Testing LCD TFT backlight with my new Siglent PSU ? - Page 1
Forum section: electronics / projects

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 02:57:44 pm
content: I thought I could test a TFT LCD backlight with the current limiting mode of the PSU.The backlight is 8 LEDs - two parallel of 4 LEDs in series.forward current = 40mA, forward voltage 12.8v.I set my PSU to those numbers, and the backlight barely lights up. It is drawing around the quoted 0.48w and being current-limited, but you can only just tell it's lighting up at all.Am I missing something? Am I correct in thinking that with no other pins connected, the screen should show up all-white, right?It's this panel:http://www.internetsomething.com/lcd/DT034XVZ01.pdf

author: prasimix
author_rank: Unknown
timestamp: July 28, 2018, 03:02:52 pm
content: If current is limited properly then you should safely set voltage to e.g. 20-22 V and see what will happen. Your assumption is correct about visible white background.

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 03:05:01 pm
content: Quote from: prasimix on July 28, 2018, 03:02:52 pmIt doesn't change, in fact the 'out' voltage never goes above 12.05v.

author: prasimix
author_rank: Unknown
timestamp: July 28, 2018, 03:08:51 pm
content: Hm, is it possible that a kind of "dimming control" is in place or that some additional DE (display enable) signal is needed? Usually that is applicable to display controller not to a string of backlit LEDs.

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 03:19:24 pm
content: Quote from: prasimix on July 28, 2018, 03:08:51 pmNot as far as I can tell. Weird.I do have another one of these panels but I'd rather not risk killing two of them!

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 03:32:09 pm
content: I verified with my meter - 38.8ma being drawn.Weird. I tried the other panel too.I'll try a proper backlight driver circuit that I put together for a different panel.

author: james_s
author_rank: Super Contributor
timestamp: July 28, 2018, 07:11:00 pm
content: It certainly sounds like the backlight is working, are you sure the panel is not all black when off?

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 08:57:51 pm
content: Quote from: james_s on July 28, 2018, 07:11:00 pmThat exactly what I am thinking currently. Now that it's darker in the room, I can see a faint glow quite a bit more.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 28, 2018, 09:03:57 pm
content: Display technology has come on leaps and bounds in the past 20 years, these days contrast ratios in the thousands isn't unheard of. That isn't so much that backlights are brighter - more that when pixels are "off" they block light more effectively, and at a wider viewing angle.You should really test the backlight when all the pixels are on (white display).

author: carl0s
author_rank: Unknown
timestamp: July 30, 2018, 06:02:28 pm
content: Quote from: Buriedcode on July 28, 2018, 09:03:57 pmIt's going to take me quite a while to get to that stage. I currently have the SSD2828 RGB to MIPI bridge on a proto-board, a -5v inverter, loads of FPC adapters etc, and while the SSD2828 is responding over SPI properly, I'm not getting any action on the screen. I'm not even sure my dev board is outputting an image on the RGB port though! So I've ordered an RGB panel for testing.I'm struggling a bit with voltages. The panel need -5v, +5v, and 1.8v. but 1.8v would be its VDDIO. The SSD2828 bridge needs 1.2v (for MIPI signalling) and between 1.8v and 3.3v for VDDIO. Problem is that the MCU board is running at 3.3v, so the VDDIO from the RGB input would be 3.3v, which then means I need to run the display's 1.8v IO at 3.3v. It does say 'absolute maximum 3.3v', which is a bit worryingEssentially I could do with dropping the 3.3v RGB & SPI from the dev board down to 1.8v, so that the SSD2828 and MIPI TFT use 1.8v IO. It's already a rat's nest of cables though.Good news though, I looked up the panel via a different number on the back, and found information suggesting it is indeed normally-black:http://www.panelook.cn/TM034XVZP01_Tianma_3.4_LCM_parameter_cn_35531.html

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2018, 06:07:47 pm
content: The way a lot of these TFT panels work, there is a specific sequence of commands you have to send to set it up and until all of that is just right it will do absolutely nothing. Once you work out the configuration then it's not too hard to make it do what you want.

author: carl0s
author_rank: Unknown
timestamp: July 30, 2018, 06:37:33 pm
content: Quote from: james_s on July 30, 2018, 06:07:47 pmYeah I know. I'm just struggling to understand how to send commands to it via the SSD2828. I roughly get that I have to put the MIPI data (command & data combined as just 'data' though.. no differentiation?) ? into register 0xBF. and the number of bytes to be sent into 0xBC. Then I presume it just sends as soon as it can. I'm a little bit confused because the datasheet for the SSD2828 tells me that data (16 bit) should always come LS-Byte first. So although the SPI is MSBit first, the data part (after the first byte command) it's split into two bytes, and the least significant byte comes first. Well it doesn't reiterate that for the MIPI stuff. So should I be putting all the MIPI data in reverse too? and the 0xBC data size register data? should I be putting 0x0 then 0x2 if there's 2 bytes to be sent.:-/I'll get there.