Low Tempco Trimpots or multiturn Wirewound Potentiometers? - Page 1
Forum section: electronics / general

author: beanflying
author_rank: Super Contributor
timestamp: February 05, 2019, 10:15:50 am
content: Hi All getting some bits together while I wait for the busy Mr Pettis to make me some 0.005% resistors for a Hamon Divider (10 and 100:1) and I have been looking at what sort of a trim arrangement to put together for it.What are the collective thoughts on using multiturn precision Potentiometers like the Vishay 536 (claimed 20ppm datasheet attached) instead of say the Bourns 3290 (50 PPM) or other similar trimpots? Part of the consideration is budget the Vishay's are 2/3 of the price and space is really a non issue for this project.Any others that should be considered in the mix with the relaxed size requirements?I have had a good search for a topic and couldn't find one but if there is let me know too

author: Echo88
author_rank: Frequent Contributor
timestamp: February 05, 2019, 01:21:51 pm
content: General advice:If youre looking at Vishay-datasheets always remember: fancy marketing-wankery headlines mean nothing, try to search for the hidden truth.In this case the Vishay Spectrol 536https://www.vishay.com/docs/57066/536.pdfclaims 20ppm/°C TCR for the wire from one end to the other only, not the taper youll use!Havent found the Taper-TCR in the DS and wouldnt recommend using it.Another Vishay-case:http://www.vishaypg.com/docs/63056/12801285.pdf"Super awesome performance" in the details: - through the wiper ± 50 ppm/°CEdwin recommend Bourns 3250W and mentions the needed minimum current for the taper:https://www.eevblog.com/forum/metrology/what-are-some-recommended-ic-opamps-for-dc-voltage-reference-circuits/msg1142447/#msg1142447https://www.mouser.de/datasheet/2/54/250-776719.pdf<- Bourns 3250WDr Frank built a Hamon Divider himself, which can be used as an inspiration:https://www.eevblog.com/forum/testgear/anyone-else-built-a-hamon-divider/msg775991/#msg775991andhttps://www.eevblog.com/forum/testgear/hp34401-measurement-of-linearity/msg358713/#msg358713

author: ArthurDent
author_rank: Super Contributor
timestamp: February 05, 2019, 02:22:13 pm
content: What I did on the ratio amp to get 10V from the 7.08xxxx of my LTZ1000 wasn’t as elegant but I needed a 4K941 ohm resistor. I used a 2ppm 4K750 ohm and three selected lower resistance WW resistors I had in series to get just slightly higher than that value. I paralleled the last resistor, which was 88R67 ohm, with a 5K 30 turn pot and 2K2 resistor in series across the low value resistor. That way the effects of the 5K pot and 2K2 resistor were minimized and gave me a 2K2 to 7K2 ohm resistor across the 88R67 ohm resistor changing its value by just over 2 ohms, more than enough to trim the output to 10.00000 volts.

author: chuckb
author_rank: Frequent Contributor
timestamp: February 05, 2019, 05:05:35 pm
content: Quote from: Echo88 on February 05, 2019, 01:21:51 pmI see that 50ppm spec in the datasheet. What are the details here?Is that spec the Voltage Temp Co (at the wiper) when the trimmer is used in a potentiometer (Voltage Divider) application, with low or no wiper current.Or is it the TCR in a rheostat (variable resistance) application?Thanks

author: beanflying
author_rank: Super Contributor
timestamp: February 05, 2019, 11:38:15 pm
content: Thanks for the information I was aware of Vishay massaging the 'truth' on some data sheets is why I asked. I won't be making final decisions for a while until I get the resistors from Edwin to finalize but having a good idea of an series to use will make it easy then.The Bourns 3290 was Edwins pick to me but the pricing available on the 3250W looks a lot better.More research

author: The Soulman
author_rank: Super Contributor
timestamp: February 06, 2019, 12:41:06 am
content: Whatever trimpot you choose, make the trim range as small as possible, it makes adjusting a whole lot easier and anyunwanted side affects are reduced by the same amount.chuckb mentioned (minimal) wiper current, this may be more significant than any tempco if the trim range is fairly small.I've used a 10x 1K ESI decade switch+resistors to create a divider with 1 thru 10 Volt tabs.The resistors where already well matched but I also added parallel resistors (just regular metal film) to match them to within 50 ppmand on top of that a 20 turn trimmer+network with approximately 70 ppm range to dial them within +/- 1ppm, the best I could do with a Wheatstone bridge configuration and a low res meter..The trimmers I've used where jelly bean bourns 20x ones.At that time I never did think of any minimal wiper current (and the smaller the trim range, the smaller the voltage drop, the less current..), but it may be good still as I don't have a high res meter to spot it from the output voltage.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: February 06, 2019, 01:56:59 am
content: You definitely do not want zero current through the wiper unless you want instability in the contact. Bourns has a booklet about trimmers and proper use of them on their website. Instruments like the HP 3456A, Fluke 720A and many others used only wire wound trimmers. These trimmers are often left alone for years after the instrument ages a bit and stay in cal for a long time, assuming the circuit is well designed.The 3290 is top of the line but just about any of the other ww trimmers Bourns makes will be okay, shop around, look for sales and sometimes they pop up on eBay for a good price....patience doesn't cost anything.

author: branadic
author_rank: Super Contributor
timestamp: February 06, 2019, 06:00:03 am
content: Quote... but life time. It's like hope dies last, but it dies.-branadic-

author: SilverSolder
author_rank: Super Contributor
timestamp: February 06, 2019, 05:43:17 pm
content: Thank you Edwin for pointing out the Bourns resource. Lots of interesting stuff there.I already learned a couple of things I didn't know:* Don't use a trimmer outside 10% - 90% of its range! Going near the end stops is bad news.* Some handheld ohmmeters can damage the wiper with over-current!https://www.bourns.com/resources/technical-library/library-documents/trimming-potentiometers-technical-library