A current source is a circuit element that delivers a constant flow of electric current regardless of what’s connected to it. While a battery (a voltage source) maintains a steady voltage and lets the current fluctuate depending on the load, a current source does the opposite: it holds the current steady and lets the voltage adjust as needed. This distinction is fundamental to how many electronic devices work, from LED lighting to the transistors inside every computer chip.
How a Current Source Works
The simplest way to understand a current source is to compare it with something more familiar. A standard wall outlet or battery is a voltage source. It pushes out a fixed voltage, and the amount of current that flows depends on what you plug in. A small lamp draws a little current; a space heater draws a lot. The voltage stays roughly the same either way.
A current source flips this relationship. It pushes out a fixed current, and the voltage across its terminals adjusts automatically to maintain that current. Connect a small resistance, and the source produces a low voltage. Connect a larger resistance, and the voltage rises to keep the same current flowing. The current itself doesn’t change.
Ideal vs. Practical Current Sources
In textbooks, an ideal current source has infinite internal resistance, arranged in parallel with the output. That infinite resistance means zero current “leaks” internally, so 100 percent of the source current reaches the load. No matter what load resistance you connect, the output current stays perfectly constant.
Real-world current sources can’t achieve infinite internal resistance. A practical current source always has some finite parallel resistance inside it. The closer that internal resistance is to the load resistance, the more the output current drops from its ideal value. A high-quality laboratory instrument might have an internal resistance of 100 megaohms, which is close enough to ideal for most purposes. A cheaper or simpler circuit might have an internal resistance of just 20 ohms, meaning its output current will sag noticeably as the load changes. The rule of thumb: the internal resistance of a current source should be much larger than the load resistance for stable current delivery.
Compliance Voltage
Every practical current source has a ceiling on how much voltage it can produce while maintaining constant current. This ceiling is called the compliance voltage. If the load resistance climbs high enough that the source would need to exceed its compliance voltage, the current starts to drop. Think of it as the source “running out of push.” A current source rated at 10 milliamps with a 50-volt compliance voltage, for example, can maintain that 10 mA into any load up to 5,000 ohms (since 10 mA × 5,000 Ω = 50 V). Go above that resistance and the source can no longer deliver the full current.
Current Sources vs. Voltage Sources
The differences between voltage and current sources come down to what stays constant and what varies:
- Voltage source: maintains constant voltage, current varies with load. Internal resistance is very low (ideally zero) and sits in series with the output.
- Current source: maintains constant current, voltage varies with load. Internal resistance is very high (ideally infinite) and sits in parallel with the output.
A voltage source with extremely low internal resistance keeps its voltage stable across a wide range of loads. A current source with extremely high internal resistance does the same for current. They’re conceptual mirrors of each other.
Independent and Dependent Sources
Current sources come in two broad categories. An independent current source outputs a fixed current that doesn’t depend on anything else in the circuit. It’s the kind described above: set it to 5 mA and it delivers 5 mA.
A dependent (or controlled) current source, by contrast, produces a current that’s controlled by a voltage or current somewhere else in the circuit. There are two types relevant here. A voltage-controlled current source adjusts its output current based on a voltage measured elsewhere. A current-controlled current source scales its output current based on another current in the circuit. These aren’t just theoretical constructs. A field-effect transistor naturally behaves like a voltage-controlled current source, and a bipolar junction transistor acts like a current-controlled current source. Nearly every amplifier circuit relies on this behavior.
How Current Sources Are Built
In practice, constant-current circuits are built from transistors. One of the most common building blocks is the current mirror, a pair of matched transistors where one sets a reference current and the other copies it to the output. Current mirrors appear everywhere inside integrated circuits. Operational amplifiers, for instance, use multiple current mirrors internally to bias their transistor stages and maintain stable performance.
For higher-power applications, a transistor combined with a feedback resistor can regulate current through a load. The resistor senses how much current is flowing, and the transistor adjusts itself to keep that current on target. This is the basic principle behind most LED driver circuits and battery chargers that need constant-current operation.
Why LEDs Need Current Sources
LEDs are one of the most common real-world applications of current sources, and the reason is straightforward. An LED’s brightness is directly proportional to the current flowing through it, not the voltage across it. On top of that, the relationship between voltage and current in an LED is exponential: a tiny change in voltage causes a large swing in current. If you drive an LED with a voltage source, small variations in supply voltage or temperature can cause dramatic brightness changes or even destroy the LED.
A current source solves this by regulating the current directly. The voltage across the LED settles to whatever forward voltage the LED needs, and the brightness stays consistent. This approach also makes dimming more predictable. Pulsing the current on and off rapidly (a technique called PWM dimming) lets you reduce perceived brightness without shifting the LED’s color, because the LED always sees the same current when it’s on.
Other Common Applications
Beyond LED driving, current sources appear in several areas you might not expect. Battery charging circuits for lithium-ion cells use constant-current mode during the bulk charging phase, because controlling the charge rate precisely prevents overheating and extends battery life. Electroplating systems use current sources to deposit metal coatings at a uniform rate. Sensor circuits often use small current sources to excite resistive sensors like thermistors or strain gauges, since a known constant current makes it easy to calculate the sensor’s resistance from the measured voltage.
Inside analog integrated circuits, current sources are everywhere. They set the operating points of amplifier stages, provide bias currents for differential pairs, and act as high-impedance loads that improve amplifier gain. A single operational amplifier chip might contain dozens of internal current sources, all implemented as transistor current mirrors.