Author Topic: Replacing a 500k resistor with a standard value one  (Read 1709 times)

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Offline maybepanicTopic starter

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Replacing a 500k resistor with a standard value one
« on: February 16, 2023, 05:29:24 pm »
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 = 4.59V
My 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!
 

Offline Kim Christensen

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Re: Replacing a 500k resistor with a standard value one
« Reply #1 on: February 16, 2023, 05:35:59 pm »
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).
 
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Offline TimFox

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Re: Replacing a 500k resistor with a standard value one
« Reply #2 on: February 16, 2023, 05:50:19 pm »
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?
 
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Offline maybepanicTopic starter

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Re: Replacing a 500k resistor with a standard value one
« Reply #3 on: February 16, 2023, 05:52:34 pm »
That's a genius solution didn't even cross my mind to use two 1M Ohms in parallel. I'll look into that.
 

Offline fzabkar

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Re: Replacing a 500k resistor with a standard value one
« Reply #4 on: February 16, 2023, 05:56:46 pm »
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.
 
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Offline maybepanicTopic starter

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Re: Replacing a 500k resistor with a standard value one
« Reply #5 on: February 16, 2023, 06:00:18 pm »
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?
 

Offline maybepanicTopic starter

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Re: Replacing a 500k resistor with a standard value one
« Reply #6 on: February 16, 2023, 06:09:25 pm »
That was my gut-feeling too but not more than that....
 

Online wraper

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Re: Replacing a 500k resistor with a standard value one
« Reply #7 on: February 16, 2023, 06:16:46 pm »
That's just a logic input, not some precision comparator. Precision is not needed there.
 
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Offline maybepanicTopic starter

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Re: Replacing a 500k resistor with a standard value one
« Reply #8 on: February 16, 2023, 06:50:06 pm »
Thanks!
 

Offline fzabkar

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Re: Replacing a 500k resistor with a standard value one
« Reply #9 on: February 16, 2023, 07:59:25 pm »
In this application ...

https://pc-clinic.bg/wp-content/uploads/2021/01/SY8288CRAC_shema.pdf

... EN2 appears to be set at 2.5V.
 

Online wraper

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Re: Replacing a 500k resistor with a standard value one
« Reply #10 on: February 16, 2023, 09:19:23 pm »
In this application ...

https://pc-clinic.bg/wp-content/uploads/2021/01/SY8288CRAC_shema.pdf

... EN2 appears to be set at 2.5V.
There is no voltage divider at all because PR4558 is not populated.
 

Offline fzabkar

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Re: Replacing a 500k resistor with a standard value one
« Reply #11 on: February 16, 2023, 10:04:16 pm »
In this application ...

https://pc-clinic.bg/wp-content/uploads/2021/01/SY8288CRAC_shema.pdf

... EN2 appears to be set at 2.5V.
There is no voltage divider at all because PR4558 is not populated.

Doh! That means it's just a high logic level, as you say.
 

Offline maybepanicTopic starter

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Re: Replacing a 500k resistor with a standard value one
« Reply #12 on: February 17, 2023, 09:11:48 pm »
Replaced it with a 470k resistor and that actually got the laptop back working again, measured 4.5V at the divider. Thanks everyone!
 


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