Equivalent for Vishay NS-2B Resistor (Non-Inductive wirewound)

[cat87]

Hi everyone,

I have a project where I need some non-inductive, wirewound, 3 to 5 Watt, 3.3 Ohms, 1%, 50ppm resistor. Vishay's site pointed me to the NS-2B series, but as luck would have it, they're extinct. Other similar wirewound  resistors from Vishay seem to have had the same fate (the non-inductive ones don't seem to be in stock)

I found a 10W, 5%, 50ppm part from Ohmite, in stock at TME. They're sold as "Audio grade"  .  I'd leave this as a very last resort, because it's a huge part, taking up a lot of board area. Just the body is 45mm long, compared to just 14mm for the Vishay one.

And seems there are sone NS-2Bs on eBay available, but they're not the right value.

So, does anyone have an idea of what other manufacturer has something similar? Might not seem like much, but it's hard to find a series of resistors that fits the bill and is also in stock with Farnell or Mouser.

This is a one-off project so I only need 10 resistors.

Thanks

[daqq]

Do they have to be wire wound? There's a lot of other choices you can make. Current thick film technology can get you great results.

[cat87]

No, it's not set in stone that they can only be wirewound, but I need  a resistor that has a low-ish ppm, to be able to handle 3-5 Watts,  have low inductance and also to have a high pulse withstand. Also, the noise from a thick film resistor tends to somewhat higher.

I've looked at what local suppliers have in stock, and no film resistor has all of the atributes. That's why I went wirewound.

I've looked at some thick film resistors in TO cases, but they didn't cut the mustard. Also, they're kind of expensive.

[floobydust]

Consider plate resistors, like Koa BPR plate resistors.
Axial wirewounds always have a fair bit of inductance due to metal (iron) endcaps and the coil winding.

Power metal film types I avoid as I have seen their resistance DECREASE with mild overloads, bad.

[calexanian]

Just did some searching. Most things like that are non stock. I would suggest paralleling a few 1 or 2 watt metal film or ceramic resistors go get what you need. 1% is really your limiting factor. They are just not as common in wire wound. You could also go flat pack, but again those are usually not off the shelf.

[cat87]

Thanks for the input everyone.

I think I'm going to have to bite the bullet and  as calexanian suggested, I'll make a series-paralell array of  resistors. As for what flavor, the only viable options are wirewound (but that's almost out the window) or metal foil. Thin and thick film tend to have higher noise. There are some thin film types that are lower noise and also have the required ppm, but I'll have some more digging to do before I can confirm this. Then, there's also the matter of stock availability

I've also looked at KOA. Those metal plate resistors are very nice, but the resistance range is kind of small. Maybe again, a series-parallel array.  But they also have some other nice stuff on their site and it looks like they have a (free ?) sample form 

[floobydust]

Yageo also make plate resistors SLR series.

[Conrad Hoffman]

AFAIK, a thin film resistor is essentially a metal film. There are very few circuits where you could tell the difference between the noise of those and a wire wound. The big feature of the wire wound is pulse handling- there's an old Ohmite paper on that. I'd parallel some conventional metal films, allowing for some extra power to handle the pulse needs. Be careful of MOX resistors as they aren't all as quiet or stable as regular metal films.

[DaJMasta]

Maybe you've already chosen, but how low inductance do you need?  Wirewounds are always going to have some, but usually it only gets big with higher value resistors (more turns).  Getting oversized resistors could also help, as the loops inside are probably physically larger (which if they're using the same restive element, means there's fewer turns), but it's not always the case... it's probably best to get a few and choose the ones that are lowest.

As an example, I've got a few 10W (2 Dales and an Orevox) and running through the LCR meter I get:
100 ohm - 5uH @ 1kHz, 3.1uH @ 10 kHz, 3.2uH @ 100kHz
50 ohm - 3.7uH, 2uH, 2uH
1 ohm - 0.3uH, 0.3uH, 0.36uH

And a 2 ohm 5W (unknown brand)- 0.2uH, 0.14uH, 0.2uH

[cat87]

I'm still hunting for  available parts right now and probably will be doing this for some time. It's pretty complicated to get the right combination of low ppm, low noise and high pulse withstand, all wrapped up in a  small as possible package.

As for the inductance I need, it's not a critical value here, but seeing as how the whole thing is going to take some pretty large pulses, I'd personally aim for a maximum value of something like 300-500 nH. Of course, the lower the better.  For the 3.3 Ohms I need, I think those numbers would be a pertinent expectation (Thank you for the measurements DaJMasta  )

Right now I'm trying to figure out the best way to do some series-parallel combination with the metal film. I can get the needed ppm with 4 resistors, for example, but not the power rating. Or, I can get the power rating with some higher power 4  films,  but the ppm will be much higher. The solution would be more series-parallel of the lower ppm kind of resistor, but having 20 or 30 resistors on the board instead of just 10 wirewound doesn't sound that tasty to me.

Thanks for the hint Hoffman, I'm going to look for that paper, to see what real difference there is between the wire wound and film when it comes to high power pulses

[cat87]

I've had a look at some application notes on pulse-withstand of different resistors and found some pretty nice things. I'm posting them here, jufr future reference and in case someone else will have the same problem in the future

There's a note from Vishay showing more maths and how pulse testing is done.
I also found this paper detailing some experiments with different kinds of resitors in pulse conditions. Now, for my application, even the metal film pulse withstand is enough, but notice the huge difference between the wirewound withstand (10's or 100's of kW) compared to the metal film withstand (10's or 100's of W). Admittedly, the paper is pretty old, but the basic principles still apply.

From Ohmite I only found an app note detailing general things to watch out for when selecting a resistor, but might be useful to others

[BU508A]

Have you checked out this website?

https://www.rhopointcomponents.com/components/resistors.html

[Alex Nikitin]

Inductance may be not that large for low ohm values, I've just measured Welwyn W21 3R3 3W W/W resistor I have here in the lab, and the inductance at 1MHz is below 100nH. If the standard version has a low enough inductance, Vishay RS series 3R3 1% is available.

Cheers

Alex

[cat87]

Thank you for that info. I originally overlooked the RS and went straignt for the NS series because of its lower inductance, but <100 nH for a plain wirewound resistor isn't that much. I mean, it's acceptable for my needs.

[cat87]

Ok, so just an update on this.
For my needs, the equivalent to a non-inductive wirewound resistor seems to be a ..... well, non-inductive wirewound resistor. The  film resistor replacement would have worked, but I also need to save space on my board, so the most compact solution is still the Dale/Vishay NS series resistors. Looks like Mouser has a few stocked, perhaps they hid them away in a crack in  a wall or something.
Thanks everyone who chimed in on this