Can we PWM an LED backlight on LCD module

[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.

[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)

[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