SOLVED: https://forum.allaboutcircuits.com/...-backlight-on-lcd-module.202269/#post-1929644
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From what I've seen so far, backlights on LCDs (often over 100mA) still consume a lot of current compared to the logic circuit (usually in single mA). I'd like to improve that.


Since LED backlights generally run on 5V without resistors (I'm assuming resistors are now built-in and cannot be "easily" disabled), I was thinking of running the backlight on 7 VDC, but with PWM.

Is there a reason why this wouldn't work just like it does for ordinary single LEDs?

In my feeble mind, LED is LED, shouldn't it act the same way?

 

I don't know what type of back light you have there. If it has a resistor, I think PWM will work. Just don't leave it at 100%.
Assume a 3V LED and 2V across the resistor. Putting 7V on the display will put 4V across the resistor and double the current. So about 50% might = full bright.
The backlight might be something different.

 

Why up the voltage? Why not PWM the 5V?

The LED may be overdriven when using 7V.

(No, PWM does not reduce the voltage, it reduces the ON time)

If you are trying to get the same light for less power, PWM will not do that either.

 

Does it have to display all the time it is powered?
If for a short period, or only when necessary, you could turn the back light off for this duration or time?

 

Welcome to AAC.

There is something you may be intending but are not saying. Using PWM is not, in itself a power saving measure (as pointed out by @BobTPH). Do you intend to dim the display to reduce power consumption?

If so, you might be better off using a constant current LED driver like the CN5711 (as implemented on this module). You can reduce the current to the LED setting it to a value compatible with your energy budget.

The chip also has an CE (Chip Enable) pin that can be used to implement PWM if desired. This would allow you to set a maximum value for current consumption and reduce it from there.

 

Does the TS even know how much current that particular backlight draws? And has a close examination revealed how many LEDs are in the display? And has the TS experimented with dropping the voltage just a bit to see what effect that has?
Besides that, how much brightness reduction can that specific display suffer before readability is impaired?? It might even be that an examination of the PCB from the back may reveal some details.
One more thing: Unless this is a portable battery powered system, how important is reducing the display power consumption???

 

I didn't have detailed datasheets.

Finally got some samples. I can control both contrast and backlight with PWM @ 1953 Hz using 5V DC with satisfying results.



LCD model 2004A V1.3
White text on blue background

CURRENT

Contrast:
2N2222A NPN with 1K on base
- 0.20mA
- 0.46mA maximum with 5K pot

Backlight:
Connected direct to 5V
- 34mA
2N2907 PNP
- 5mA night
- 15mA day
- 34mA maximum


CURRENT ON BASE:

Contrast:
- 1.5mA
- 3.8mA maximum

Backlight:
- 0.5mA
- 4.0mA maximum

* Notes *
Recommended, by me, with sample LCD on hand.
Maximum, with pot turned at maximum.

 

Is the push to display (i.e. use back light by selection) feasible?

 

Is the push to display (i.e. use back light by selection) feasible?

Not really. This is for a flight sim console, and I have my hands busy on the yoke and adjusting other controls (throttle, flaps, radio, lights and what not). My goal was to control contrast and backlight every once in a while as lighting changed (day to night, into storm clouds, etc), and this setup does that.

Finally got it set up to control contrast and backlight on multiple LCDs from 1 PIC mcu. I don't like that viewing angle on the white-text-on-blue, much more narrow than the black-on-white.

(dog doesn't like thunder)

 

...how important is reducing the display power consumption???

Initial plan was to run my console off USB power, so limited to 500mA. Plan B was using a 12V 3A wall adapter (9V would be better) with 7805 1.5A voltage regulator. Plan C was using stronger parts, but price goes up quickly when you want to mass fabricate.

Ideally I want it to run off USB to limit wires. I will have 3 parts to this product, I want to avoid a ball of spaghetti wires.