Measuring power (in watts) in a low voltage (up to 50 volts) system.
The Watt meter is of simple design, consisting of 2 cheap multi-meters, the first connected up in series with the circuit to measure current, the second connected in parallel to measure voltage.
To use the watt-meter you connect the wires on one side (the left) of the meters to your power source and the other set of wires on the other side (right) to your circuit.
To find wattage you simply multiply the 2 number together, and voila the wattage!
The cost. If one can assume that the person building the Wattmeter is a hobbyist of some sort,
I would assume that he/she has a bunch of wire lying about, so the only cost would be in the meters.
You need 2! so that would be $11.98. BUT Harbor Freight always has these on offer at $3.99, sometime less. And once in a while, they give them away FREE. Yes you read that correctly FREE. In fact the total cost to me for the Instructable was ZERO. Don't you just love that number $0.00 !!!
Warning Dr Smith!!!!. Please read the warnings on the Instructable WRT plugging in the test probes, etc.
WRT = with respect to
Harbor Freight is a tool supply company in the USA.
Any meter would do, these where chosen as they are very inexpensive, and the shape makes them easy to glue together.
In the photo, a small 3 AA cell battery is on the left, the meters are in the middle, and a simple LED circuit on the right.
With the LED light, the Voltage is 5.89 and the current is 0.01 Amps, giving a power of 5.89 x 0.01 = 0.0589 Watts
This meter has better accuracy when used with higher currents, such as 0.5 Amps or above.
The wires in the photo are different to the ones shown in the instructable, this type of wire (solid core) was chosen so it could be used with breadboards.