Resistor: What is it & What Does it Do? (Examples Included)

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Key learnings:

Resistor Definition: A resistor is defined as a two-terminal passive electrical element that provides electrical resistance to current flow.
Primary Function: Resistors limit and regulate current flow in electrical and electronic circuits.
Measurement Unit: Resistance is measured in Ohms (Ω), which can be converted to milliohms, kiloohms, and megaohms.
Circuit Connections: Resistors can be connected in series or parallel, affecting the overall resistance differently.
Resistor Diagram: The circuit symbols for resistors include a zig-zag line (North America) and a small rectangle (Europe and Asia).

What is an Electrical Resistor?

A resistor is a two-terminal passive electrical element that inserts a set electrical resistance into a circuit. Resistance is the opposition to current. A larger ohm value lets less current flow for the same voltage. Catalogue types of resistors include the thermistor and many fixed parts.

Its job in a circuit is to oppose electric current and drop voltage. Circuit diagrams use conventional current. In metals the carriers are electrons, moving the other way.

A resistor is passive: it cannot generate electrical energy. Current through it dissipates power as heat (Joule heating, P = I²R).

Resistors limit current and create voltage drops. Stock values run from milliohms to many megohms.

For an ohmic resistor at a fixed temperature, ohm’s law is V = IR. Voltage across the part is proportional to current through it, and R is that ratio.

What Does a Resistor do?

Uses include current limiting, voltage division, signal-level setting and bias of active devices.

One common case is a single series resistor that limits current through a light-emitting diode (LED). More cases follow.

Protect Against Voltage Spikes

An RC snubber circuit places a resistor and capacitor in series, then that pair across a thyristor. The network limits the rate of voltage rise so the device is not forced on by a high $\frac{dv}{dt}$ .

The same series resistor also keeps LED current inside the datasheet limit. LEDs fail from excess current, not from voltage alone.

Provide Proper Voltage By Creating Voltage Drop

A series dropper or a divider can set the voltage that a load sees. A switch does not need a dropper to “work”; it is either open or closed.

What is Electrical Resistance Measured in (Resistor Units)?

The SI unit of resistance is the ohm (Ω), named for Georg Simon Ohm. A resistor is specified in ohms: the same unit as the property R.

One ohm is one volt per ampere:

$\begin{align*} 1\,\,Ohm = 1 \frac{Volt}{Ampere} \end{align*}$

Resistance is also written as V/A. The stored identity 1 Ω = 1 V/A is unchanged.

Preferred prefixes cover that span: milliohm (1 mΩ = 10^-3 Ω), kiloohm (1 kΩ = 10³ Ω) and megaohm (1 MΩ = 10⁶ Ω).

Electrical Resistor Circuit Symbol

Two resistor symbols are in use. The zigzag is IEEE/ANSI practice, common in North America.

IEC 60617 uses a rectangle. That rectangle is the usual symbol in much of Europe and Asia.

Both symbols are shown below.

Series and Parallel Resistors

Resistors in Series Formula

The circuit below shows n resistors in series.

For series-connected resistors, equivalent resistance is the sum of the individual resistances.

In equation form:

$\begin{align*} R_e_q_. = R_1 + R_2 + ........ + R_n \end{align*}$

$\begin{align*} R_e_q_. = \sum_{i=1}^{n} R_n \end{align*}$

The same current flows through every resistor in a series string.

Example

Three series resistors are 5 Ω, 10 Ω and 15 Ω. Find the equivalent resistance.

Solution:

Given Data: $R_1 = 5 \,\,\Omega, R_2 = 10 \,\,\Omega$ and $\,\,R_3 = 15 \,\,\Omega$

According to formula,

$\begin{align*} \begin{split} & R_e_q_. = R_1 + R_2 + ........ + R_n \\ & = 5 + 10 + 15 \\ & R_e_q_.= 30\,\,\Omega \end{split} \end{align*}$

The series equivalent is 30 Ω.

(The figure marks 25 Ω. That label is wrong. 5 + 10 + 15 = 30 Ω.)

Resistors in Parallel Formula

The circuit below shows n resistors in parallel.

For parallel-connected resistors, 1/Req equals the sum of the reciprocals of the branch resistances.

In equation form:

$\begin{align*} \frac{1}{R_e_q_.} = \frac{1}{R_1} + \frac{1}{R_2} + ........ + \frac{1}{R_n} \end{align*}$

$\begin{align*} R_e_q_. = \sum_{i=1}^{n} \frac{1}{R_n} \end{align*}$

The same voltage appears across every resistor in a parallel group. Voltage does not “flow.”

Example

Three parallel resistors are 10 Ω, 20 Ω and 30 Ω. Find the equivalent resistance.

Solution:

Given Data: $R_1 = 10 \,\,\Omega, R_2 = 20 \,\,\Omega$ and $\,\,R_3 = 30 \,\,\Omega$

According to the formula for parallel resistance,

$\begin{align*} \begin{split} & \frac{1}{R_e_q_.} = \frac{1}{R_1} + \frac{1}{R_2} + ........ + \frac{1}{R_n} \\ & = \frac{1}{10}+ \frac{1}{20} + \frac{1}{30} \\ & = \frac{6+3+2}{60} \\ & \frac{1}{R_e_q_.} = \frac{11}{60} \\ & R_e_q_. = \frac{60}{11} \\ & R_e_q_. = 5.455 \,\,\Omega \end{split} \end{align*}$

The parallel equivalent is 5.455 Ω (10∥20∥30 = 60/11 Ω).

Resistor Circuits (Example Applications)

LED Current Limiting Resistor

An LED needs its forward current held at or below the datasheet rating. A series resistor is the usual cheap way to do that.

Connect the limiter in series with the LED, as in the figure below.

led current limiting resistor — LED – Current Limiting Resistor Circuit

Calculate the Necessary Value of Current Limiting Resistor

You need three numbers for the resistor:

LED forward voltage (from datasheet)
LED maximum forward current (from datasheet)
V_S = supply voltage

Forward voltage VF is the drop when the LED is on, often about 1.8 V (red) to 3.3 V (blue/white) for small indicators. Continuous IF is often 20 mA for 5 mm parts; use the datasheet.

The resistor is then

$\begin{align*} R = \frac{V_S - V_F}{I_F} \end{align*}$

Where, $V_S$ = Supply Voltage

$V_F$ = Forward Voltage

$I_F$ = maximum forward current

Worked numbers:

Example

The figure uses a 3.8 V battery, VF = 3.1 V and IF = 30 mA. Find the series resistor. (30 mA is high for a small indicator; the arithmetic still uses 0.03 A as stored.)

Solution:

Given Data: $V_S = 3.8\,\,V$ , $V_F = 3.1\,\,V$ , $I_F = 30\,\,mA = 3*10^-^3\,\,A = 0.03 \,\,A$

Substitute into the equation:

$\begin{align*} \begin{split} & R = \frac{V_S - V_F}{I_F} \\ & = \frac{3.8 - 3.1}{0.03} \\ & = \frac{0.7}{0.03} \\ & R = 23.3 \,\,\Omega \end{split} \end{align*}$

R = (3.8 − 3.1) / 0.03 = 23.3 Ω. Pick the next standard value above that, and check wattage (I²R ≈ 21 mW here).

Pull-up Resistors

A pull-up resistor holds a logic node at a defined high when nothing else drives it.

With no other driver, the node sits high through the resistor. A pull-down does the same job to logic low, toward ground.

Open inputs on ICs, microcontrollers and digital logic gates float unless biased. A pull-up (or the chip’s internal pull-up) sets a known level.

A typical use is a switch or transistors that connect a node to ground, while the resistor ties that node to V_CC. The figure below is that arrangement.

pull up resistor circuit — Pull-up Resistor Circuit

With the switch closed, V_in at the gate is at ground. With the switch open, V_in is pulled to the high rail shown as V_in in the figure (the pull-up supply).

When the switch is open the resistor biases the pin. With no pull-up the pin floats (high impedance) and can read randomly.

4.7 kΩ is a common pull-up. The value is a trade-off between speed, power and noise; 1 kΩ to 100 kΩ is a usual band.

Voltage Drop Across a Resistor

The voltage drop across a resistor is the potential difference between its two ends. In DC circuits it is also called the IR drop.

Ohm’s law gives that drop: V = IR for an ohmic part.

In symbols:

$\begin{align*} V (Voltage \,\, Drop) = I * R \end{align*}$

Sign for IR Drops (Voltage Drops)

The sign of that drop follows the assumed current direction (passive sign convention).

Take current I through R from A toward B, as in the figure.

A is then at the higher potential. Walking A to B is a fall of IR (often written −IR in KVL). Walking B to A is a rise +IR.

Reverse the assumed current and the signed drop reverses.

Resistor Color Codes

Axial through-hole resistors often use coloured bands for value and tolerance. Surface-mount parts usually print digits instead.

A four-band part has three bands grouped, then a gap, then the tolerance band.

4 band resistor color code — 4 Band Resistor Color Code

From the left (away from the tolerance band): two digits, then the multiplier, then tolerance. Five-band codes add a third digit.

5 band resistor code — 5 Band Resistor Color Code

The table below is the usual 4-band colour map.