An Introduction to Current Sources

Most of us understand voltage sources. Whether you plug only one device or several devices into your wall outlets, the voltage stays the same. When you put two or three batteries in series, you double or triple the voltage.

Measuring voltage is easy. If you have a voltmeter, set it to the proper scale and put the two probes into the wall outlet (carefully, of course) or across the battery.

Current, however, is less familiar. Even some technical people don’t really understand current or know how to measure it. Even fewer understand current sources or even know they exist. In this article, let’s quickly review the basics of current before jumping into current sources.

Voltage vs Current

Diving in, let’s look at Figure 1, which compares electric current to water flow.

Voltage is the pressure generated by the pump, and current is the flow. Adding a resistance (restriction) reduces the current (flow). If the circuit is broken (pipe is blocked), there still will be voltage (pressure) but no current (flow).

To measure water flow, a flowmeter is added in series in the pipe. Likewise, to measure current, an ammeter is added in series, as shown in Figure 2.

The circuit must typically be broken (a wire cut or a connection opened) to add the meter. This is different from voltage, which can be measured by just touching the meter probes between the two measurement points.

As an aside, there are clamp-on current meters that wrap around a wire and measure current indirectly by measuring the wire’s magnetic field. These do not require breaking the circuit. These are analogous to clamp-on flowmeters that use an ultrasonic beam to measure water flow.

Current Source vs Voltage Source

Next, take a look at Figure 3, which compares voltage and current sources.

Adding loads in parallel to a voltage source increases the current but does not change the voltage unless the total current is more than the source (generator or battery) can supply.

In the example of a current source, shown in Figure 3b, the loads are added in series. If additional light-emitting diodes (LEDs) are added, the resistance changes. If the motor is removed (replaced with a short circuit), the voltage will change, but not the current. The current will stay constant unless the total load voltage exceeds what the current source can supply.

Current Source Applications

Current source circuitry is widely used in industrial control systems. Current, rather than voltage, is used to transmit analog measurements over long distances. Current transmission has advantages over-voltage signals. The current is not affected by the added resistance of long wiring. Also, current signals are less apt to be affected by electrical noise or electromagnetic interference (EMI).

Industrial Two-wire Transmission

In industrial applications, an elevated-zero signal with a current range of 4 to 20 mA DC is typically used. The elevated-zero signal means that 4 mA represents the low (typically zero) end of the range. The high, full-scale end of the current range is 20 mA. For example, 4 to 20 mA might represent 0 to 250 degrees from an angle position sensor. Figure 4 shows a typical 4–20 mA industrial current communication system.

Another advantage of current sources is that, in some systems, power can be sent over the same two wires as the current signal. Industrial two-wire transmitters control the current in the loop to be proportional to the measurement and “steal” their operating power from the loop current.

Current Source and LED Drivers

LEDs are often driven by current sources. As mentioned above, one, two, or several LEDs can be wired in series, and the current will not change. The current sources usually are specialized LED driver ICs, not simple circuits you can design yourself. Often, they use pulse width modulation (PWM) to maintain efficient operation with changing loads and supply voltages. Many are available, with varying features for different applications.

To learn a bit more, here are links to a simple IC from Diodes Inc. and one from onsemi.

Currents Sources in Integrated Circuits

Current sources are also used in the internal circuits of ICs, especially analog circuits. The current sources are also referred to as current mirrors. Popular current mirror designs include the Widlar current source, named after the legendary analog designer Bob Widlar.

How to Design a Current Source?

Take a look at Figure 5, which illustrates a simple, one-transistor current source circuit.