Find right resistor value for a project - Page 1
Forum section: electronics / beginners

author: Darximor
author_rank: Contributor
timestamp: June 27, 2020, 04:50:25 pm
content: Hello,As a beginner, I would like to do a light panel like in this video :In summary, it require a 12v adapter and 6v to power on a screen.I understand why he use two resistor of the same value (180ohms) to split the voltage and get 6v from the 12v adapter.But I don't have any 180ohms lying around. What will happen if I use two resistors with another value (two resistors of 9 ohms for example ?)

author: KMoffett
author_rank: Regular Contributor
timestamp: June 27, 2020, 08:12:00 pm
content: 220Ω and 1KΩ in parallel are common and close...180.32786885246Ωor 390Ω and 330Ω...179

author: Benta
author_rank: Super Contributor
timestamp: June 27, 2020, 08:29:30 pm
content: Nice idea, I like itBut frankly, I'd forget about the resistors and look for the +5 V supply trace on the PCB (probably called VDD or VCC) and connect the two inputs to that directly. Much easier.

author: Fredderic
author_rank: Regular Contributor
timestamp: June 27, 2020, 09:00:42 pm
content: Just put 40 of your 9Ω resistors in a big long string, and it'll be fine…!But a little more seriously, if it's just a digital on/off input, then you generally want the biggest resistors that'll deliver enough power to turn it on reliably, and a couple hundred ohms is a pretty safe bet, poking it with a generous fist full of mA… If you can get away with a pair of 1KΩ's, even better… the more resistance you can put between those wires the better.If it's a current-dependant line (can't think of any reason why it would be), than you can probably go up or down from that a little, but will want to stick pretty close. I'd say he just picked 180Ω because it's a reasonably safe bet for inputs like that, or because he just had a whole bunch of them laying around, and they happened to work.And, as Benta said… even better if it's already available right there on the board somewhere.

author: Darximor
author_rank: Contributor
timestamp: June 28, 2020, 09:12:10 pm
content: Thanks all for your answers. KMoffett, I don't manage to find any of these expect 1KΩ but I get the idea and I found many others on few dead power supply.I put my multimeter on Ohmmeter mode, these are the value I've found, with the 2000Ω calibration for most of them.https://ibb.co/23sRPTQhttps://ibb.co/5xMK32PFind +5v on the pcb seems to be a really good idea, but I'm questionning myself about why the guy doesn't do this in the video, and if the only purpose of+5v is to power on all the circuitry, will it work or not. Maybe I will give it a try before using resistor !Fredderic your answer help me to get the motivation of desoldering some others resistors, and I manage to find a couple of them of 600Ω. But theyare a little bit different than the other (all 600Ω only), a little bit transparent I'd say. Are they really resistors, maybe zener diode ?

author: Benta
author_rank: Super Contributor
timestamp: June 28, 2020, 09:39:52 pm
content: Quote from: Darximor on June 28, 2020, 09:12:10 pmThat's two questionsWhy the guy doesn't do it in the video? Probably because he didn't think about it. Or had too little knowledge.Second, the idea is not to power the circuit via the resistors, the circuit handles this itself from the +12 V supply. The purpose is just to connect the two logic inputs to a low-power +5 V source from somewhere. The best source being the board itself.BTW, the guy should take a soldering course. I've seldom seen such lousy joints.

author: Darximor
author_rank: Contributor
timestamp: June 29, 2020, 12:56:06 pm
content: Thanks Benta, good news, I managed to find all the pins I needed, I think. Here is a photo of what I will wire, is this correct ?https://drive.google.com/file/d/1atnCrcAdGbxw2ciX4ajEPEwjV8mTj4zC/view?usp=sharingIn my case :PWM = PWM_LEDLED_EN = LED_ENINVLED = VLEDVDD = VDDIN (I think that VDDIN is what I'm looking for, and AVDD is not, I'm not sure)Ground are really easy to find, there is a lot gnd1, gnd2... I suppose I can wire a random oneI manage to found datasheet of the panel I use, I have 5 of them (http://www.hbx168.com/upload/file/21.pdf)According to this, VDD should be able to supply 4.0v

author: Darximor
author_rank: Contributor
timestamp: June 30, 2020, 09:30:50 pm
content: Update : I don't found any +5v on the board for now so I used USB power for the two PWM and LED_EN, it doesn't work either.The srceen I use is a slim design screen, with this datasheet :https://datasheetspdf.com/pdf/1139039/BOE/NT156WHM-N32/1I tried with another screen that is more close to the original from the guy in the video, (I think this is the most common type of 15.6 LED from laptop in the last decade), here it is:https://datasheetspdf.com/pdf-file/705591/AUO/B156XW02-V2/1And it worked very well ! With the same wiring and same power supplies.This is weird because they seems to have same behaviour according to the datasheets. (expect from the connector).There should be something I missed, any idea ?