Reducing 3.3V down to 2.8V - Page 1
Forum section: electronics / beginners

author: Yatoto73
author_rank: Newbie
timestamp: March 24, 2024, 04:14:32 pm
content: Hello everyone,I need a bit of help, and I hope everyone will understand me.My goal is to build a pcb for a bare displayILI9341.I'm fairly confident I got the traces correct just few things that are bugging me- Reducing the voltage down.My microcontroller board is ESP32 and it has 3.3V outIn the datasheet of the display it says it needs 2.8V VCC. So I thought to use a diode for 0.5V drop 1N4007, and one person suggested to me to place a capacitor 100nF to filter/ stabilize the voltage.-Now I used Easy EDA to draw my first schematic, and saw that there is an Simulation option. So i decided to draw a circuit with 100nF capacitor and 1N4007 diode, battery, and led diode and test it. The issue is the voltage was way below 2.8V.-Now I think that this idea with diode wont work at all, because simulation showed way low result...The other issue, the backlight operating voltage is 3.2V-3.4V. - So i decided to use two resistors 1.1k +32ohm. Again when I made a circuit for simulation it didn't work at all.I have no experience in building circuits, but can someone have a look at my schematic

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 24, 2024, 04:45:48 pm
content: You don't need 2.8V, 3.3V is fine, see theILI9341 datasheet(Page 236).Also theER-TFT032-3.1 datasheetshows this (Page 10).I don't understand the 1K1 resistor in the backlight circuit.The datasheet states the backlight start conducting at 3.2V, so the voltage you need to care about is 3.3-3.2 = 0.1V.For 90mA the resistor will be 0.1/0.09 = 1R.But the working voltage is so close that it might barely light up at 3.3V, even with no resistor.Better to power the backlight from the ESP 5V power input, calculating the resistor as (5-3.2)/0.09 = 20R = ~22R

author: Yatoto73
author_rank: Newbie
timestamp: March 24, 2024, 05:05:04 pm
content: Thank you David.I was worried about running at max voltage shortening its life.Now I used Ohm Law for backlight LED (90mA-100mA 3.2-3.4V)(3.3V-3.2V)/0.095A =~1 ohm resistorI will run a simulation as you suggested with 100R and lower.EDIT: I will test with 5v as suggestedThis is what i got with 5V

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 24, 2024, 05:26:17 pm
content: Bad simulation, a red led is not a white led.Red leds have 1.6-1.8Vf (Showing 2V in your simulation), while white leds have 2.9-3.2V.So for the red led: (5-2)/22=136mABut again, for the backlight: (5-3.2)/22=82mAAlso don't exceed 100mA, it's thw maximum allowable as stated in the datasheet.

author: Yatoto73
author_rank: Newbie
timestamp: March 24, 2024, 05:46:13 pm
content: Thank you once again!i didn't even know about that i thought it was all the sameI think I will have to probe that with different resistors when I make the board. This Easy EDA doesn't offer white led, and when i change between color that they offer blue, red, green, yellow the values are all the same.

author: BTO
author_rank: Frequent Contributor
timestamp: March 24, 2024, 07:54:05 pm
content: Quote from: Yatoto73 on March 24, 2024, 05:46:13 pmMaybe something to try, When i was much younger in electronics, i got aWhite LEDRed LEDGreen LEDOrange LEDYellow LEDBlue LEDNow, Use ohms law to calculate the resistance that gives you say 20mA of current through the LED (say the RED one)Now use that same resistor for all the rest of them and see what happens.They're not all the same, i can assure you

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 24, 2024, 08:01:50 pm
content: Just start with 47R, measure the current, then try lower values until getting around 90mA.

author: Yatoto73
author_rank: Newbie
timestamp: March 24, 2024, 10:01:37 pm
content: Thank you! i will order different types of resistors with the board, and try to measure with different types of leds

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 24, 2024, 10:32:36 pm
content: You can't push 90mA into a normal led, will instantly burn it, just test the backlight, no need to simulate anything.

author: tooki
author_rank: Super Contributor
timestamp: March 25, 2024, 12:44:11 am
content: Quote from: Yatoto73 on March 24, 2024, 05:05:04 pm3.3V is the maximum of therecommendedvoltage. If you look at the Absolute Maximums, the values beyond which damage is likely, it's 4.6V, far beyond 3.3V.Also, I've used that display before, and it's perfectly happy with 3.3V.

author: tooki
author_rank: Super Contributor
timestamp: March 25, 2024, 12:46:08 am
content: Quote from: DavidAlfa on March 24, 2024, 10:32:36 pmIt's not a "normal LED", it's 5 LEDs in parallel, as stated in the datasheet, 90-100mA recommended current.And also, 100mA LEDs aren't rare anymore. Heck, I've used 1.5Asingle-die LEDsbefore.

author: Irilia
author_rank: Regular Contributor
timestamp: March 25, 2024, 04:48:54 am
content: Starting with a LCD without the adapter board is fun, really recommend to have a programmable power supply. Also I recommend to have a component tester. For the LEDs blue white green, red yellow orange, infrared, ultra Violet (different groups = different voltage.

author: tooki
author_rank: Super Contributor
timestamp: March 25, 2024, 08:39:22 am
content: Quote from: DavidAlfa on March 24, 2024, 05:26:17 pmThe display’s datasheet says Vf = 3.2-3.4V. No need to use guesstimates when you have specs.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 25, 2024, 01:28:38 pm
content: Quote from: tooki on March 25, 2024, 12:46:08 amI know. But going for this. He's trying something else:Quote from: Yatoto73 on March 24, 2024, 10:01:37 pm