FAQ on voltage and current sources: part 3

Voltage and lesser-known current sources provide important IC, circuit, and system functions. This section continues the discussion of current sources and related topics.

Q: How do you construct a better current source than a voltage source and a resistor?
A: There are several ways, but the most common is, ironically, to start with a voltage reference, which can be a diode of some type or a more sophisticated voltage reference. Then, you use a transistor and the well-known relationship between base-emitter voltage V_BE and collector current I_C to transform this voltage into a current, as illustrated in Figure 1.

Q: How much current can a current source deliver?
A: As with the voltage source, it depends on the design. A precision current source can usually deliver up to 100 mA, while a high-power current source using transistors as a current booster can provide amps.

Q: Just as a voltage source is limited in how much current it can deliver, what’s the corresponding limitation for a current source?
A: It’s called compliance voltage. Here’s the scenario: a current source delivers the specified current to the load. By Ohm’s Law, the voltage across the load and thus at the terminal of the current source is simply V = I × R. If the load is a diode, it would be about 1.6-1.8 volts per diode). As R increases or the number of LEDs in a series string increases, so will the voltage.

This reaches a point where the current source cannot deliver the expected current while complying with the voltage value. Inadequate compliance voltage complements a voltage source delivering its rated voltage until it can no longer supply the current at that voltage.

Q: Do voltage and current sources have “fixed” output values?
A: Depending on the application, they sometimes do and sometimes do not. There are many situations where a fixed-in-advance value is desired and many where the value needs to be adjustable by the circuit or a user.

There are also “dependent” or controlled sources, where the source value is a function of another circuit voltage or current, as follows:

Current-controlled current source
Current-controlled voltage source
Voltage-controlled voltage source
Voltage-controlled current source

Q: How are they represented in a schematic diagram?
A: In most cases, the circle of the current or voltage source is replaced by a diamond, as shown in Figure 2.

Q: Where would a controlled current source be used?
A: They are widely used in transducer-sensing and actuator-control loops in industrial settings. The industry standardized many years ago on using a 4-to-20 mA loop in this application, with 4 mA representing zero-point on the scale and 20 mA for full scale.

The reason for using current is that the loop’s current value at the load is unaffected by the resistance of the loop wires, which can be tens or even hundreds of feet long. In contrast, driving a voltage down long wires means the voltage reaching the load would be less than the source voltage, and that loss would be a function of wire gauge, loop distance, and other factors, thus ruining the accuracy of the signal.

For example, for sensing in an industrial loop that sends temperature information, the temperature reading is converted into a current between 4 and 20 mA, corresponding to the minimum and maximum temperature range. It is then “read” by the controller.

In the complementary actuator direction when controlling a motor or valve, the system controller, such as a programmable logic controller (PLC), provides a signal from 4 mA for zero speed or position and up to 20 mA for full speed or position. Again, as a current loop, there is no drop in the wires, so the control signal reaches the actuator at the original value.

Q: Do you have to build your voltage or current source?
A: It depends on the amount of voltage and current needed, as well as accuracy, precision, stability, and cost factors. It’s fairly easy to build a basic voltage or current source using a few discrete components and transistors, or you can buy moderate-to-high-performance ICs that provide the functions. For laboratory-level work where precision and repeatability are critical, you can buy a chassis-based voltage or current source or even a single “box” like the one in Figure 3 that offers both as side-by-side units.

Summary

Voltage and current sources are important building blocks at many levels in instrumentation, transducers, and industrial systems. Voltage sources are initially defined by their voltage output and how much current they can deliver at that voltage, while their current output and compliance voltage defines current sources. Although current sources are less well-known to newer engineers, they have many uses within circuits and in larger systems.