APPENDIX A ■ APPENDIX ALet’s say you find a 220 W resistor. You measure the voltage by touching the multimeter probes to each
end of the resistor and it reads 3.52 volts.
current in amps = 3.52 volts / 220 ohms
0.016 A = 3.52 V / 220 W
In case you’ve forgotten, you can multiply amps by 1000 to get milliamps.0.016 A × 1000 = 16 mA
16 mA must be passing through the path of the resistor, regardless of the stuff in the circuit before the
resistor or after the resistor. Why? Mr. Ohm (Georg S. Ohm) guarantees it or double your money back!
The Key Point of Ohm’s Law
Anytime you say to yourself, “I sure wish I knew A, but I can only measure B and C”, think of Ohm’s law. To
assist you, I’ve created an online tool (see Figure A-2) where you can enter two measurements and the site
calculates all of the other values. Visit: http://www.robotroom.com/Calculators/Ohms-Law/Ohms-Law-
Voltage-Current-Resistance-Calculator.aspx
Figure A-2. Online Ohm’s law calculator
The Larger Implication of Ohm’s Law
There is an undeniable reality to Ohm’s law: In a circuit, you can’t change just one of the three terms (either
voltage, current, or resistance). Any change in one causes at least one of the others to change to balance.
This also means you can purposely alter one term to force a change in another.