V.

So are you driving the LED directly from an arduino output pin?
If so that is your problem as it can't supply the current.

If not try measuring the voltage across the resistor, then from ohms law you can calculate the current.

Quote
with a higher-valued resistor the LED should just lit darker" (correct?).

Yes quite correct.

BenF

#2
Mar 15, 2010, 04:46 pm
Quote
My questions is: Am I measuring wrong?

Not necessarily. Although forward voltage drop for the backlight is specified with min/max, the LCD module will typically have a series current limiting resistor fitted already. This would give you a higher voltage reading than you expect as it includes the voltage drop across the LCD plus the voltage drop across the resistor.

In general you can choose any series resistor you like as long as max current is not exceeded. One way to pick the "right" value would be to use a pot meter (e.g. 100 Ohm in this case) and turn it until you're happy with the backlight level. Then either leave the pot in permanently or measure the resistance across pot wiper to ground and replace it with a fixed resistor as close in value as you can find.

Another alternative altogeher is to use PWM to adjust backlight brightness through software.

gurka

#3
Mar 15, 2010, 06:13 pm
I'm using the 5V power pin (not a digital output/input pin). Shouldn't it be able to give atleast 120mA?

I just noticed that the data sheet says "BKL to be driven by pin1,pin2,or pin15,pin16,or A,K"

So, instead of dealing with resistors and two cables hooked to pin15 and pin16 I could make the backlight be driven by pin1 and pin2 (same pins that drives the LCD). But I can't find where it says HOW I make the display use pin1 and pin2 for backlight..

Any ideas?

Couldn't find anything in the sheets but I might missed something. These are the two data sheets:

MarkT

#4
Mar 15, 2010, 06:13 pm
Also don't forget that forward-biased diode voltages depend on temperature as well as current.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

gurka

#5
Mar 15, 2010, 06:29 pmLast Edit: Mar 15, 2010, 06:29 pm by gurka Reason: 1
Hum.. "LED Forward Voltage" means the voltage it needs to work, right? Or does it mean how much it will drop (from 5V)? I'm confused, haha.

BenF

#6
Mar 15, 2010, 06:34 pm
Quote
BKL to be driven by pin1,pin2,or pin15,pin16,or A,K

This translates to "you can connect BKL directly to +5V and ground" which is what you have on pin1 and pin2. That is pin1 goes to pin16 (K) and pin 2 goes to pin15 (K).

If you want to reduce backlight level however, you will still need  a series resistor.

Grumpy_Mike

#7
Mar 15, 2010, 08:42 pmLast Edit: Mar 15, 2010, 08:44 pm by Grumpy_Mike Reason: 1
Quote
One way to pick the "right" value would be to use a pot meter (e.g. 100 Ohm in this case) and turn it until you're happy with the backlight level.

That is a good way to blow up the pot, having 120mA going through the last bit of the pot's travel.

Quote
If you want to reduce backlight level however, you will still need  a series resistor.

You still need a series resistor to get the maximum brightness, typically series resistors are not fitted to an LCD backlight.

floresta

#8
Mar 16, 2010, 05:57 am
Quote
Hum.. "LED Forward Voltage" means the voltage it needs to work, right?

Not exactly.  It won't work when supplied with less than that amount of voltage, but you cannot connect the LED to a supply that is equal to the LED forward voltage and expect it to survive for long.

The LED forward voltage is the voltage that will appear across the LED when it is operating at it's rated current.  You typically start out with a supply voltage greater than the LED forward voltage and choose a series resistor to limit the current to the rated value.

Example:  A typical red LED has a forward voltage of 1.7v at 20 mA.  If it is operating from a 5v supply then 3.3 v (5 - 1.7) will be dropped across the series resistor.  For this to happen the resistor must be 165 ohms.  You would use the next higher standard resistance which is 180 ohms.

The calculation for the backlight isn't this simple.  The LEDs aren't red so the forward voltage is different.  There are typically several LEDs in series so Vf is not 1.7v, and there are typically several parallel groups of LEDs so the required current isn't 20 mA.

If you want to experiment with a variable resistor then make sure you also use a fixed resistor in series with it so you don't burn it out as Mike was mentioning.

Don

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