Constant current source

MC7805

The 7805 (here in its MC7805 version) is a three-terminal linear voltage regulator from the well-known 78xx family. It delivers a fixed 5 V output at up to 1 A with adequate heat sinking. It usually comes in a TO-220 package.

Although its most common use is as a voltage regulator, the very same part works as a constant current source by simply adding a resistor, as the schematic shows.

How the circuit works

The key is the regulator’s fundamental property: it always keeps 5 V between its output pin (OUT) and its common pin (GND), regardless (within its limits) of what is connected.

If we place a resistor R between OUT and GND, the regulator itself ensures the voltage across it is exactly 5 V. By Ohm’s law, the current through it is set by that voltage and that resistance, and is practically independent of the load: no matter what we connect in series at the output, the regulator adjusts its output voltage to keep those 5 V across R and, with them, the current.

That current flows out of the GND pin toward the load. The input capacitor C1 improves stability and filters supply noise, just as in a conventional regulator.

Application design

The output current follows directly from the reference voltage (the 5 V output) divided by the resistor, plus the regulator’s own small quiescent current:

$$I_{out} = \frac{V_{out}}{R} + I_{Q} = \frac{5\ \text{V}}{R} + I_{Q}$$

For example, with $R = 100\ \Omega$ we get about $50\ \text{mA}$. The 7805’s quiescent current $I_{Q}$ is in the range of 4 to 8 mA and adds to the output current, so this circuit is simple and robust but less accurate than the LM317 equivalent (whose reference is only 1.25 V and whose quiescent current barely matters).

When choosing the resistor, keep in mind the power it dissipates, since it takes the full 5 V:

$$P_{R} = \frac{V_{out}^{2}}{R} = \frac{25}{R}$$

With $R = 100\ \Omega$ that is $0.25\ \text{W}$, so a resistor rated for at least $0.5\ \text{W}$ is advisable. The input voltage must also be high enough: as a rule of thumb, $V_{in}$ has to exceed the voltage dropped across the load plus the 5 V reference plus the regulator’s dropout voltage (typically around 2 V).

Other applications

Beyond the constant-current source explained here, the 7805 shows up in many circuits: fixed 5 V supplies, local on-board regulation or adjustable regulators. Some of the most common:

• 5 V supply for logic and microcontrollers. Its classic use: delivering a stable 5 V to digital circuits from a larger unregulated source.
• Local on-board regulation. A regulator next to each board or block of the system, instead of a single central supply, to cut down noise and wiring drops.
• Adjustable output regulator. By lifting its common pin with a divider or a diode, the output rises above the nominal 5 V.
• Basic bench power supply. With a transformer and a bridge rectifier, it provides a ready-to-use 5 V output for the workbench.