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- November 7, 2023
- PCB knowledge
- (0)
- 05 mins
Current Limiting Resistors – Principles, Functionality and More
Current limiting resistors are simple yet important circuit components that restrict current flow to safe levels in electronic systems.
This guide provides a comprehensive overview of what current limiting resistors are, how they work, why we use them, how to select appropriate values and examples of their key applications.
What is a Current Limiting Resistor?
A current limiting resistor is a passive component placed in a circuit to intentionally limit and control the amount of current able to flow through a portion of the circuit. By inserting a resistor with calculated resistance, the current can be constrained to safe magnitudes that prevent damage to components.
Resistors inherently impede current flow to some degree as described by Ohm’s law. But dedicated current limiting resistors are deliberately sized to set a ceiling on the current, protecting sensitive electronic elements. Values are chosen based on the power supply voltage and load properties.
When a power source is first connected or a fault occurs, the current-limiting resistor cushions the inrush or spike of current by dropping some voltage. They also prevent excess current to components like LEDs during normal operation.
What is the Purpose of the Current Limiting Resistor?
Current limiting resistors have two primary purposes in circuits:
1. Protecting Components from Excess Current
Many electronic components are vulnerable to damage or destruction from current spikes or sustained overcurrent. Resistors used for current limiting insert impedance that keeps the current below damaging thresholds.
2. Safety and Reliability
Unconstrained inrush current poses safety risks like overheating and fire. Current limits also improve reliability by preventing intermittent failures related to temporary current spikes.
Current limiting resistors add a level of insurance and redundancy. For a modest cost, they greatly improve robustness. Components protected include LEDs, integrated circuits, motors, speakers, sensors, and more.
How Does a Current Limiting Resistor Work?
The current limiting function relies on a straightforward principle – using Ohm’s Law to calculate the required resistance to reduce current to a desired level, and installing a resistor with that value.
For example, consider a circuit with a 9V battery and an LED rated for 20mA maximum current. Using Ohm’s Law, the resistor value needed is:
R = V / I
R = (9V – 1.8V) / 20mA
R = 7.2V / 20mA
R = 360Ω
Inserting a 360Ω resistor between the 9V source and LED limits the current through the LED to the safe 20mA, preventing damage while allowing normal operation.
Higher resistances more severely limit current. Lower resistances allow more current to flow. Selecting the ideal tradeoff between current reduction and circuit voltage drop is key.
What is an Example of a Current Limiting Resistor?
A classic example of utilizing a current limiting resistor is powering a light emitting diode (LED).
LEDs require stable, restricted currents to operate efficiently. But their low forward voltage would allow excessive current to flow if directly connected to a higher voltage source.
Inserting an appropriate resistor limits the LED drive current to safe levels per the diode specifications, maximizing brightness while preventing burnout.
For instance, driving a typical red LED with 2V forward voltage from a 5V source would require a resistor value of:
R = (Vsource – VLED) / ILED
R = (5V – 2V) / 20mA
R = 3V / 20mA
R = 150Ω
So a 150Ω resistor between the source and LED would limit the current to 20mA, protecting the LED.
Why Do LEDs Need Current Limiting Resistors?
LEDs require current limiting resistors specifically due to their nonlinear current-voltage relationship.
An LED’s forward voltage remains fairly constant across a wide range of currents. Minimal resistance within the LED means high currents can flow if the supply voltage exceeds the forward level.
This unrestricted current rapidly overheats and destroys the LED junction. Resistors restrict LED current to safe operating ranges, maximizing light output without allowing thermal runaway.
LEDs need these supplemental resistors to:
●Limit current to avoid thermal damage.
●Maintain safe forward voltage and power dissipation.
●Ensure consistent luminosity by stabilizing current.
Choosing the correct resistor value prevents LED failure while allowing the desired light output.
What is the Difference Between a Fuse and a Current Limiting Resistor?
While both protect circuits, fuses and current limiting resistors provide protection in different ways:
Fuses act as a hard limit – when current exceeds the fuse rating, they permanently open the circuit by melting internal connections. Fuses are one-use only.
Current limiting resistors continuously restrict current flow to safe levels without opening the circuit. They can endure extended overcurrent situations without failure.
Fuses are best for detecting faults and isolating failures. Current limiting resistors help prevent damage in the first place. Using both provides layered protection.
How Do You Choose a Current Limiting Resistor?
To select an appropriate current limiting resistor, follow these steps:
1. Identify the power source voltage and target device specifications.
2. Calculate the maximum safe operating current for the device based on ratings.
3. Use Ohm’s Law to determine the resistance needed to sufficiently drop voltage and limit current.
4. Select a standard resistor value that provides the required resistance, ensuring adequate power handling capabilities.
5. Verify performance through testing and monitoring. Optimize if needed.
Choosing conservative resistor values provides a safety margin against normal variations and intermittent spikes. Over-specification does not negatively impact functionality.
Does a Current Limiting Resistor Drop Voltage?
Yes, inserting a current limiting resistor in a circuit unavoidably drops some voltage across the resistor according to Ohm’s Law.
The voltage dropped equals the current flowing through the resistor multiplied by the resistance. Higher resistance values induce larger voltage drops.
This voltage drop is necessary to create the impedance that restricts current to the desired level. But if excessive, it can negatively impact circuit performance.
Carefully calculating resistor values to drop just enough voltage to achieve adequate current limiting preserves functionality of the powered components.
Conclusion
In conclusion, current limiting resistors provide inexpensive but invaluable protection and safety assurance in a wide range of electronic systems. Selecting appropriate resistor values requires understanding the principles of Ohm’s Law along with the source voltages and device operational parameters. With the right design and integration of these simple resistors, damaging overcurrent conditions can be sustainably prevented.
A current limiting resistor is a passive component placed in a circuit to intentionally limit and control the amount of current able to flow through a portion of the circuit. By inserting a resistor with calculated resistance, the current can be constrained to safe magnitudes that prevent damage to components.
1. Protecting Components from Excess Current
2. Safety and Reliability
LEDs need these supplemental resistors to:
●Limit current to avoid thermal damage.
●Maintain safe forward voltage and power dissipation.
●Ensure consistent luminosity by stabilizing current.
