Resistors

In one line

A resistor pushes back on current. How hard, in ohms (Ω), is set by Ohm's law: V = I × R — voltage equals current times resistance.

How it works

Resistors do two everyday jobs:

• Limit current so a downstream part isn't destroyed — e.g. the small resistor in series with an LED so it glows instead of burning out.
• Divide voltage. Two resistors in series split a voltage in proportion to their values (a voltage divider). This is the trick that turns a variable resistance — a temperature- or pressure-sensitive element — into a voltage a chip can read.

Where it shows up in digital health

• A thermistor (temperature-dependent resistor) in a patient thermometer or incubator sits in a voltage divider; the chip reads the divided voltage and converts it to °C.
• Pull-up / pull-down resistors hold a digital line at a known state so a button press or an alarm contact reads cleanly.
• Current-sense resistors measure how much current a motor or pump is drawing — a way to detect an occlusion (a blocked infusion line) from the electrical side.

Watch for

Real resistors have a tolerance (±1 %, ±5 %) and a power rating (how many watts they can dissipate before overheating). In a measurement front-end, resistor tolerance directly limits how accurate the reading can be.

Series, parallel, and reading the value

Combine resistors and the maths is simple but essential: in series they add (R₁ + R₂ + …) and split voltage in proportion; in parallel the total is smaller than either and they split current. The value is marked by colour bands (or printed digits on tiny surface-mount parts), and the tolerance band says how far the real value may stray — which, in a measurement divider, sets the floor on how accurate the reading can be.

Key takeaways

• A resistor opposes current; Ohm's law (V = I × R) sets how much.
• Its two everyday jobs: limit current and divide voltage (the voltage divider).
• A voltage divider is how a variable-resistance sensor (thermistor) becomes a readable voltage.
• Tolerance and power rating matter — tolerance caps measurement accuracy.