How to Calculate DC Currents

While not commonly required, it's sometimes necessary to know how much voltage or power is being pulled through a device when working with electronics. This specialized knowledge is usually reserved for circuit designers, engineers and electricians. But the tinkerer and do-it-yourself builder can run across this need to know. For instance, if building a custom speaker system for a window-shaking, floor-thumping stereo, power output can be calculated before any connections are soldered.

Difficulty: Moderately Easy

Instructions

1. Determine the voltage and resistance of the circuit. Calculation of current is basic electronics theory. These two variables must be determined before current can be calculated.

2. Convert all variables to the proper unit designations. Volts and Ohms must be converted to like units. Most DC circuits use small voltages; a car electrical system is 12 volts. Resistors, however, can range from single digits to mega-ohms (millions of ohms). A 1K resistor would be 1000 ohms. This is the number to use in the calculation of current.

3. Calculate current (amps) by dividing volts by resistance. Ohm's Law states any element of a DC electrical circuit can be determined, so long as the other two variables are known.

4. Expect the amperage to be small in a DC circuit. For instance, a 9-volt input across 1K of resistance pulls .009 amps (9 milliamps) of current.

5. Check your answer. Multiply the calculated amps by the measured volts. The answer should be the known resistance of the circuit.

Tips & Warnings

• Power (wattage) can also be determined through slightly more complex calculations. Use components and power sources with known values to ease the determination of current usage.
• Most electronic circuits are terribly complex. It really doesn't take much--a few resistors in a series and parallel--to create a current calculation equation to boggle the mind.

References

• Basic Electronics DC Circuits
• Ohm's Law with a Twist