Measure Amps With a Sensor

Measure Amps With a Resistor

Measure Amps With A Meter

Free DC Circuit Simulation

# Measure / Calculate Current and Power in an Electrical Circuit.

Below is diagram showing how to measure current using  a shunt resistor.

This is an example of how a 0.05 Ohm precision resistor can be used to measure / calculate AC or DC current and power produced from your bicycle pedal power generator.  This is an electrical diagram (see schematic below), however you don't need to be an electrical engineer to understand it.  One wire of the pedal power bicycle generator is connected to a light bulb in this case.

The other wire of the pedal power bicycle generator passes through the precision resistor and connects to the light bulb.    You will then have to measure two things at the SAME time to get your power calculation. So you might guess that you would need two voltage meters ,  or two data acquisition LabVIEW channels to your computer.

The first thing you measure from you pedal power bicycle generator is the voltage coming off the generator.  The second thing is the voltage across the leads of the precision 0.05 Ohm resistor.   The current running through the resistor and the light build is calculated by using a form of Ohm's Law. V=IR or voltage = current x resistance.  You can express the formula another way to solve for current which is I = V/R  or current = voltage divided by resistance. In other words  you divide the voltage from the resistor by 0.05.   Then you use the power formula P=IV or power = current x voltage.   (See the formula above)

WARNING - your precision power resistor can get very hot if you are running a lot of voltage and current through it.  Before choosing a precision resistor you must calculate how much power it will be dissipating. Then choose a resistor and heat sink that has the proper wattage rating.  The Caddock part above has a large heat sink attachment area so you can bolt it to a piece of metal to dissipate the heat generated from it.

WHAT IF YOU DON'T HAVE A PRECISION RESISTOR??

If you do not have a precision resistor then you can use just a calibrated section of wire with about one or two feet length.  The following table shows resistance per foot of wire by wire size.  Typically the wire in your house wall receptacles is 12 AWG.  So if for example you are trying to measure current from supplied by a 24V power source, and you had a volt meter that measured down to a 0.001 Volt resolution, you could hook install a 2 foot section of 12 gauge AWG wire into your supply line, and put your voltage meter probes on either end of the 2 foot section of wire.  You would then take your reading and divide it by (2 x .00187 ohms) to get your current.

### AWG Wire Gauge Resistance per foot from  (http://www.radiolocman.com/electrical-engineering/circuit-cache.html?di=18899)

```4   .000292

6   .000465

8   .000739

10  .00118

12  .00187

14  .00297

16  .00473

18  .00751

20  .0119

22  .0190

24  .0302

26  .0480

28  .0764

```

You can also use another approach to measuring current.  Instead of a precision resistor you can use a hall effect transducer which can measure DC or AC current.   Click HERE to read about that. Other Topics