Electronics > Beginners

Voltage controlled resistor options?

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IDEngineer:

--- Quote from: Caliaxy on February 11, 2020, 06:47:19 pm ---Maybe you can find a voltage controlled PS, or find a way to control your PS with a voltage rather than a resistor?
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Since 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.

rwgast_lowlevellogicdesin:
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.

IDEngineer:

--- Quote from: rwgast_lowlevellogicdesin on February 11, 2020, 08:49:30 pm ---Wow I actually never even thought about injecting voltages at the taps.... 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.
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OK, 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....

Jwillis:
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.pdf

Anyway  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.pdf You should be able to achieve 1ohm increments. 
Or as  a  voltage divider https://www.onsemi.com/pub/Collateral/AND8420-D.PDF this might work for the current control.

Caliaxy:

--- Quote from: Jwillis on February 12, 2020, 10:31:12 pm ---
Anyway  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.pdf You should be able to achieve 1ohm increments. 


--- End quote ---

That'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.

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