does increasing the voltage in a circuit increases the current or decreases it?

i have read that increasing the voltage will decrease the current as P = VI. this is assuming a constant power output in the circuit ,but i have also heard that increasing the voltage will increase the current pulled, say to a fan, thereby increasing the power. so are there two different scenarios, constant power, and non constant power in a circuit? if yes, can you give me an example of each of those. 
also, how is the circuit in our homes different from a simple circuit which follows ohms law, and increasing the voltage will increase the current, as V=IR

A resistor R is an example of a non-constant-power load, as I = V/R and P=V*I = V*V/R. When V increases, I and P increase. When V decreases, I and P decrease.

A switching power supply is an example of a constant-power load. The power Pin drawn by an efficient switching power supply depends upon Pout the power output by the supply, and not upon the input voltage Vin. Thus the input current Iin = Pin/Vin. When Vin increases, Pin remains the same and Iin drops. When Vin decreases, Pin remains the same and Iin increases.
Another constant-power load is the input of an automatic-tap-changing transformer. Control circuitry changes taps in the transformer to keep the transformer's output voltage constant as the transformer's input voltage changes. The transformer is efficient, so shows similar behavior to the behavior of a switching power supply.
Constant-power loads exhibit negative dynamic resistance (voltage reduction causes current to increase), which can cause instability and oscillation: A voltage reduction causes the load current to increase, causing more voltage reduction, causing more current increase, etc.

thanks, that cleared it up.

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In the case of a fan, you have three physical domains to consider.  1) The electrical domain.  Here power is voltage (V) times current (I).  2) The rotating mechanical domain.  Here power is rotational speed (radian/s) times torque (twisting force)  (N*m or Nm), and 3) The aerodynamic domain.  Here, power is pressure rise (N/m^2) times flow rate (m^3/s).  So in the fan, the relationship between voltage and power very much depends upon how the input voltage and current translate to output speed and torque of the motor.  And the output of the motor depends on how the input speed and torque of the fan blades translate to output air speed and pressure.In many simple motors (think permanent magnet DC), output torque is proportional to input current and output speed is proportional to input voltage.  Increasing speed without increasing torque will cause the input power to increase proportionally with voltage.  Increasing torque without increasing speed will cause input power to increase proportionally with torque.Then the question comes down to what is the speed/torque relationship of the fan blades?  With fans, this varies a lot with the design of the fan.  From https://fluidflowinfo.com/fan-performance-and-fan-laws/, we see that increasing fan speed will generally cause the air speed to increase proportionally.  But the pressure increases with the square of the speed so the power increases with the cube of the speed.In this idealized case, since the speed increases directly with voltage, theoretically the power will increase with the cube of the voltage, thus the current increases with the square of the voltage.  In a real fan, there are of course losses electrically and magnetically so these relationships will be altered and depending on the design of the fan the aerodynamic load curves may be different. 
Thus, the fan is a very non-constant power circuit.  With the fan, the current goes up with the square of the voltage and the power with the cube of the voltage.  With a resistor, the current only goes up proportionally with voltage (I=V/R) and power only with the square of voltage P=V^2/R).As to the question of the circuits in our homes, it all depends on the types of loads connected.  Electric heaters (irons, space heaters, ovens, toasters, etc) are simple resistive loads so V = IR.  But a fan is a fan, as detailed above.  And, we must remember that our homes are powered by AC and in many AC motors  frequency plays a big roll in determining speed (and thus load).  So to speed up a fan with an induction motor, you must not only increase voltage but also frequency.  However, the power supply for our homes is of constant voltage and frequency.  Thus, only current varies as the load changes (ignoring voltage sag with large currents or undersized wiring, or brown outs).   So converters of various types and complexity are needed to change voltage and frequency.  

that was really helpful, thanks.