Shure Tech Tip: A Small Slice of Ohm's Law
The sound system designer required the power consumed (in watts) from each Shure product in the design. This information was required to satisfy the local electrical inspector that the sound system would not overload the electrical circuits in the vintage building. In most cases, the watts rating can be found on the Shure User Guide for the product. If it is not listed, it can be calculated; all that is required is simple multiplication. First, here is a lesson in basic electronics using water as the analogy:
Current, measured in amperes, is the amount of water flow.
Voltage, measure in volts, is the water pressure.
Power, measured in watts, is the amount of work the water can do.
Water power is "water pressure" multiplied by "water flow."
Electrical power is "voltage" multiplied by "current."
Example 1: A kitchen toaster needs 120 Vac (volts alternating current) to operate and consumes 10 amperes. 120 multiplied by 10 = 1,200 watts.
Example 2: The Shure UA844SWB needs 18 Vdc (volts direct current) to operate and consumes 3 amperes. 18 multiplied by 3 = 54 watts.
Example 3: The Shure ULX4 needs 15 Vdc to operate and consumes 0.550 ampere (550 milliamps.) 15 multiplied by 0.550 = 8.25 watts.
The math also works in this way:
Example 4: The SCM800 is rated at 24 watts when connected to a 120 Vac source. 24 divided by 120 = 0.2 ampere or 200 milliamps.
If the voltage and amperes are known, the wattage can be calculated. If the wattage and voltage is known, the amperes can be calculated.