How to choose a resistor? - Page 1
Forum section: electronics / beginners

author: redgear
author_rank: Frequent Contributor
timestamp: November 01, 2019, 10:37:46 am
content: I am trying to recreate a reference circuit that I found online. The circuit has a BOM which specifies the resistances to use.I know the value of the resistor, case code and the tolerance. I assume case code won't matter much as long as I can solder the components properly(correct me if I am wrong). There is no info about the power rating, temperature coefficient and voltage rating.How do I choose a resistor without these?Thanks

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2019, 10:55:28 am
content: You need to understand how the circuit works. A well-designed circuit will minimise the dependencies, but can't eliminate them.If you can't understand the (analogue?) circuit, you could simulate with, say, LTSpice. You could manually tweak component values, or set it up to do monte-carlo simulations.

author: redgear
author_rank: Frequent Contributor
timestamp: November 01, 2019, 11:21:10 am
content: Quote from: tggzzz on November 01, 2019, 10:55:28 amThanks.The resistors are directly connected to the pins of the IC. I only have access to the block diagram of the IC. I don't think I simulate it. Can I? The Datasheet is in Chinese which makes things more difficult. I am attaching the circuit below for reference.EDIT: The power traces are thicker that the others, how do I determine the thickness that I would require?

author: tggzzz
author_rank: Super Contributor
timestamp: November 01, 2019, 11:46:33 am
content: Most of those resistors maximum possible power dissipation can be easily calculated: V2/R, where R is the supply voltage. For R2/R7, reduce V by one LED forward voltage drop.RNTC is interesting; NTC may mean negative temperature coefficient, which implies a thermistor. Unfortunately thermistors are made with differing tempcos, so the best bet would be one with a tempco in the middle of the possible values.Google can be useful when looking at foreign language datasheets.

author: redgear
author_rank: Frequent Contributor
timestamp: November 02, 2019, 06:48:07 am
content: Quote from: tggzzz on November 01, 2019, 11:46:33 amDo you meanVis the supply voltage?Quote from: tggzzz on November 01, 2019, 11:46:33 amYep, RNTC is a thermistor and they are 100K @ 25deg with a material constant of 4200Quote from: tggzzz on November 01, 2019, 11:46:33 amI have been using Google Translate. But a lot of information is given as a image and I am not able to translate them.

author: tggzzz
author_rank: Super Contributor
timestamp: November 02, 2019, 08:37:34 am
content: Quote from: redgear on November 02, 2019, 06:48:07 amYes, VGiven that information, use DigiKey to find a thermistor.Bummer.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 02, 2019, 08:57:52 am
content: Quote from: redgear on November 02, 2019, 06:48:07 amTry the Google translate app for IOS/Android, it translates using the camera. I tried it on the text in the diagram and it works fine.

author: Gallardo
author_rank: Newbie
timestamp: November 08, 2019, 05:19:31 am
content: Hope it helps youR1 to VOUT1, R1 to VOUT2, R1 to VBUS, R1 to VIN four lines need to be separated from the R1 pad, the current path of the four lines can not have any coincidence, otherwise it will affect the multi-port change single port is automatic recovery fast charge function .Note that the power path traces need to be as short and wide as possible, and the line width needs to be at least 80 mils.When the NTC function is not used, R12 is replaced with 51KOhm, and RNTC does not need to be installed.

author: Gallardo
author_rank: Newbie
timestamp: November 08, 2019, 05:31:12 am
content: for reference

author: redgear
author_rank: Frequent Contributor
timestamp: November 08, 2019, 10:54:15 am
content: Quote from: Gallardo on November 08, 2019, 05:19:31 amThanks.But from the connections shown R1 is not connected to either of VIN, BUS, VOUT1 or VOUT2.I still have few more questions on the same project. Would be great if you can help me.I have created anew threadfor the same.