Non-availability of low-ohm 0.1% resistors? - Page 1
Forum section: electronics / projects

author: motocoder
author_rank: Unknown
timestamp: January 14, 2016, 11:50:35 pm
content: I have an IET RS-200 Resistance Substitutor that I bought some time ago on eBay. It seemed that some of the resistors in this thing were not up to the 1% claim, perhaps they had been replaced by the eBay seller. So over the US holidays, as a small project, I upgraded the resistors inside this thing to 0.1% resistors.Each range only uses 1x and 2x values (so 1 and 2 ohm for ohms range, 10 and 20 ohm for 10-ohm range, etc.). So I bought a few extras of every value, and was able to select the resistors such that the accuracy on all ranges except the ohms range is better than 0.06%. The total cost of this was $50 for the used RS-200, plus $30 for the resistors, so for $80 I have the equivalent of the more expensive ($330+) RS-201.However, I was not able to find 1 and 2-ohm 0.5W 0.1% resistors. There were some 1W 0.1% resistors that I found, but they were very expensive ($8-$9 in QTY 1). Does anyone have a source for reasonably priced 0.1% 1 and 2-ohm resistors? Either 1/2W or 1W would fit. Film resistors preferred.Can someone explain why 0.1% resistors in this particular range are so hard to find? It seems like you can find these in value below one ohm, and 10 ohms or above, but there is this gap from 1 - 9 ohms.

author: motocoder
author_rank: Unknown
timestamp: January 15, 2016, 12:32:28 am
content: Quote from: AcHmed99 on January 15, 2016, 12:23:49 amI think that would be feasible, but definitely not something I would be interested in doing. I would just live with the 1 ohm range precision as it is now. I'm mostly just interested in understanding why there is this gap in available resistor values at a particular precision.

author: dom0
author_rank: Super Contributor
timestamp: January 15, 2016, 10:06:27 am
content: Unless you are using Kelvin connections a 0.1 % 1 ? resistor just doesn't make a lot of sense with every 4 mm connection having 2-3 m?.

author: tszaboo
author_rank: Super Contributor
timestamp: January 15, 2016, 10:25:05 am
content: Recently I also had to select precision shunts in this ~1 ohm range. It is very difficult to select, I contacted a dozen manufacturer about the task, and only three come up with even remotely close to what I needed, one of those ended up being worse than they claimed.For resistors like this, you need to go with a bulk metal foil resistor from -you know who-. Even the Zeranin resistors exhibit 0.1% stability over 1000 hours.I dont think you should expenct the 0.1% accuracy with the bulk metal, without special, stress relief construction, and probably some ageing or burn in after soldering the resistor in place.If you ask me, you should be glad, with whatever accuracy you get, and calibrate the resistor yourself. 8 USD is not expensive for these.

author: Fungus
author_rank: Super Contributor
timestamp: January 15, 2016, 01:11:58 pm
content: I think the temperature coefficient of a 1 ohm resistor will be awful. It will heat up really easily.

author: dom0
author_rank: Super Contributor
timestamp: January 15, 2016, 03:51:26 pm
content: Isabellenhütte makes precision shunt resistors. PBV for example is stocked by some, has quite good parameters and costs about 9 € ea. I think their lowest TK resistor has 3 ppm/K guaranteed, the PBV has 30 ppm/K.But as I said above - without a Kelvin setup these precision resistor don't make any sense, which is why they all are Kelvin resistors after all.

author: tszaboo
author_rank: Super Contributor
timestamp: January 15, 2016, 04:07:07 pm
content: QuoteSo, even though they sell 0.5% upon request, It is not going to be 0.1%. I dont want to bring up the Arrhenius Equation, because it is a lie for electronics, let just say the hysteresis is going to be a huge issue for this resistor. Maybe the PBV-Z might be OK, if you are lucky. Made out of the Zeranin, mentioned above.

author: Vgkid
author_rank: Super Contributor
timestamp: January 15, 2016, 06:05:32 pm
content: Unfortunately precision low ohms resistors are uncommon, so look into 2-3w wirewounds. Or for even lower values, shunt resistors.

author: motocoder
author_rank: Unknown
timestamp: January 16, 2016, 01:43:30 pm
content: Thanks everyone for the replies. It seems that it's not practical to make the metal film resistors down at this low ohm range, and the range is too high for a simple wire shunt, so that leaves wirewound resistors, which are apparently inherently more expensive to make.