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Proportional valves are drived by current because we need a steady force and since resistance changes with temperature if they were driven by voltage this force wouldn't be steady.

Ok, this is what I read. What I don't understand is how can a device be driven only by current and not by voltage if voltage is the cause of current?

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    \$\begingroup\$ The answer is on the wiki third paragraph. There are several implementations on it as well. \$\endgroup\$ – Huisman Mar 6 at 8:51
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A voltage regulator allows the load to take the current it wants, while forcing the terminal voltage to remain constant.

A current regulator allows the load to determine the terminal voltage it wants, while forcing the current to remain constant.

Either can be implemented with a mainly voltage-output stage (like an emitter or source follower) or mainly current-output stage (like a collector or drain). The only difference is which of the output voltage or output current is sensed and fed-back to control the output stage.

To address your main point, voltage is not the 'cause of current'. At least, not in the sense that stops 'current is the cause of voltage' also being true at the same time. You set up the conditions, and current flows. While you can calculate numerically what's happening, arguing around the word 'cause' is going to confuse you.

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"Controlled by current" does not mean that no voltage is required (although that might be true in some specialized situations), it means that current is what determines the action of the valve. So you should adjust the voltage in order to maintain a certain current if you want the valve "position" to be relatively stable. That adjustment is done automatically by a "constant current" circuit.

In the case of an electromagnet as in such a valve (or a relay for that matter) the mechanical force is proportional to the current through the coil**. The voltage, for a given current, will change depending on the temperature of the coil, so a constant current circuit should not only start off with a voltage dependent on the coil temperature, but it should increase the voltage across the coil as the coil self-heats.

** There will also be some change with the temperature of the materials involved in the magnetic circuit, but usually that's relatively small.

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A voltage regulator will adjust the current while keeping the voltage constant.

A current regulator adjusts the voltage to keep the current constant.

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