Can we PWM an LED backlight on LCD module

[Demon]

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?

[Ioannis]

Without knowing the specs of the LED's is not easy to say if it is OK to drive the LED string with 7V instead of with 5V as it was designed initially, even with PWM control.

Besides that PWM may stuck in the ON state and burn everything...

Ioannis

[Demon]

... PWM may stuck in the ON state and burn everything...

That is my biggest concern with this plan.


My design now has 3 enclosures, with power being fed into the center unit. I'm now thinking of using 3 separate voltage regulators; one per unit.

That way each 7805 doesn't get as hot as a single device trying to feed all three. Less complicated (less parts, simpler circuitry) than trying to "cheat the system" so to speak.

[Ioannis]

Why not use SMPS? No heat generated, no heatsink needed.

Unless you need a clean +5V for ADC purposes. In all my cases though, a switching did not make any difference in the ADC.

Ioannis

[Demon]

(post removed, because I can't tell the difference between a circuit that provides power to PIC, and a circuit that provides power while relying on PWM from a PIC)

[Demon]

I'm currently using these buck converters to drop from 12V from wall adapter down to 5V:

https://www.amazon.ca/gp/product/B0B5GQTS64/


These go for $2 each and they are only rated at max 3A with warning to remain under 2.5A.

[Ioannis]

I would not trust those chinese modules for serious projects. Rather design your own based on the TI or Microchip App. Notes for you specific needs.

The modules are just bad copies of western know chips with too cheap inductors and capacitors.

From various module test I did, some were so bad that on power up they just burned(!) others worked ok with a lot of noise and ripple, and all those that worked are way to lower on power output in relation to the advertised power.

Ioannis

[Demon]

...The modules are just bad copies of western know chips with too cheap inductors and capacitors....

A lot of those converter circuits look like they were done in some cave. This one looks surprisingly well done:

https://www.aliexpress.com/item/1005001629723875.html


The 5V pro variety comes down to $0.69ea in packs of 20.

I'm going to look for those app notes.

[Demon]

I would not trust those chinese modules for serious projects. Rather design your own based on the TI or Microchip App. Notes for you specific needs. ...

Well, that's disappointing. I looked only at Mouser, the IC alone is over $6 and most of the discrete components are not available. I don't know enough if I can replace them with components.

8V–24V to 5V/3A Buck Converter, through-hole, Page 7

https://www.microchip.com/content/da.../AN15-APID.pdf


And the SMD designs had a LOT less power (my first choice, to have them fully assembled by JLCPCB). Microchip's app note dates from March 2001, gonna check out TI...

[Ioannis]

Another idea would be to use a low drop linear regulator for 5 volt and use the cheap switch mode ones to lower the 24 volts down to 6-7 volts. That way power losses on the low drop would be minimal and still you will have good quality noise free 5v line.

Ioannis

[Ioannis]

...This one looks surprisingly well done:

https://www.aliexpress.com/item/1005001629723875.html

Well, 5 Amps? No way the ... baishundianzi will perfom a 99,9% conversion...! Get ready a fire extinguisher when testing 24->5 @5A.

Ioannis

[Demon]

Ooops, just to clarify, that 24V is the title of that schematic.

I will be using 12V wall adapters, maybe even 9V to minimize heat (if I can find a good deal).

My circuits are exclusively 5V.

So far I have:

- 3 LED strips consuming about 175mA (525mA)
- 4 LCDs consuming about 130mA, datasheets are vague, just received some samples for testing (520mA)
- 21 conventional LEDs (420mA)

So about 1500mA, but I don't like running components at peak. I prefer to run stuff at 50% at the most, ideally 20-30%.

Without intense searches, 12V 3A goes for $6, 12V 5A goes for nearly double at $12:
https://www.aliexpress.com/item/32859196804.html

I will be making a RFQ on Alibaba later; they tend to be 5-10 times less, but you have to buy more.

[Demon]

Short answer, yes, probably. I only got it working on contrast so far.

The top left pot controls contrast.




I used a 2N2222A transistor with 100R resistor on base (yeah, forgot my pull-up resistor, although I'm going to change that to pull-down).

[Ioannis]

If you replace the 2222 with a MosFet, then the 100 ohm resistor is OK. But for an NPN bipolar, I think it is too low! A 1K to 4k7, depending on the Ic current and the Hfe of the transistor would be better. You will feed too much current into the base junction.

Also the 10K is not needed there, as the PWM singnal is a 0-5 volt.

Better use a N-MosFet for the control, to have a very low ON resistance and almost negligible power loss.

Ioannis

[Demon]

LCD model 2004A V1.3

White text on blue background

Contrast:

- 2N2222A NPN with 1K on base
- 0.20mA with PWM @ 1953 frequency (recommended)
- 0.46mA with PWM @ 1953 frequency (maximum with 5K pot)


Backlight:

- 34mA direct 5V

- 2N2907 PNP with 1K on base
- 5mA with PWM @ 1953 frequency (recommended for night)
- 15mA with PWM @ 1953 frequency (recommended for day)
- 34mA with PWM @ 1953 frequency (maximum)


CURRENT ON BASE:

Contrast:

- 1.5mA with PWM @ 1953 frequency (recommended)
- 3.8mA with PWM @ 1953 frequency (maximum)


Backlight:

- 0.5mA with PWM @ 1953 frequency (recommended)
- 4.0mA with PWM @ 1953 frequency (maximum)

[Ioannis]

As long as you have the same module you will be ok.

I think I use the same one from Ali express. About 3.5€

Ioannis

[Demon]

As long as you have the same module you will be ok

That's the thing. These are from AliExpress so I can do initial testing.

I'm waiting for a manufacturer to send an order of samples. It's the same product, but the version might be different, and it's not the same colours (that might have an impact on current).

It looks promising, but I still have to repeat these tests with the final product, just to be sure.

[Ioannis]

The white LED on blue background are the most efficient. Green LED seems the most current hungry, 10 fold up.

Ioannis

[Demon]

I'm waiting on red text on black and white text on black. I expect them to be about 130-150mA backlight.

This batch of white on blue have terrible viewing angle. I bet they are designed to be on the left side of a photocopier, cause the display sucks once your head passes beyond the LCD on the left.

Mine are supposed to be 6 o'clock. I need even viewing angle left and right.

[Ioannis]

That is strange... Usually they have narrow angle on the vertical axis, not on horizontal.

White on blue is most interesting for the user, at least that is what I receive from customers.

Ioannis

[Demon]

Agreed, white-on-blue has a lot of interest amongst hobbyists. Here's a video alongside a black-on-white. Notice the better viewing angle on B-on-W.




(still have to make current tests on B-on-W)


We're having thunder, the dog wants to climb on my lap.

[Ioannis]

We have not seen rain for months... Very dry here and many wildfires.

Indeed the b on w is very good. Hope it has same LED current needs as w on b. After all, both have white LEDs.

Maybe I can switch to this on my future projects.

Ioannis

[Demon]

Comparison between LCDs from AliExpress (first batch), and LCDs from Alibaba. The viewing angle on the Alibaba units is really good.

Note that the contrast and backlight are not tuned for the Alibaba units. The (rough) adjustments were done using the AliExpress units.

[HenrikOlsson]

Oooh, me like the blue-ish/white on black in the second half, almost looks like VFDs.

[Demon]

Oooh, me like the blue-ish/white on black in the second half, almost looks like VFDs.

Yup, they are just awesome! I'm so happy I went with this supplier. If you ever want to order a large batch, let me know and I'll give you the rep's contact info over at https://cle.en.alibaba.com. They have other colours, the black-on-yellow background was real nice.

I still have current tests to do, as well as fine-tune the contrast and backlight adjustment (I want to limit the range to a safe usable range that won't fatigue the LEDs).

[Ioannis]

I am interested in the supplier please.

Ioannis

[Demon]

First test with 4 dual-encoders with LCD driven from 16F18877.

Barely 25mA to drive entire circuit, including backlight.




I still have to do more contrast & backlight tests on these, but this is awesome. I expected this to be at least 150mA.

[Demon]

I suspected my voltmeter gave a faulty reading, so I connected 2 ammeters in series (removed my yellow power LED at the bottom to give a more true reading).



14.5mA! That's just crazy low. :O

[Ioannis]

That's incredible!

Amazing LCD's...!

Ioannis

[tumbleweed]

That red one looks more like an OLED to me, and not an LCD.
a lot of them have much lower current unless you light all the pixels.

Got a datasheet?

[Demon]

...Got a datasheet?

How's your chinese?


Trying to send by message. I can't attach a PDF here.

EDIT: Nope. Don't see any way to attach in PM either.