Replacing single adjustment pot with concentric pair coarse/fine - Page 1
Forum section: electronics / projects

author: perdrix
author_rank: Frequent Contributor
timestamp: February 21, 2023, 11:54:00 am
content: I'm thinking of modifying a bench PSU I have to allow a finer adjustment of output voltage.Several questions spring to mind - lets assume for the sake of argument that the existing pot. is 10K1) Coarse adj. pot also 10K I assume - what should fine adj. pot value be?2) How should this be wired up3) Where might I find such a dual pot. and knobs for same?ThanksDavid

author: jonpaul
author_rank: Super Contributor
timestamp: February 21, 2023, 12:20:07 pm
content: Concentric pots and knobs are specially designed for the equipment eg HP and TektronixSuggest 10..20 turn wirewound Bourns/ Beckman Helipot available surplus or ebay.alternative 2 separate controls.Jon

author: themadhippy
author_rank: Super Contributor
timestamp: February 21, 2023, 12:39:47 pm
content: Quoteand are often available off the shelf at your local guitar emporium

author: artag
author_rank: Super Contributor
timestamp: February 21, 2023, 01:40:40 pm
content: You could consider a 10-turn pot instead. It's slow to adjust coarsely but does give you a fine adjustment as well as a full range. It also avoids the problem of concentric pots that when adjusting the fine pot you reach the end of travel and have to make a coarse adjustment which stops a smooth increase or decrease.

author: TimFox
author_rank: Super Contributor
timestamp: February 21, 2023, 02:53:46 pm
content: On my small -hp- bench supplies, the concentric coarse/fine pots work well, but I always get frustrated when making a delicate adjustment (against a good DVM) by the relatively narrow range of the "fine" pot. I get to end of range, then try to make a small change in the "coarse" pot. Since they are both wirewound (for good tempco), there is a finite resolution due to resistive wire size.I prefer the 10-turn pots on my other supplies.When looking at 10-turn pots, Bourns and other good manufacturers specify the "resolution" due to the wire size, which is coarser for small values.Expensive pots smoosh a conductive plastic layer over the wires, which helps with contact noise and smooths out the wire bumps, but at a substantial price increase.

author: Gyro
author_rank: Super Contributor
timestamp: February 21, 2023, 06:21:25 pm
content: Quote from: themadhippy on February 21, 2023, 12:39:47 pmYes, you can see them on ebay too but they are mostly high resistance and beware that most (all?) are audio / log taper.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 21, 2023, 08:22:08 pm
content: I have 20 turns pots for voltage and current, love them until you have to go full range lollI did saw in the past some mechanical "speed" you had one stage for full range in 1 turn, you pull and you get the 1/10 dividerI never found out where they came, been tossed long time ago, maybe ham stuff ??

author: Benta
author_rank: Super Contributor
timestamp: February 21, 2023, 08:55:49 pm
content: Quote from: coromonadalix on February 21, 2023, 08:22:08 pmYep, seen those as well, really neat!It's basically a knob with a 1:1/1:10 switchable gearbox. It was from one of the knob manufacturers, but the name eludes me right now.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 22, 2023, 02:40:36 am
content: https://www.royalsignals.org.uk/jackson_bros_caps/4832unit.htmhttps://www.ebay.co.uk/itm/292850016222?https://www.royalsignals.org.uk/jackson_bros_caps/4511drf.htmhttps://www.orenelliottproducts.com/product-category/planetary-reduction-drives/??Some of them where called dual speed knob