LCD LED backlight question

[stoneattic]

Hi,

I just installed this LCD:

http://www.mouser.com/catalog/specsheets/M16327UYT.pdf

to replace the dead one in an Alesis QuadraVerb (this doesn't really
matter, it's more of a generic question).

The LCD display itself work fine but to get power to the backlight I
ran the 5V source and ground that supplies the LCD part to the LED
blacklight and it works too. However I'm wondering if I need to add a
resistor to limit the current to the backlight like I normally would
with regular 5mm LED. I've read the specs on the backlight but they
aren't given as simply as:

# 30mA (max)
# Typical Voltage is 2.0, with a maximum voltage of 2.8V

like I'm used to.

Can anyone point me in the correct direction?

Thanks!

Reply »

 

[James T. White]

See page 11 of the spec sheet. VF(typ) = 4.1V @ IF = 112ma.

So you will need a series resistor sized to drop .9V at 112ma.

 

[stoneattic]

See page 11 of the spec sheet. VF(typ) = 4.1V @ IF = 112ma.

So you will need a series resistor sized to drop .9V at 112ma.

Thanks! That's actually what I thought but I wasn't sure I was reading
that correctly on the spec sheet. So an 8 ohm resistor is the way to
go? I assume a 1/4w would be fine?

Thanks again

 

[James Sweet]

Thanks! That's actually what I thought but I wasn't sure I was reading
that correctly on the spec sheet. So an 8 ohm resistor is the way to
go? I assume a 1/4w would be fine?

Thanks again

In a pinch you can start with a 500 ohm or so pot, then set it to max
resistance, measure the current with a DMM and adjust the pot to get the
desired current/brightness and then disconnect the pot, measure the
resistance of that and choose a fixed resistor as close as you can get.
1/4W should be plenty, 1/8W should work too.

 

[stoneattic]

In a pinch you can start with a 500 ohm or so pot, then set it to max
resistance, measure the current with a DMM and adjust the pot to get the
desired current/brightness and then disconnect the pot, measure the
resistance of that and choose a fixed resistor as close as you can get.
1/4W should be plenty, 1/8W should work too.

Since I use this thing in the dark mostly I would lean towards the
brightest I could get out of it which would be at a resistance of 0.
hehe Obviously that's not a good idea. Do you think the 8 ohms would
give me the best brightness and still protect the LED enough? I've got
10ohms resistors (1/8 and 1/4W) laying around so I may toss one of
those in and call it done. How does that sound?

Thanks again!

 

[M Berger]

You don't need it at its brightest if you use it in the
dark.

 

[James Sweet]

Since I use this thing in the dark mostly I would lean towards the
brightest I could get out of it which would be at a resistance of 0.
hehe Obviously that's not a good idea. Do you think the 8 ohms would
give me the best brightness and still protect the LED enough? I've got
10ohms resistors (1/8 and 1/4W) laying around so I may toss one of
those in and call it done. How does that sound?

Thanks again!

Measure the current, don't exceed 20mA or the LED will not last long.

 

[stoneattic]

Measure the current, don't exceed 20mA or the LED will not last long.

Don't I want (nominally) 112mA?

 

[stoneattic]

Don't I want (nominally) 112mA?

I put a 10 ohm resistor in and get 90mA on the ammeter. Nice bright
backlight too.