APPENDIX A ■ APPENDIX A
resistance in ohms = voltage in volts / current in amps
o r...
voltage in volts = resistance in ohms × current in amps
All three formulas are identical. They’ve only been moved around algebraically.Ohm’s Law is Helpful in Selecting a Current-Limiting Resistor
Here’s a practical example. According to some datasheet, the maximum current flow that some component
can accept is 0.25 amps. You’ve got a 12-volt battery. What size resistor should you select to limit current flow
to 0.25 amps or less?
According to the Ohm’s law, if you know two parts, you can calculate the third. You know 0.25 amps and
you know 12 volts.
resistance in ohms = 12 volts / 0.25 amps
48 W = 12 V / 0.25 A
A 48-ohm resistor will never allow more than 0.25 amps to flow through it when you’re using a 12-volt
battery.
Help in Determining Current from a Voltage
You’re worried about the robot’s motors stalling and burning out. Based on the motor datasheet, the motor’s
stall current is 0.5 amps, but you’ve decided to have the robot shut off motor power if it reaches 0.4 amps,
just to be safe. Unfortunately, almost all of the chips and microcontrollers only seem to be able to read
voltage, not current.
Ohm’s law guarantees that if you know the voltage and the resistance, then you can calculate the
current. The solution is to put a known value of resistor in the same path as your motor and have the robot
read the resistor’s voltage to calculate the current in that path. Installing a low-value power resistor, say 0.1 W,
should have little effect on the motor’s performance.
voltage in volts = 0.1 ohms × 0.4 amps
0.04 V = 0.1 W × 0.4 A
When the chip detects 0.04 volts or more, then 0.4 amps or more are pouring through the motor. This
must be true, since the resistor is in the same path as the motor, so any electricity passing through the motor
must pass through the resistor.
Remember, this is magical, unbeatable Ohm’s law. There’s absolutely no way to get more than 0.4 A
through a 0.1 W resistor unless the voltage dropped across the resistor is 0.04 V or more.
Multimeter Voltage to Current Trick
Ohm’s law is helpful for measuring current on a soldered circuit board. You can’t measure current directly
with the multimeter unless you desolder one end of a wire somewhere to connect it through the meter in
amp mode. But, if you can find a resistor with a value printed on it, then you can calculate the current after
measuring the voltage across the resistor.