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I have a variable resistance, 1K to 1M. As it varies due to external factors, I want to be able to pick a X value in between 1K and 1M so that once it reaches X even if the resistance increases the resistance is capped to X.

For example. I want to limit the upper bound to 1.7K. As the variable resistance crosses 1.7K I want the resistance the circuit sees to stay at 1.7 instead of keep increasing.

What kind of circuit can I resort to in order to achieve that behavior?

Requirements:

  1. When variable resistance value is below upper bound X the value seen should match the value of the variable resistance.
  2. The resistance is probed keeping voltage constant
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  • \$\begingroup\$ I thought about that, but, wouldn't that make it so when the variable resistance is below 1.7K, let say 1.2K, the circuit would not see 1.2K but 703.45 ? I am looking for a way to limit the upper bound while keeping the behavior below the upper bound the same.... \$\endgroup\$ – stoic Feb 26 at 17:40
  • \$\begingroup\$ Makes sense. So I am looking for a constant voltage scenario as far as the probing goes... \$\endgroup\$ – stoic Feb 26 at 17:54
  • \$\begingroup\$ Use two pots in parallel. I'd put course pot in series with 1k resistor. Other becomes a fine. \$\endgroup\$ – StainlessSteelRat Feb 26 at 18:45
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Look at it this way: if the resistance of your sensor increases above 1.7 kohms, you want your complete circuit to still load the probing voltage source with the same current as if it were a 1.7 kohm resistor.

So essentially you want a current source that only turns on if insufficient current is flowing through the variable resistor:

schematic

simulate this circuit – Schematic created using CircuitLab

Choosing the correct op-amp, transistor, and reference voltage depend on knowing the stimulus voltage, which you haven't shared. Also on how quickly the variable resistor value might be changing.

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