Practically differentiating AC current from DC current
Theoretically AC current is one which when passed through a conductor the polarity of the current at the two ends of the conductor keeps changing with a fixed frequency i.e. at a time if current is flowing in one direction then after sometime(a fraction of second) it will be flowing in the opposite direction and so on. While in case of an DC current polarity of the current at the two ends of the conductor remains same throughout the time i.e. current flows only in a single direction(from positive terminal to negative terminal).We can use this property, of changing the conduction direction, for detecting whether the current flowing in a conductor is AC or DC. There are many many ways of doing this in which some easiest ones are listed below which can be done very easily and also can be understood how they work. These methods require the instrument which are easily available in market , very cheap, one can use them easily with basic electronic knowledge:
Before going for the testing you must set the multimeter is appropriate rage for example if the input supply(to be tested) is estimated to about 80- 100 V you should set it in 200 V(AC) range, if it is estimated to be around 200- 250V you should set it in 1000V(AC) range, remember you always have to keep multimeter in AC voltage ranges and not in current mode otherwise multimeter may be damaged.
1) The easiest method is to use an diode because it is an unidirectional device. For this you need to have a multimeter, be it digital or analogue and a diode 1N4008 because its reverse break down voltage is 1000 V. Now connect one terminal of diode with one end of input supply and other one with the meter(set in appropriate range). Now connect other end of input supply with remaining terminal of meter and see whether meter is showing any voltage or not. Now reverse the terminals of diode and see whether meter is showing any voltage or not. If meter shows voltages in both the cases then it is AC current in input supply otherwise if it shows voltage in only one case(first or second) then it is DC current.
2) This method requires no diode but applicable manly for small voltages. Here to must have a analogue multimeter and all you need to do is that set the meter in 50V(DC) range and connect it directly to the input supply. if the needle of meter remains still at a level (further or behind the 0 reference level) then it is DC and if at keeps fluctuating or vibrating near 0 reference level then it is AC.
3) This method requires a speaker of about 25 ohm, and a variable resistor which is available in market and known as vilume. Take a volume of resistance around 10-100 ohm. Connect the terminals of input supply to the two extreme ends of volume and also connect the first and last terminal of volume with speaker. Rotate the volume knob if any beep sound is heard then it is AC but if only a single click sound is heard then it is DC. There are also many other ways for differentiating AC from DC current but the methods listed above are simplest ones
