Author Topic: Replacing MELF resistor  (Read 381 times)

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Offline XaviPacheco

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Replacing MELF resistor
« on: July 07, 2019, 07:47:04 pm »
There's an array of 10 MELF resistors arranged as a voltage divider for high DC voltage sensing. All of the resistors are 400k (1/4W or 1/2W not sure). One of them is open. I'm trying to find the same replacement in Mouser/Digikey but I don't see them available. The closest one I saw was 402k but they are non-stocked. What do you suggest in this case? Using another package? The board is pretty delicate, that's why I'm trying to find the same replacement in order to avoid doing some ugly tweaking.
 

Offline shakalnokturn

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Re: Replacing MELF resistor
« Reply #1 on: July 08, 2019, 12:00:59 am »
How delicate is your board?
What tolerance do the original resistors have?
The board must be very delicate if you need to worry about a +0.05% up.
Assuming the resistors are all series and the one you're replacing is not the divided output one, I'd just throw the 402k in...
 

Offline XaviPacheco

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Re: Replacing MELF resistor
« Reply #2 on: July 08, 2019, 12:30:10 am »
How delicate is your board?
What tolerance do the original resistors have?
The board must be very delicate if you need to worry about a +0.05% up.
Assuming the resistors are all series and the one you're replacing is not the divided output one, I'd just throw the 402k in...

Yes, all of them are in series. The 402k is non stocked. What about 392k?
 

Offline jaromir

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Re: Replacing MELF resistor
« Reply #3 on: July 08, 2019, 06:40:25 pm »
392k is 10k lower, but you may compensate it by removing another 402k resistor from string and replacing it with 412k resistor, so that total resistance of string will remain the same. 412k resistor will have a few percent higher power loss and voltage across it; but any sensible design should be de-rated so it shouldn't matter much.
412k in E96 series too, so you may be lucky and find it in stock. If not, you may try to find another combination, but do not deviate much from 402k, because of voltage across resistors and power loss.
My hobby projects: https://hackaday.io/jaromir ----------- http://jaromir.xf.cz/
 

Offline XaviPacheco

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Re: Replacing MELF resistor
« Reply #4 on: July 09, 2019, 04:23:59 pm »
I'm having hard time trying to find those MELF resistors 400k, 0204 size. Can't I just use SMD chip resistors? 1206 size maybe?
 

Online capt bullshot

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Re: Replacing MELF resistor
« Reply #5 on: July 09, 2019, 05:05:43 pm »
Depends. Ten MELF resistors in series might form some kind of rugged high voltage divider chain, in which case I would recommend against replacing them with 1206 chip resistors since these aren't as robust as MELF.
Safety devices hinder evolution
 

Offline eliocor

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Re: Replacing MELF resistor
« Reply #6 on: July 09, 2019, 10:30:44 pm »
Is it for a photomultiplier dynodes voltage divider?
Buy 10 pieces of the nearest value ones (392k or 402k or ...) and replace ALL of the resistors in the voltage divider. The ratio will be the same!
Also follow what capt bullshot says.
 

Offline langwadt

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Re: Replacing MELF resistor
« Reply #7 on: July 09, 2019, 10:48:57 pm »
How delicate is your board?
What tolerance do the original resistors have?
The board must be very delicate if you need to worry about a +0.05% up.
Assuming the resistors are all series and the one you're replacing is not the divided output one, I'd just throw the 402k in...

0.5% ;) but yeah unless the original was 0.1% it is likely good enough
 


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