Electronics > Beginners
Voltage controlled resistor options?
rwgast_lowlevellogicdesin:
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!
GerryR:
Might these be of any help to you?
https://micronor.com/product/mr266/
0-10 V or 4-20 mA Control
IDEngineer:
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.
rwgast_lowlevellogicdesin:
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!
IDEngineer:
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.
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