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 »

"stoneattic" wrote in message
oups.com

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.

--
James T. White

James T. White wrote:
"stoneattic" wrote in message
oups.com

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.

--
James T. White


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

stoneattic wrote:
James T. White wrote:

"stoneattic" wrote in message
groups.com

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.

--
James T. White



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.

James Sweet wrote:
stoneattic wrote:
James T. White wrote:

"stoneattic" wrote in message
groups.com

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.

--
James T. White



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.

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!

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

stoneattic wrote:

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!

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.

James Sweet wrote:

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.

Don't I want (nominally) 112mA?

stoneattic wrote:
James Sweet wrote:

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.

Don't I want (nominally) 112mA?

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