Instructables

Step 12: Reading amps. (current draw)

Although often used interchangeably in this Instructable, and wrongly so, "current" and "voltage" are actually different from one another. Voltage concerns the pressure at which electrons flow, like water pressure in a pipe.  Current (amps. or amperes) deals with the volume of electrons flowing at the operating voltage. A device may appear to work, but makes an unusual noise or quickly overheats. A check of the amps. drawn by the circuit can tell you if there is a problem, even though it will not identify the exact problem.  

First, check the device specifications.  Look for a plate or label on the back or bottom of the device. It may tell you the device is designed to draw (for example) 2.3 amps. at 120 volts. Or, it may tell you the device uses 276 Watts at 120 volts. (Watts equals volts multiplied by amps., so divide Watts by volts to determine the proper amps.) If this device were found to draw (for example) 4.5 amps., you would know immediately something is wrong.

Reading amps. is different from reading voltages. Voltage readings are the drop in electrical "pressure" across two points in a circuit, or a whole circuit. The meter is not part of the circuit, but reads what happens across or between two points in the circuit.  When reading amps, the meter must become a link in the circuit, just like a link in a chain. See the graphic showing how you can make a sandwich with two conductors and a piece of plastic between them. This sandwich can be placed between two batteries in your device to see what the current draw is. The alternative is to break the circuit by cutting a conductor and connecting the meter to the ends of the cut conductor. You would need to reconnect the cut connector when you are finished. Set the selector for DC amps in the desired range.

You may need to change the holes into which the probes connect on the meter. See the second photo.
 
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