Voltage controlled resistor options?  - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 10, 2020, 09:51:24 pm
content: I would like to build a Voltage or Digitally controlled resistor for controlling older analog equipment from a micro or PC. The only methods I know of that can get this done is using digipots, jfets/mosfets, or relay controlled resistors.My application needs to control a PSU remote port which requires 1k ohm to produce 1v on the PSU out put. This means I need 1ohm to 20k ohms with a 1ohm resolution, which imeaditly rules out a digipot due to resolution. Using a relay controlled decade box is a pretty decent way to get good acurracy and resolution, but it is kind of clunky. This leaves the FET route, I've never actually used a FET as a replacement for a rheostat, so is it possible to get the range I'm looking for and set it in 1ohm steps? There are also sense lines I can use to read the output voltage of the PSU and use that as a feedback for the FETs gate voltage which I'm guessing would be controlled by a 16bit dac in order to get good resolution. Only question is if it is possible can anyone recommend a FET (peferably one fairchild makes) low power is just fine as long as it can handle 25v.Maybe there is another way to go about this, besides a stepper/pot and the ways I listed above. I seems like digitally controlling resistance at 1ohm steps would be more common that I thought!

author: GerryR
author_rank: Frequent Contributor
timestamp: February 10, 2020, 10:49:29 pm
content: Might these be of any help to you?https://micronor.com/product/mr266/0-10 V or 4-20 mA Control

author: IDEngineer
author_rank: Super Contributor
timestamp: February 11, 2020, 12:55:09 am
content: Do you realize you're asking for (1/20000 =) 0.005% resolution increments? How important are 1) accuracy and 2) repeatability?In the digital realm, 1/20K steps requires 15 bits of resolution.If that reference to 25V is your max output voltage, then 1 LSB will represent (25 / 20K =) 0.00125V (1.25mV). And the control side will likely be a much lower voltage, with correspondingly lower signal levels. How noisy is your environment?As far as the closed loop for FET control, you'd be better off keeping that in the analog domain. Let the digital side set the target voltage, then let the analog side keep it there. It will have better performance (due to bandwidth) AND cost less to implement.None of the above is impossible, but it's not just throwing together a few components and expecting it to work. The first thing I'd reconsider is whether you really, truly need 1/20K resolution.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 03:56:43 am
content: wow those pots look cool but no listed price, nor does it say the # of turns.The PSU outputs 1volt for 1k ohm (using remote terminal), it's precision ovenized with 1mV or . 001 volts resolution, so that is 1ohm steps per mV. I have measured it's noise at 200uV with a proper scope.Actually I think I goofed and only need to do 10k ohms. It's max output is 20v but there is a selector for either 1mv-10v and 10.001-20.000. So I guess I could use a 10k ohm restiance it 1ohm steps and a relay to switch the 10/20v selector.I want to program it in 1mv steps in order to use it for automated component testing, curve tracing etc... I figure it's noise/drift/resolution are pretty damn good so it's probably suited well for the task, much better than anything I could build, only problem is computerizing it for tests, batt charging etc... Im pretty sure and digitally programmable PSU with it's specs is also well out of my price range!

author: IDEngineer
author_rank: Super Contributor
timestamp: February 11, 2020, 04:35:49 am
content: Nice that the power supply itself may be quiet and have the resolution/repeatability, but my comments were more about the circuit you want to connect to it to control it. THAT circuit will need the kinds of accuracy, repeatability, and noise performance that I mentioned. Otherwise, in essence, you will be commanding your super power supply with noisy garbage - and if it's good enough it might just give you exactly what you're asking for! {grin}BTW, I'm not enthusiastic about a motor-driven rheostat being able to give you anywhere near the performance you're seeking. It might be "quiet" but I think you'll be dismayed at its absolute accuracy and repeatability.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 06:01:14 am
content: so what's the problem with just relay controlling a decade box built with one precent resistors? I really wanted to try the get route controlled with a 16 bit dack but I have no idea what get to choose

author: GerryR
author_rank: Frequent Contributor
timestamp: February 11, 2020, 12:15:10 pm
content: A Kelvin-Varley divider would get you the accuracy you need, but you would have to work out a way to activate each section. I've attached a couple of info sheets. They are rather pricey if purchased new, but like anything else, Ebay can be your friend. (I picked up a 10 K unit, good for 5 W and 250 V for under $50!) Just another thought.Edit-added:Here is a different setup found on Ebay:https://www.ebay.com/itm/KELVIN-VARLEY-K175E-RESISTIVE-VOLTAGE-DIVIDER-TESTED/252021811403?hash=item3aadaba8cb:g:jzoAAOSw6JNci7oe

author: ogden
author_rank: Super Contributor
timestamp: February 11, 2020, 12:35:02 pm
content: Quote from: rwgast_lowlevellogicdesin on February 11, 2020, 03:56:43 amCould you provide more info about PSU? Model, brand, datasheet, picture?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 06:15:50 pm
content: thanks for the Kelvin barley divider ideait's a power designs pd2005a which ive rebuilt and recalibrated/re trimmed using a 100uv scope and good 6.5 digit meter. All I'm looking for is a way to generate 1ohm steps for the remote input in hopefully a compact way.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 11, 2020, 06:47:19 pm
content: Quote from: rwgast_lowlevellogicdesin on February 11, 2020, 06:15:50 pmUnlikely with a digipot, as the series resistance of most CMOS analog switches is >10 ohms (at best). Maybe you can find a voltage controlled PS, or find a way to control your PS with a voltage rather than a resistor?

author: IDEngineer
author_rank: Super Contributor
timestamp: February 11, 2020, 07:04:05 pm
content: Quote from: Caliaxy on February 11, 2020, 06:47:19 pmSince he's "rebuilt and recalibrated/re trimmed" the supply, it sounds like he's familiar with its workings. Perhaps this external resistance is simply used to set an internal voltage, and he can bypass this pursuit of resistance and just drive a voltage into a specific node to achieve the control he's seeking. Things would get a lot simpler.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 08:49:30 pm
content: Wow I actually never even thought about injecting voltages at the taps. See my idea is to make an add on box with which allows to up the max voltage and current (obviously at extended power rangers current I will see more noise which is probably ok for most higher power stuff), make a split rail selector, and a digital control along with a serial port and a 16/24 bit adc to interface to a PC with some custom software to run smu functions etc.The thing is I've been thinking about how to keep everything as external to the supply as possible so it would be easy for anyone with the same psu to duplicate. The thing is there is no remote port for the 0-500ma control so I guess I was going to have to connect wires to the internals anyway. I know what I'm trying to accomplish isn't easy and I'm going to end up doing a lot of low noise work etc, but I have all the parts together to build a 16 bit four channel 105msample (each) SDR with fpga and usb3. 1, so Id better be able to deal with 1mv of DC or I'm going no where. I was honestly thinking using a fet as the resistor with feedback was the answer, but never used a get as a load and just wasn't sure if I could get resistance as low as an ohm and as high as 10 or 20k. I have a bunch of 30n06ls and 2n60s which I'm guessing aren't ideal, I would need something way more linear and ideally small signal.

author: IDEngineer
author_rank: Super Contributor
timestamp: February 11, 2020, 09:41:41 pm
content: Quote from: rwgast_lowlevellogicdesin on February 11, 2020, 08:49:30 pmOK, but you may have to dig inside to understand HOW to accomplish what you want from the outside.It's possibly you can have what you want. If voltage injection (instead of resistance) can control the supply - even if it's nonlinear - then you can inject through the external port. The other side of your feedback loop is the normal output itself. Then an external circuit would connect to both, with a third connection to whatever "control" input you desire (serial, dipswitches, pot, whatever). Thus you have your target input, your actual output, and an input for closed loop control.Time to open the enclosure again and start probing around....

author: Jwillis
author_rank: Super Contributor
timestamp: February 12, 2020, 10:31:12 pm
content: In the schematic of the power designs pd2005a it looks like a step voltage control. So its not linear ? Is that correct?https://nscainc.com/wp-content/uploads/pdf/POW_2005A.pdfAnyway I just thought that digital pots can be paralleled , stacked or cascaded to give better resolution.https://www.analog.com/media/en/technical-documentation/application-notes/AN-582.pdfYou should be able to achieve 1ohm increments.Or as a voltage dividerhttps://www.onsemi.com/pub/Collateral/AND8420-D.PDFthis might work for the current control.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 13, 2020, 03:23:48 pm
content: Quote from: Jwillis on February 12, 2020, 10:31:12 pmThat's an interesting idea worth exploring, especially if you don't mind (1) not being able to go all the way to 0 and (2) getting lower resolution at lower resistance values (when the wiper resistance becomes more relevant). It seems to me that OP's remote control resistor is supposed to be used in rheostat (2-pin) mode, in which case the wiper resistance (typically 10-100 ohms) is the big limiting factor.

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2020, 10:45:42 pm
content: If the lack of low value digital potentiometers is an issue, an op-amp can be used to divide it by a fixed value.·Note this circuit probably needs some frequency compensation and will need a low offset voltage op-amp. Figuring out how it works and what resistor ratios to use, is an exercise for the reader.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 14, 2020, 02:38:23 am
content: How about a step motor operated ten turn pot

author: GerryR
author_rank: Frequent Contributor
timestamp: February 14, 2020, 09:31:07 pm
content: Quote from: Ground_Loop on February 14, 2020, 02:38:23 amGood idea. I had thought about mentioning a 10 (or 15 or 20) turn pot driven by a micro-stepper; you can get all the resolution you need, but I am unsure of the wattage available in those units. Most of what I have used were 2 watts.

author: Vgkid
author_rank: Super Contributor
timestamp: February 15, 2020, 02:16:15 am
content: What about making a precision dc load. Valhalla made some resistance calibrators that worked that way.