Replacing a 500k resistor with a standard value one - Page 1
Forum section: electronics / repair

author: maybepanic
author_rank: Contributor
timestamp: February 16, 2023, 05:29:24 pm
content: Hey,I have a broken laptop with a (possibly) faulty resistor which is part of a voltage divider that is connected to an enable pin of a Step-Down Converter (JW5068A). The laptop schematic says its supposed to be a 500k resistor which I can't buy. The closest readily available ones seem to be 470k and 510k.How do I select the right one? The difference in voltage is not that huge spec: V_in = 19V, R_2 = 150k R1 =500k => V_out = 4.38 ; for R_1 = 510k => V_out = 4.31 ; for R_1 = 470k => V_out = 4.59VMy plan was to check the datasheet of the Step-Down Converter to see what kind of voltage range it expects as a high signal but I am unable to find one with enough info just very basic data sheets.I am 99% sure either the 470k or the 510k should be safe to use here, but is there anything else I could do to confirm this? Or maybe someone has some general advice for this like "between 3.3V and 5V is in general expected on such an enable pin".Thanks!

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 16, 2023, 05:35:59 pm
content: Well, since it's a 499K 1%, then anything between 494K and 504K should be good. Maybe put two 1M resistors in parallel? (Stack one on top of the other).

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2023, 05:50:19 pm
content: Usually, a 470 k or 510 k resistor has a +/-5% tolerance, so the total range of both would go from 448 k to 535 k, well outside the 499 k +/- 1% band.Include that in your calculation of the range of voltages to be expected from the converter.As +/- 1% resistor values go, 499 k is quite popular: where have you looked for a replacement of the small part?

author: maybepanic
author_rank: Contributor
timestamp: February 16, 2023, 05:52:34 pm
content: That's a genius solution didn't even cross my mind to use two 1M Ohms in parallel. I'll look into that.

author: fzabkar
author_rank: Super Contributor
timestamp: February 16, 2023, 05:56:46 pm
content: If the difference between enable and disable is 1%, then something is seriously wrong with the design of the IC. I would use either of 470K or 510K. They should both be OK.

author: maybepanic
author_rank: Contributor
timestamp: February 16, 2023, 06:00:18 pm
content: I am from germany so digikey and mouser are a bit of a pain to order from because of the shipping cost and import tax. I used "conrad" a local electronics hobbyist supplier and had a look at these "resistor book collections" on ali / ebay. I was only able to find 470 and 510 there. Thanks for the hint with the 5% range totally forgot about that +-5% is quite the range. So your opinion is definitely go for the correct 500k +-1% replacement value here?

author: maybepanic
author_rank: Contributor
timestamp: February 16, 2023, 06:09:25 pm
content: That was my gut-feeling too but not more than that....

author: wraper
author_rank: Unknown
timestamp: February 16, 2023, 06:16:46 pm
content: That's just a logic input, not some precision comparator. Precision is not needed there.

author: maybepanic
author_rank: Contributor
timestamp: February 16, 2023, 06:50:06 pm
content: Thanks!

author: fzabkar
author_rank: Super Contributor
timestamp: February 16, 2023, 07:59:25 pm
content: In this application ...https://pc-clinic.bg/wp-content/uploads/2021/01/SY8288CRAC_shema.pdf... EN2 appears to be set at 2.5V.

author: wraper
author_rank: Unknown
timestamp: February 16, 2023, 09:19:23 pm
content: Quote from: fzabkar on February 16, 2023, 07:59:25 pmThere is no voltage divider at all because PR4558 is not populated.

author: fzabkar
author_rank: Super Contributor
timestamp: February 16, 2023, 10:04:16 pm
content: Quote from: wraper on February 16, 2023, 09:19:23 pmDoh! That means it's just a high logic level, as you say.

author: maybepanic
author_rank: Contributor
timestamp: February 17, 2023, 09:11:48 pm
content: Replaced it with a 470k resistor and that actually got the laptop back working again, measured 4.5V at the divider. Thanks everyone!