Do current sources have voltage? Yes, but with a critical caveat: the voltage across a current source is not generated by the source itself, but is instead forced into existence by the load it is driving. A current source is an active circuit element that forces a specific, constant current through a load, with the voltage across it determined entirely by the load's resistance rather than the source itself. What people most commonly confuse this with is a standard voltage source, where the voltage is fixed and the current fluctuates based on the load. In a real circuit, a current source changes the fundamental behavior of the loop by dynamically adjusting its output voltage to maintain a rigid current flow, regardless of whether the load resistance increases or decreases.
To visualize this, think of a voltage source like a municipal water tower providing fixed pressure, whereas a current source acts like a positive displacement pump providing a fixed flow rate, where pressure only builds when the water meets pipe resistance.
The Core Difference: Voltage Sources vs. Current Sources
When you connect a 12V battery (a voltage source) to a 6-ohm resistor, Ohm's Law dictates that 2 amps will flow. If you swap the resistor for a 12-ohm one, the voltage remains 12V, but the current drops to 1 amp. The source dictates the voltage; the load dictates the current.
A current source flips this relationship. If you have a 2-amp current source connected to a 6-ohm resistor, the source will automatically generate 12V to push exactly 2 amps through it. If you swap in a 12-ohm resistor, the source will instantly raise its output to 24V to maintain that exact 2-amp flow. According to Electronics Tutorials, an ideal current source has infinite internal resistance, meaning it will generate whatever voltage is necessary to maintain its set current.
| Characteristic | Ideal Voltage Source | Ideal Current Source |
|---|---|---|
| Fixed Output Variable | Voltage (V) | Current (I) |
| Dependent Variable | Current (determined by load) | Voltage (determined by load) |
| Internal Resistance | Zero ohms | Infinite ohms |
| Short Circuit Behavior | Infinite current (theoretical failure) | Zero volts, normal current flow |
| Open Circuit Behavior | Normal voltage, zero current | Infinite voltage (theoretical failure) |
Worked Example: Calculating Compliance Voltage in a Real Circuit
In the real world, infinite voltage is impossible. Practical current sources have a compliance voltage range, which is the minimum and maximum voltage the device can generate while maintaining its constant current regulation. Let us look at a real-world scenario using a Mean Well HLG-120H-C1400 LED driver, which is a dedicated constant current power supply.
Model: Mean Well HLG-120H-C1400
Rated Output Current: 1,400 mA (1.4 A)
Compliance Voltage Range: 54 VDC to 114 VDC
Load: 20x Cree XLamp XP-L High-Power White LEDs wired in series.
LED Forward Voltage (Vf): 3.2 V at 1.4 A
First, we calculate the total forward voltage required by the LED string. Since they are in series, we multiply the individual Vf by the number of LEDs: 20 LEDs x 3.2 V = 64 V. Because 64 V falls perfectly inside the driver's 54 V to 114 V compliance window, the driver will output exactly 64 V to push 1.4 A through the string. The total power consumed is 64 V x 1.4 A = 89.6 W.
Now, what happens if we add 10 feet of 18 AWG copper wire to connect the driver to the LEDs? The wire adds approximately 0.13 ohms of resistance. Using Ohm's Law (V = I x R), the voltage drop across the wire is 1.4 A x 0.13 ohms = 0.18 V. The current source instantly detects this added resistance and raises its total output voltage to 64.18 V to ensure the LEDs still receive exactly 1.4 A.
However, if you only wired 10 LEDs in series, the total Vf would be 32 V. Because 32 V is below the driver's 54 V minimum compliance limit, the driver cannot lower its voltage enough to maintain regulation. The internal switching circuitry will fail to stabilize, causing the LEDs to flicker or the driver to enter a protective 'hiccup mode' and shut down. This is a massive real-world gotcha for DIY lighting builders who assume a higher-wattage current source can simply 'step down' to any voltage.
Where You Meet Current Sources in Practice
While textbooks treat current sources as abstract mathematical models, you interact with practical current sources constantly in electrical and electronics work.
- High-Power LED Drivers: As demonstrated above, high-output LEDs require strict current regulation to prevent thermal runaway. Dedicated LED drivers operate exclusively as current sources within their compliance voltage windows.
- 4-20mA Industrial Control Loops: In industrial automation, sensors transmit data over long distances using a 4-20mA current loop. The sensor acts as a variable current sink/source. Because it is a current loop, the voltage drops across long wire runs do not degrade the signal, provided the loop power supply has enough compliance voltage to overcome the total wire and shunt resistance.
- Laboratory Bench Power Supplies: Most modern bench supplies feature a CC/CV (Constant Current / Constant Voltage) crossover. When you set a 5V limit and a 2A current limit, the supply acts as a voltage source until the load attempts to draw more than 2A. At that exact threshold, the supply seamlessly transitions into a current source, dropping its voltage to hold the current at exactly 2A.
- Battery Charging (CC Phase): Lithium-ion and LiFePO4 charge controllers utilize a Constant Current (CC) phase during bulk charging. The controller acts as a current source, steadily raising its voltage as the battery's state of charge increases, until it hits the absorption voltage limit.
Frequently Asked Questions
Does an ideal current source have infinite voltage?
In pure circuit theory, yes. If you connect an ideal current source to an open circuit (infinite resistance), Ohm's Law (V = I x R) dictates that the voltage must become infinite to push the fixed current through the infinite resistance. In reality, practical current sources are bound by their compliance voltage limit and the physical breakdown voltage of their internal components. When faced with an open circuit, a real current source will simply max out at its compliance voltage limit and stop regulating.
Can a current source have zero voltage across it?
Yes, but only under a specific condition: a dead short circuit. If you short the output terminals of a current source with a zero-ohm wire, the resistance is zero. Therefore, V = I x 0, resulting in 0 V across the terminals. An ideal current source would happily push its rated current through the short at zero volts. A practical current source will also output near-zero voltage, though it will still dissipate some internal heat due to non-ideal internal switching losses.
What happens if a current source is open-circuited?
If you disconnect the load from a practical current source while it is energized, the resistance of the air gap is effectively infinite. The source will rapidly ramp up its voltage to its maximum compliance limit attempting to force the current across the gap. If the compliance voltage is high enough, it will arc across the disconnected terminals. If not, the voltage will simply peg at the maximum limit, and the supply's Over-Voltage Protection (OVP) circuitry will typically trip, shutting the output down to protect the internal MOSFETs and capacitors.
How do I measure the voltage of a current source with a multimeter?
You cannot accurately measure the operating voltage of a current source by simply probing its open terminals. If you put your multimeter in voltage mode and touch the disconnected output leads, you are creating an open circuit, and the meter will simply read the supply's maximum open-circuit compliance voltage or OVP limit. To measure the actual working voltage, you must connect the load first, and then place your multimeter probes in parallel across the load or across the source's output terminals while the current is actively flowing through the closed circuit.
