Please help with an idea for a variable miliohm resistor - Page 1
Forum section: electronics / projects

author: cat87
author_rank: Regular Contributor
timestamp: May 29, 2015, 10:02:22 am
content: Hello everyone,For work, I need to make a variable resistor, from 10miliOhm to 100miliOhm range, preferably something that I can control with a voltage so I can automate a test procedure around this.The most obvious idea is to use a MOSFET as a variable resistor, but I did not find anything suitable on the net that goes down to that low a resistance value.I need this setup to simulate the internal impedance on a Litium-Ion Iron Phosphate battery. Right not I'm doing this by connecting a bunch of wires of different lengths together and getting the needed value that way, but this is just something I came up with on the spot. Like I said, I need something that can also be automated during testing.I did think of a decade resistance box with some very low Vishay metal foil resistors with a MUX and a bunch of relays, but I'm afraid the contact resistance of the relays will mess things up. Or maybe replace the relays with some ADG-XXXX switchesOf course that also implies to connect it to a PC and feed it the data for switching the appropriate value resistors that way, which is ok, but not ideal.So, if anyone has any ideas about this or if anyone has come across this problem before, please replyHave fun and be safe

author: dom0
author_rank: Super Contributor
timestamp: May 29, 2015, 10:09:29 am
content: If the required linear bandwidth is low you can do this with a op amp controller ; just like an constant current source, just that your reference isn't fixed but proportional to the voltage across the source, i.e.:

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2015, 01:09:11 pm
content: If you need a true variable resistor rather than current source then the best plan is probably switched resistors in parallel.Making low value resistors by switching in higher value resistors in parallel is probably the best bet for minimising the effect of switching elements (be they relays or fets). You can make each or the parallel resistors from multiple higher value resistors too to ease sourcing / accuracy and minimise the effect of wiring and connections.Add resistors and switches as needed for your needed resolution.

author: dom0
author_rank: Super Contributor
timestamp: May 29, 2015, 05:32:13 pm
content: Quote from: Gyro on May 29, 2015, 01:09:11 pmThe circuit above isn't a current source.

author: Gyro
author_rank: Super Contributor
timestamp: May 29, 2015, 05:42:21 pm
content: Ah, yes sorry, I didn't read your two lines properly!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2015, 06:03:38 pm
content: The MOSFET / opamp circuit shown above is still only working with positive voltage. The simple parallel combination of resistors (and MOSFETs as switches) can work with AC voltage too. As long as the voltage stays below about 0.5 V single MOSFETs can switch in both directions. For fine tuning a MOSFETs can also be used in linear mode (e.g. changing R_on with gate voltage between 4 and 10 volts) - however this part is not very temperature stable, thus it's likely for fine tuning only.

author: Marco
author_rank: Super Contributor
timestamp: May 29, 2015, 06:06:59 pm
content: He's emulating a battery, so positive is enough.

author: woodchips
author_rank: Frequent Contributor
timestamp: May 29, 2015, 06:30:46 pm
content: Another possibility is a water bath with something like salt in it. Its resistance is proportional to the area of the two plates and inversly proportional to there spacing. The ultimate resistancevalso depends on the strength of the salt, or ion, solution.These can be very large for load testing very big batteries. The heat is simply dissipated in the water.Snag is the variable part is probably a motor and leadscrew moving one plate, like an old floppy disc.Sorry if it is a bit old fashioned!

author: zapta
author_rank: Super Contributor
timestamp: May 29, 2015, 07:03:35 pm
content: How about varying the temperature? Will this give you the resistance range you need?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2015, 07:37:54 pm
content: In the mohms range water would not help very much - mercury might. But this is very old style.Changing the temperature usually only gives small changes - maybe a factor of 2 or 3.At DC the feedback circuit above should be OK. Depending on the current one might need more that one such circuit in parallel. The nice thing is that it needs only one (possibly more if paralleling is needed) low ohms reference resistor - something in the 5 mOhms range, that is still available as a shunt. One can also compensate for the cable resistance by measuring the voltage outside - kind of 4 wire ohms like.

author: dom0
author_rank: Super Contributor
timestamp: May 29, 2015, 08:08:14 pm
content: Also since the shunt amplifier and the reference divider (not in the schematic) are both ground-referenced this makes adjusting them with digital pots very easy.

author: cat87
author_rank: Regular Contributor
timestamp: May 30, 2015, 12:28:19 am
content: Wow, so many responsesThanks for your ideasReading through them, I forgot to mention that the battery is about 700mAh capacity, so the currents involved aren't that big. I've tried playing with a MOSFET i.e. just hooking it up and give it some juice directly into the gate, without the op-amp, and only got to about 300 miliOhms drain to source. The RDS of the MOSFET I tested was spec'ed at around 20 miliOhms, but since the current was small, I didn't have high hopes of it going down to those 20 miliOhms.Didn't have the time to test it with an op-amp, will try that next.And yes, the bandwidth is so low, it's not even taken into consideration. Apparently the only thing of interest is simply simulating the battery's internal impedance ans seeing if the DUT it's connected to correctly reads that impedance.Did think of the parallel switched resistors, but turns out that it's a bit of a hassle because of the many components involved and also the controlling scheme.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 30, 2015, 01:15:53 am
content: you can use a line of stable resistance wire and basically stretch it out, put a ruler next to it and essentially make a sliding rheostat. You can do something with a motor/belt system too...its very repeatable with good wire n a good non oxidizing connector (as shown by robrenz (eevblog user) on his youtube channel video regarding micro ohm meter testing).not too many good solutions to this problem

author: Marco
author_rank: Super Contributor
timestamp: May 30, 2015, 03:35:20 pm
content: Quote from: cat87 on May 30, 2015, 12:28:19 amI'm not aware of non-linear effects in the triode region ... unless you actually measured 20 mOhms at higher currents my first assumption would be that you did something wrong.BTW, Digikey has 100s of MOSFETs with sub 10 mOhm resistance ... a lot of which are under 1 buck in singles.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2015, 04:30:16 pm
content: On resistance depends on the gate voltage. So one may need more than 5 V at the gate to really get the quoted R_on. To a certain degree this might be used, if thermal drift is not critical. With the right MOSFET it's well possible to go below 5 mOhms.

author: Kalvin
author_rank: Super Contributor
timestamp: May 30, 2015, 05:08:10 pm
content: 1 milliohm @ 1V = 1000A10 milliohms @ 1V 100A100 milliohms @ 1V = 10ACombining a small battery of 700mAh with a 20 milliohms load ...

author: Marco
author_rank: Super Contributor
timestamp: May 30, 2015, 08:43:06 pm
content: Presumably it's meant to go in series with a (two quadrant) power supply for simulating a battery.

author: Kalvin
author_rank: Super Contributor
timestamp: May 31, 2015, 12:51:34 pm
content: Quote from: Marco on May 30, 2015, 08:43:06 pmYes, i understand that. However, a small battery capacity of 700mAh makes me believe that the battery may have an internal resistance in range of 100 milliohm ... 1 ohm.If the purpose is to simulate real battery model using the mosfet-circuit presented in the 2nd posting, then mosfet with 20 milliohm on-resistance looks good to me: For example, in order to get 100 milliohm  resistance, the combined resistance of (Rs + Rmosfet) needs to be 100 milliohms.Selecting the Rs to be 50 milliohms, selecting mosfet with Ron less than 50 milliohms, selecting the op amp with the offset voltage << voltage across the Rs, and using an e-potentiometer for the gain selector, it should give a good starting point for creating an adjustable series resistor from 100 milliohms ... few ohms.Of course, there are many mosfets with Ron close to 1 milliohm, so it should not be too difficult to obtain minimum resistance of tens of milliohms. The voltage across Rs will be quite small, so the op amp should have a very low offset voltage.

author: cat87
author_rank: Regular Contributor
timestamp: June 08, 2015, 05:51:04 am
content: Sorry for my absence from this post recently, but I've been away this last week.I've given a lot of thought to this and finally decided to go with an array of 10 10 miliOhm resistors, in series, and each decade tapped off with a MOSFET and their gates connected to a MUX. So basically, with digital control, I can switch to 10 miliOhms, 20 miliOhms and so on all the way to 100 miliOhms by choosing the appropriate address / mosfet. To me this looks like the most accurate and stable method.The circuit with the MOSFET tied with the op-amp and Rs resistor is way simpler and all though it works, I have my doubts that it will be stable from a temperature point of view i.e. a few degrees rise or fall in temperature will significantly alter the resistance and since repeatability is also important, I've decided on the method described earlier.Will update on the progress and results as things progress.

author: Kalvin
author_rank: Super Contributor
timestamp: June 08, 2015, 08:17:56 am
content: Quote from: cat87 on June 08, 2015, 05:51:04 amIt will be stable if the resistor Rs is stable. The feedback loop wil take care of any MOSFET's temperature effects.

author: moffy
author_rank: Super Contributor
timestamp: June 09, 2015, 01:57:32 am
content: You could use a brute force 100W/1ohm variable resistor:http://www.vishay.com/docs/31841/avt.pdf

author: cat87
author_rank: Regular Contributor
timestamp: June 09, 2015, 10:09:10 am
content: Interesting bit of kit, that variable resistor, but I'd need it to go lower than 100 miliOhms. I've tried looking for something like this, but didn't had much luck.

author: JuKu
author_rank: Frequent Contributor
timestamp: June 09, 2015, 10:45:24 am
content: If stepped values are sufficient, I would go with gold plated relay or switch contacts and resistors.