Can we PWM an LED backlight on LCD module

[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.