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Old 17th Jun 2019, 5:15 pm   #1
ColinTheAmpMan1
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Default Close-tolerance resistors

I am looking for a couple of 200 ohm, 0.1% resistors with a power capacity of 1 watt or perhaps more. If they are non-inductive, that would certainly be a plus. I know I can buy them from Mouser, but the postage is greater than the cost of the resistors and the inductance is unknown. These are to replace a couple of burnt resistors in a WPA N71 resistance box and voltage divider.
TIA, Colin.
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Old 23rd Jun 2019, 12:46 pm   #2
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Default Re: Close-tolerance resistors

I am surprised that so many people have read this, but no-one knows where I might get such resistors.
Colin.
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Old 23rd Jun 2019, 1:39 pm   #3
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Default Re: Close-tolerance resistors

500mW ones at Farnell for 5 and a bit quid.

Maybe a pair of 100 Ohm in series would give you a bit more choice?

David
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Old 23rd Jun 2019, 2:08 pm   #4
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Default Re: Close-tolerance resistors

RS have them, but not cheap!
https://uk.rs-online.com/web/p/throu...stors/1580306/
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Old 23rd Jun 2019, 6:04 pm   #5
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Default Re: Close-tolerance resistors

I have found a Vishay item which would be just the ticket. The manufacturer's part number is:

Y0090200R000B0L

and it is 200 Ohms ±0.1% 1W Through Hole Resistor Axial Non-Inductive Metal Foil. It is orderable from Digi-Key, but has a lead-time of 17 weeks and a minimum order of 500. That gives a total value of nearly $6000 and a wait of over four months.

The ones available at RS could well do the job (though the tolerance is even closer at 0.05% and the power-rating is also more than I need at 1.5W). I believe that metal foil resistors are non-inductive, but the datasheet for this one doesn't specifically say so. I might well go for two of these, if nothing closer to my requirements/cheaper doesn't come up. Thanks sortedradio, I learned something about the RS ordering system, too.

Does anyone know if 200 Ohms resistors in a decade box, at 1W (max current 70mA) and 0.1% tolerance really need to be non-inductive? The highest range on this particular decade box is hundreds of ohms.

TIA, Colin.
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Old 24th Jun 2019, 6:23 am   #6
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Default Re: Close-tolerance resistors

Wouldn't have thought it matters Colin, all the wire and and resistor leads in the box would surely have more inductance than one low inductance resistor.

Andy.
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Old 24th Jun 2019, 9:04 am   #7
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Default Re: Close-tolerance resistors

Five 1kΩ ±0.1% 0.25W in parallel?
https://uk.rs-online.com/web/p/throu...stors/1372753/
£4:60 for 20 with free next day delivery.
David
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Old 24th Jun 2019, 12:18 pm   #8
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Default Re: Close-tolerance resistors

There are two threads about this, and I suggested in the other one five 1k in parallel. But it would appear that there is insufficient room for that solution in the cramped innards of the resistance box.

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Old 24th Jun 2019, 12:29 pm   #9
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Default Re: Close-tolerance resistors

I don't think that there is enough room for any more than two resistors in parallel anywhere. The arrangements for each decade are that there is one of the smallest resistances and four that are twice that size. The lower value resistor gets shorted out or not by a second wafer on the switch (I think so; but I'm not sure as I can't see the second wafer easily). The only instance where this is different is on the tens decade, where the 10 Ohms resistor is actually two 20 Ohm resistors in parallel.
The 0.1 Ohm and 1 Ohm decades use open wire-wound resistors; the other two decades may use wire-wound resistors, but they are encased, as can be seen in the photo of the "hundreds" decade. The two burnt 200 Ohm resistors can be seen. The switch is shown in the 500 Ohm position.
The wiring is minimal and uses thick wire, so I doubt that there is much stray inductance, but that is just a supposition on my part and may be way off. Does anyone know any more about the construction of close-tolerance decade resistance boxes?

Colin.

Crossed with Craig. You are quite right that there are two threads, but the other relates to a Muirhead A-25-K, where I am sure that I want to use Muirhead replacements, if possible, due to the specific winding type. This thread relates to a WPA N71 Resistance Box and Voltage Divider.
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Old 24th Jun 2019, 3:10 pm   #10
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Default Re: Close-tolerance resistors

Quote:
Originally Posted by ColinTheAmpMan1 View Post
I don't think that there is enough room for any more than two resistors in parallel anywhere.
A pair of 200Ω ±0.1% 0.25W in parallel gives 100Ω. Two pairs in series gives 200Ω:
https://uk.rs-online.com/web/p/throu...stors/1372771/
£3:10 for a pack of 10

In my experience, for most purposes the inductive impedance of metal film resistors is negligible compared to the resistive and capacitive impedances.

David
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Old 24th Jun 2019, 3:47 pm   #11
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Default Re: Close-tolerance resistors

Yes, but as I have shown, there isn't really that much room to put a resistor matrix. Unless my arithmetic is wrong, your suggestion of a series/parallel quartet will still only have a power-rating of 0.5W.

I can tolerate the price that Mouser are asking, but I would like to be better informed that the inductive impedance isn't going to spoil the 0.1% tolerance at any frequencies that I might want to use the decade box at. I take your point about the inductive impedance of metal film resistances, but that still leaves the puzzle of why metal-film and fancy wire-wound resistors with the inductance reduced as near as possible to zero are made. I can see that they are not made in huge numbers and that makes them a bit uncommon and eye-wateringly expensive.

I feel that there must be information somewhere on the design of resistance (and I suppose, capacitance and inductance) decade boxes, in terms of why stray inductance in a precision decade box needs to be reduced or eliminated and what strategies might be used. I have noodled around on the web without getting much more than school-level explanations.

Colin.
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Old 24th Jun 2019, 4:28 pm   #12
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Default Re: Close-tolerance resistors

You might try https://www.charcroft.com/ or https://www.rhopointcomponents.com/

Craig
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Old 30th Jun 2019, 4:41 pm   #13
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Default Re: Close-tolerance resistors

I looked at those two sites, Craig, without much result.
I have purchased four of the resistors from Mouser that I mentioned in message #1 in this thread. I think two of those will probably do nicely, at least until something with no inductance pops its head up at a reasonable price.
Thank you all for the suggestions.
Colin.
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