41099 Chap.1 Circuit fundamentals: voltage, current and Ohm's law
Circuit fundamentals: voltage, current and Ohm's law
What this chapter covers
- 01
Charge, current, voltage, resistance and power as one connected set
- 02
Where each quantity is measured, and why the meter goes in a different place each time
- 03
Ohm's law written in the direction the question leaves open
- 04
Sizing a component against a supply, then living with the stock value
- 05
Power dissipation and the current a single output pin can honestly supply
Size a limiting resistor and recompute the real current
- 2Subtract the component's own drop from the supply to find what the resistor must absorb.
- 2Divide that voltage by the design current to get the required resistance.
- 1Round to a stock value in the safe direction and recompute the resulting current.
Key terms
- Current
- Charge passing a point in a circuit per second, measured in amperes.
- Voltage
- The energy difference per unit charge between two points, always measured across something.
- Resistance
- How strongly a component opposes current for a given voltage across it, measured in ohms.
- Electrical power
- Energy converted per second in a component, equal to the voltage across it times the current through it.
- Tolerance band
- The marking on a resistor stating how far the real value may sit from the printed one.
- Standard value
- A resistance from the commercially stocked series, which is what a design must be rounded to.
Circuit fundamentals: voltage, current and Ohm's law FAQ
Why does a component need a resistor beside it rather than a smaller supply voltage?
Lowering the supply is not available on this board, because a digital output can only produce its own supply voltage or ground. The resistor absorbs whatever voltage the component does not, and the value you choose sets the current in the whole series loop. That makes the current a design decision instead of an accident of what the supply happened to deliver.
Which form of the power relation should be used when two parts share a current?
Use the form with resistance and current squared. Two components in series carry the same current, so heating is proportional to resistance alone and the larger resistor gets hotter. Reaching for the voltage squared form inverts that answer, because parts in series do not share a voltage. Decide what is held common first, then pick the expression whose other variable is that shared quantity.
What happens if three indicators are connected to one output pin?
Their currents add at that pin, because the pin is a node. Three at roughly 13.6 milliamps each total about 41 milliamps, which already exceeds the guidance figure of 40 milliamps quoted in the course materials. Spreading them across three separate pins keeps each one well inside the limit and costs nothing except two more wires.
How much does a five percent resistor tolerance actually change a design?
For a protective resistor, very little. A nominal 220 ohm part may genuinely measure anywhere from 209 to 231 ohms, which shifts a 12.7 milliamp design by well under a milliamp. For a sensor divider it matters far more, because the same spread moves a threshold. Measure the part with a meter, disconnected, whenever the value is doing something precise.
Assessment move
Work each chapter question in the order supply, subtraction, division, then stock value, and carry units on every line. Volts divided by milliamps lands in kilohms and volts divided by amps lands in ohms, so the arithmetic checks itself. Before wiring anything, add the currents at each output pin and compare the total against the guidance figure.
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