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# A circuit that can adjust a resistance and probing a voltage node

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#### henry kissinger

##### Member level 2
I am trying to design a circuit that has function as below.
There is a current source of 1A.
This circuit is able to adjust resistor R and probe VA at the same time, until VA reaches Vx given.
The goal is get the resistance R.
Any idea is appreciated about how to implement this circuit function.

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Hi,

I don't understand of what you really want to achieve.
If you really want to "R" then it needs to be a big adjustable resistor with wiper

Resistor:
* size of a beer bottle
* maybe wire wound
* motor driven wiper

Alternative: build R as an array of resistors (series or parallel wired) that individually can be switched ON/OFF to get the desired value.

Klaus

Looks like a variant of a high-side current source using a Vref, PMOS, current-sensing resistor, and op amp to extrapolate a voltage as a resistance value. And anyway, I'm pretty sure that someone already told you how to do it in the first thread you posted on this topic...

People use a CD4017 as a simple DAC.

You don't say the range of your resistance, but you may need a resistor(s) the size of a beer keg, never mind a bottle. A 100 ohm resistor will dissipate 100 watts.

And I, too, have no idea what you're trying to accomplish here. You're setting some voltage Vx and then trying to find a resistor that will produce that voltage with 1 amp? Why?

I think they are asking for an adjustable constant current sink but it's a strange way to do it and is reliant on the 1A being from a constant current source itself.

Brian.

Suppose you put the 1A current source in a current mirror, namely the first column? Give it a supply of 1 or 2 volts (therefore at 1 or 2 Watts which is eay to work with)...

Then create a second column for your current mirror...
selecting values so that it derives, say, 1/10 Ampere...
to be applied to your resistor-under-test. It generates a proportional voltage reading. Less watts wasted.

A fourth current mirror for 1 mA. Scale your meter readings accordingly.

Sorry, but why are you opening a second thread within this forum to discuss the design of one circuitry?
There are even three closely related threads, applying a "variable resistor" in different configurations. You may think that the questions should be discussed separately due to the circuit differences. But the discussion is overlapping in several regards and the possible solutions are also quite similar. If you are able to distinguish the threads now, an Edaboard user reviewing in a month or year will be most likely confused.

That's why we have this forum rule
Select the proper section and don’t post duplicates
Please post your question in the proper forum section, this will help you get replies faster. If your question can fit in more than one category then select the section that seems more appropriate. If you're not sure where to post, ask a moderator first.
Duplicates or cross posting are not allowed, avoid creating multiple threads with similar questions, ask all related questions in one thread.

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