If the voltage in a circuit is halved, what will happen to the circuit current?
If the voltage in a circuit is halved, what will happen to the circuit current?
It depends entirely upon the circuit. In circuit consisting of nothing but linear resistances with no significant temperature caused resistance change, half the voltage will result in half of the current. That is what Ohm’s Law is based upon, linear resistances.
If it is a resistive circuit but there is a temperature induced change, the current may drop to something more than half. Most heating elements and all incandescent light bulbs have a positive temperature coefficient. In other words, resistance rises with rising temperature.
So at half the voltage, the resistive element won’t heat up as much, so the resistance will be lower. The current will still be less than it would be at full voltage, but more than half. Toasters, ovens, soldering irons, electric water heaters, and electric dryers, for instance.
An LED with a simple resistor to limit current will drop to less than half the current. This is because the voltage drop across the LED remains relatively constant over a wide range of current, so the voltage drop across the resistor will be such that it has less than half of the original voltage dropped across it.
An LED with a simple linear constant current limiter will continue to draw the same current, as long as the new voltage is within the limits of control. If it drops below that point, the current may drop to zero or nearly zero.
If it is something powered by a switch mode power supply, the current will rise. This is because the device is still providing a constant voltage to the circuits it powers. So it must draw twice the current when the voltage is halved. This includes computers, chargers, LED lighting, etc.
For a simple DC motor, it is hard to say what the current will do. It will likely be less, but it depends. If the motor can no longer provide enough torque to turn against the load, it may stall and then the motor will draw its short circuit current. This may be more than the current draw at full voltage. If the load is light and based on the speed, for instance a propeller, the current draw will probably be less than half of the original current.
For an AC induction motor, current goes UP with drop in voltage. This is because the rotational speed is set by the frequency of the voltage, so it will draw more current with less voltage. This often why breakers pop after power comes back on after a blackout, because the lines get loaded down by all of the refrigerators and freezers coming back on line, that drops the line voltage, the motors draw more current, and your breaker opens up.
