Author Topic: Can backlight pins really be powered without series resistor on this display?  (Read 732 times)

0 Members and 1 Guest are viewing this topic.

Offline VinzC

  • Regular Contributor
  • *
  • Posts: 139
  • Country: be
  • See you later, oscillator.
Hi.

I'm using a 240x320 display from Newhaven and now I've got a last minute stress question: though the data sheet indicate the backlight LEDs can be powered directly from anything between 2.8 and 3.5V I am not sure if I should put a series resistor or if I stress for nothing because the display has one. Does anybody know?
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 2547
  • Country: gb
I'd use caution (as you are). That spec might well be the forward voltage drop of the backlight LED at normal rated current. I would at least power it from a bench PSU and series resistor until you are sure of its characteristics.
« Last Edit: July 19, 2017, 05:13:58 AM by Gyro »
Chris

"Victor Meldrew, the Crimson Avenger!"
 
The following users thanked this post: MK14

Offline VinzC

  • Regular Contributor
  • *
  • Posts: 139
  • Country: be
  • See you later, oscillator.
I'd use caution (as you are). That spec might well be the forward voltage drop of the backlight LED at normal rated current.
That is indeed what I thought. Fortunately I left room for a series resistor on my PCB though I had planned to use it as a jumper. I'll double check on a bench. Thanks for your insights.
 

Offline ajb

  • Super Contributor
  • ***
  • Posts: 1049
  • Country: us
It seems like most LCD manufacturers Just gloss over this in their datasheets, but as the schematic indicates, the four backlight LEDs are brought directly out with no internal current limiting (other than that provided by the resistance of the miniscule FPC traces to them).  So as with any white LED, you probably *could* get away with running them directly from a 3.3V supply, but it's probably not the best idea.  Anyway, resistors are cheap, so you may as well put at least one on the board, even if you eventually decide to make it a 0R.
 
The following users thanked this post: MK14

Offline Dave

  • Super Contributor
  • ***
  • Posts: 1058
  • Country: si
  • I like to measure things.
Put four individual resistors on the cathodes if you want good current balancing between the LEDs.
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 
The following users thanked this post: MK14

Offline Hero999

  • Super Contributor
  • ***
  • Posts: 9008
  • Country: gb
One of the problems with LEDs, including all diodes, is the forward voltage drop has a negative temperature coefficient, so the current tends to rise, as it heats up.  If a constant voltage source is selected to give the rated current, when power is first applied, then the current will rise, causing excessive power dissipation, resulting in destruction of the device.

You could power it from a voltage which would result in a forward current below the maximum rating, even when the device is operating at the maximum resulting junction temperature, but that could be tricky as the forward voltage for different LEDs varies.
 

Offline VinzC

  • Regular Contributor
  • *
  • Posts: 139
  • Country: be
  • See you later, oscillator.
Just a quick feedback on this topic: I've made a PCB and put a 2.2 ohms series resistor that drives all the backlight LED's, as specified in the datasheet. Lighting has been consistent throughout all of my tests so far and the LED's seem to be perfectly matched so there's no need to balance the current individually.

As a side note, the flex cable connector I was using has given me headaches as it corroded with time, also causing shorts in the backlight and power circuitry. I went cheapo on this one but I do realize that's a bad idea. I'll opt for another one from Molex or JST in the hope it won't rot this time.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf