No, LEDs are not ohmic devices; they are non-ohmic because their current-voltage (I-V) relationship is highly non-linear, meaning their internal resistance changes drastically with small changes in applied voltage. Unlike a standard wirewound resistor where doubling the voltage exactly doubles the current, an LED's current increases exponentially once it crosses its forward voltage threshold. This fundamental reality dictates everything about how we wire, drive, and dim solid-state lighting in real-world circuits.
The Physics of Non-Ohmic Behavior
To understand why an LED defies Ohm's Law ($V=IR$ with a constant $R$), you have to look at the PN junction inside the semiconductor die. An LED requires a specific amount of energy to push electrons across the depletion region. Below this threshold (the forward voltage, or $V_f$), almost zero current flows. Once the applied voltage overcomes this barrier, the junction conducts heavily.
This exponential behavior is described by the Shockley diode equation. Because the current spikes so violently with minor voltage increases, an LED cannot regulate its own current. If you connect an LED directly to a voltage source that perfectly matches its nominal $V_f$, minor fluctuations in the power supply—or minor drops in the LED's $V_f$ as it heats up—will cause the current to skyrocket and destroy the component.
| Feature | Ohmic Device (Carbon Resistor) | Non-Ohmic Device (LED) |
|---|---|---|
| I-V Relationship | Linear (Straight line on a graph) | Exponential (Hockey stick curve) |
| Resistance Profile | Fixed (Constant across voltages) | Variable (Drops as voltage rises) |
| Primary Control Method | Voltage determines current | Current must be forced/limited |
| Thermal Coefficient | Positive (PTC - resistance rises with heat) | Negative (NTC - resistance drops with heat) |
Worked Example: The Ohmic Assumption Trap
Let's look at what happens when a beginner treats an LED like an ohmic resistor. We will use a standard Kingbright WP7113SRD 5mm red LED. The datasheet specifies a typical forward voltage ($V_f$) of 2.0V at 20mA.
The Flawed Ohmic Calculation:
If the LED were ohmic, we could calculate its fixed internal resistance using Ohm's Law at its operating point:
$R = V / I = 2.0V / 0.020A = 100\Omega$.
If you believed the LED was a fixed $100\Omega$ resistor and connected it to a 5V Arduino GPIO pin, you would assume the current would be $I = 5V / 100\Omega = 50mA$. You might think this is fine for a brief test.
The Non-Ohmic Reality:
In reality, the LED does not have a $100\Omega$ resistance. At 5V, the PN junction is fully saturated. Without an external current limiter, the LED will attempt to draw hundreds of milliamps, instantly exceeding the 20mA maximum continuous forward current and the 50mA peak limit. The die will overheat, the bond wire will melt, and the LED will fail in a fraction of a second.
The Correct Circuit Design:
Because the LED is non-ohmic, we must use an ohmic component (a resistor) to absorb the excess voltage and stabilize the current.
- Supply Voltage ($V_s$): 5.0V
- LED Forward Voltage ($V_f$): 2.0V
- Target Current ($I_f$): 20mA (0.020A)
We calculate the required series resistor by subtracting the LED's voltage drop from the supply, then applying Ohm's Law to the resistor:
$R = (V_s - V_f) / I_f = (5.0V - 2.0V) / 0.020A = 150\Omega$.
With a 150Ω resistor in series, if the 5V supply sags to 4.8V, the current only drops to 18.6mA. If it spikes to 5.2V, the current rises to 21.3mA. The ohmic resistor acts as a shock absorber for the non-ohmic LED.
Where You Meet This in Practice
The non-ohmic nature of LEDs forces specific design choices in commercial and DIY lighting installations. Here is where this theory dictates your hardware choices on the jobsite or workbench.
12V LED Strip Architecture
If you cut open a standard 12V SMD 2835 LED strip, you will notice the copper cut-lines are spaced every three LEDs, accompanied by a tiny surface-mount resistor. Why three? A white LED has a $V_f$ of roughly 3.0V. Three in series equals 9.0V. The remaining 3.0V from the 12V supply is dropped across the resistor. This design relies on the non-ohmic behavior of the LEDs: because their voltage drop remains relatively clamped around 9.0V, the resistor safely limits the current even if your 12V power supply actually outputs 12.5V under light load.
Constant Current vs. Constant Voltage Drivers
For high-power COB (Chip on Board) LEDs used in floodlights or grow lights, series resistors are too inefficient. Because the LED's resistance drops as it gets hotter (a negative temperature coefficient), it will draw more current, get hotter, and draw even more current—a destructive loop called thermal runaway. According to the U.S. Department of Energy's SSL guidelines, high-power solid-state lighting requires Constant Current (CC) drivers. A CC driver actively monitors the circuit and adjusts its output voltage dynamically to maintain a strict current (e.g., exactly 700mA), entirely bypassing the LED's non-ohmic volatility.
PWM Dimming Over Voltage Reduction
If you want to dim an incandescent bulb (which is roughly ohmic at stable temperatures), you just lower the voltage. You cannot do this with an LED. Because of the exponential I-V curve, dropping the voltage by just 10% might cut the current—and therefore the light output—by 50%, while also shifting the color temperature. Instead, we use Pulse Width Modulation (PWM). We keep the voltage constant and rapidly switch the LED on and off at frequencies above 1kHz, relying on human persistence of vision to perceive a dimmer light while maintaining color accuracy.
Common Confusions: Static vs. Dynamic Resistance
The most frequent mistake hobbyists make is trying to measure an LED's resistance with a digital multimeter. If you set your meter to the Ohms ($\Omega$) range and touch the probes to an LED, you will likely get an "OL" (Over Limit) or infinite resistance reading. This is because the multimeter's resistance mode uses a very low test voltage (often under 0.5V) to protect sensitive components. This voltage is far below the LED's $V_f$ threshold, so the junction remains closed, and no current flows.
Even if you use the multimeter's Diode Test mode (which applies enough voltage to forward-bias the junction), the meter displays the forward voltage drop (e.g., "1.850"), not the resistance.
Engineers sometimes refer to an LED's dynamic resistance ($r_d$), which is the slope of the I-V curve at a specific operating point ($\Delta V / \Delta I$). For a typical indicator LED at 20mA, the dynamic resistance might be around 10Ω to 15Ω. However, this value is only useful for calculating high-frequency AC impedance or noise ripple in advanced power supply design; it is entirely useless for basic DC circuit sizing. Always treat the LED as a fixed voltage drop in DC calculations, not a resistor.
Frequently Asked Questions
Can I use Ohm's law to calculate LED resistor size?
Yes, but you apply Ohm's Law to the current-limiting resistor, not the LED itself. You must first subtract the LED's non-ohmic forward voltage drop ($V_f$) from your total supply voltage ($V_s$). The remaining voltage is what the resistor must drop. The formula is $R = (V_s - V_f) / I_{target}$. You cannot use Ohm's law to find the LED's internal resistance because it does not have a fixed one.
Why does my multimeter show infinite ohms when testing an LED?
Multimeters use a very low test voltage (usually under 0.5V) when measuring resistance to avoid damaging components. Because an LED is a non-ohmic diode that requires a specific forward voltage (typically 1.8V to 3.3V) to begin conducting, the multimeter's low test voltage cannot open the PN junction. The meter reads this lack of current flow as infinite resistance. Use the dedicated Diode Test mode (symbolized by a diode arrow) to check LED health instead.
Are incandescent bulbs ohmic or non-ohmic?
Incandescent bulbs are technically non-ohmic, but in a completely different way than LEDs. The tungsten filament has a positive temperature coefficient (PTC). When cold, its resistance is very low (causing a massive inrush current when you flip the switch). As it heats up to 2,500°C, its resistance increases by a factor of 10 to 15. However, once at a stable operating temperature, it behaves much more linearly than a semiconductor diode, which is why simple voltage-reduction dimmers work fine for them.
What happens if I wire LEDs in parallel without individual resistors?
Because LEDs are non-ohmic and have slight manufacturing variances, no two LEDs have the exact same forward voltage. If you wire them in parallel on a single shared voltage source, the LED with the lowest $V_f$ will hog the majority of the current. It will overheat, fail short or open, and then the remaining LEDs will be forced to absorb the excess current, leading to a cascading failure. Always use a dedicated current-limiting resistor for every parallel branch, or wire the LEDs in series.
