Do sub 0.1 ohm variable resistors exist? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: July 28, 2019, 07:09:04 pm
content: At that level are mechanical wipers still used?I could imagine in the old days of no heath and safety you would have electrodes dipping into mercury in a trough and you would vary the distance between the electrodes.Looking for a way to vary the current through a 0.38 ohm load at 14 amps fed by recified AC.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 28, 2019, 07:34:55 pm
content: If you're using rectified A.C. then I assume there is a transformer connected to line voltage with about 6+ volt output to work with that load. I would use a variac on the primary side. That would give you far better control of the load current and use parts you can easily find.Years ago they would use large carbon pile resistors to adjust low voltage/high current. A number of about 2" square slabs of carbon were pinched together in what amounted to a vise and the more pressure, the lower the resistance. The speed pedal on some sewing machines also worked this way.Another way that is far more complicated would be to wire a high value pot across the .1 ohm resistor and have the voltage on the wiper go to an opamp that could control a circuit similar to a lamp dimmer on the transformer primary.

author: coppercone2
author_rank: Super Contributor
timestamp: July 28, 2019, 07:45:49 pm
content: custom jobwith a glass seal and a magnetically coupled motor, there is no reason you can't do the mercury thing safely so long you follow the part

author: hagster
author_rank: Frequent Contributor
timestamp: July 28, 2019, 08:59:51 pm
content: Maybe this mercury alternative will do.https://en.m.wikipedia.org/wiki/Galinstan

author: ejeffrey
author_rank: Super Contributor
timestamp: July 28, 2019, 09:27:56 pm
content: I agree that a variac is probably the easiest and best option. A PWM controller or a linear transistor regulation would be my second choices.I've never seen a pot that handles that sort of current/resistance range but the resistance element can easily be made from a suitable gauge nichrome heater wire. Getting a wiper to run along it with relatively constant contact resistance is more of a challenge and depends on the application. How many cycles does it need, how accurately do you need to set it?

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2019, 09:42:20 pm
content: Quote from: e100 on July 28, 2019, 07:09:04 pmJust one question: why?There should be no reason to vary the current shunt resistance. Use a much higher value potentiometer to adjust the reference voltage, to the input of the error amplifier and a fixed shunt resistance.

author: coppercone2
author_rank: Super Contributor
timestamp: July 28, 2019, 09:47:17 pm
content: Quote from: hagster on July 28, 2019, 08:59:51 pmI think because it wets so much you would need to actively monitor impedance to adjust it (it would not be adjustable via a fixed scale). When you start out, the wire would only have gallistan up to where it was dipped, but after its undipped there will be galistan on the surface acting as a shunt across the resistor to some regard.I have a feeling it would not be easy, I never even had good luck with those thermometers that have mercury replacement in them, it seems to stick to the glass. I think mercury would be more stable.

author: bdunham7
author_rank: Super Contributor
timestamp: July 28, 2019, 10:27:02 pm
content: The only device I personally have seen that would fit the bill would be a very old automotive load tester. I used to have one like this:https://www.ebay.com/itm/SNAP-ON-MT-552-CHARGING-CIRCUIT-BATTERY-LOAD-TESTER-MT552/183798734213?hash=item2acb421d85:g:pMAAAOSwhh9czbcFThis one has two load ranges for testing battery and generator outputs. As I recall, my unit had pretty good control over the lower currents, so I'm sure the resistance on the lower range can easily be controlled above 1 ohm or so, the minimum would be about 0.1. The battery (500A) range would be overkill for you. Newer testers won't work if they don't have a traditional carbon pile.Seriously though, a variac or iso-variac like the BK 1653 would be a much better and cooler option.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 28, 2019, 10:36:07 pm
content: Ohmite has the 210-series, but the lowest range they offer is 1 ohm.https://www.ohmite.com/catalog/210-series?filter=ohms%3A2%3A1%20OHM#cds-attribute-ohmsAs others have mentioned, most likely a custom job.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 28, 2019, 10:54:05 pm
content: There was such a thing as acarbon pile rheostat, or something like that. The stack-of-discs version of the carbon granule microphone, rated for more power and probably more stability, and controlled by a clamp rather than by a diaphragm.For practical purposes today, I would suggest, I think, using the maximum acceptable value and adjusting the voltage reading across it with a larger value pot (or better yet, designing the surrounding circuit to have a controlled reference or gain); or if it's used as a resistor in and of itself, not for sensing or whatever, then perhaps a "power DAC" is acceptable (a few resistors, switched by MOSFETs), until the first method becomes applicable. (Example: a dynamic range of orders of magnitude, like for a SMU; the ranges could be switched by resistor selection, then each individual range is electronically variable.)Tim

author: digsys
author_rank: Unknown
timestamp: July 29, 2019, 01:09:47 am
content: Easy - pick up one of these and rewind it to whatever range / value / current you want. If it needs constant adjustment, get one with a wiper arm. Done it in the past.

author: Gyro
author_rank: Super Contributor
timestamp: July 29, 2019, 03:27:00 pm
content: Assuming that this is a one-off application... You can still find good old fashioned cylindrical rheostats at low resistance and high current. Eg.https://www.ebay.co.uk/itm/Tubular-Resistor-Rheostat-10A-5-Ohm-5P-60-A348367-61-RAF-Vintage-Aircraft-Spare/323713607813?hash=item4b5ed5a885:g:A-8AAOSwqKFdE2RAI have a 3R3 one at 10A (330W) that I use as a variable load. You won't find as low as sub 0.1R but you could use a higher value one in parallel with one or more low value fixed shunt resistors to do fine adjustment.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 29, 2019, 03:41:36 pm
content: I have used an 1 Ohm 50 Watt rheostat. It doesn't have many steps though... It's wound with ribbon instead of wire.This oneI think.If you're looking for sub 1 Ohm rheostats, you're probably doing it wrong though.I specifically needed for a test scenario to vary the load on a digital current source. Sometimes they are also used as correction for current transformer in generators. Funny stuff they've invented before the age of semiconductors.