FAQ on voltage and current sources: part 2

Voltage and the lesser-known current sources provide important IC, circuit, and system functions.

Current sources

We have examined voltage sources, and now we can examine their less-known but necessary complement, the current source.

Figure 1. The schematic symbol for a current source can be either a circle with an internal arrow for current flow or two overlapping circles with an external arrow. (Image: Electrical Technology)

Q: What is a current source?
A: As the name implies, the current source is like a voltage source but different. It delivers a constant current to the load regardless of the load impedance.

Q: What is the schematic symbol for a current source?
A: There are two symbols in common use (Figure 1).

Q: What does an ideal current source look like?
A: Obviously, it is a source of current but with infinite internal resistance (Figure 2).

Figure 2. The ideal current source has infinite internal resistance and provides a constant current into the load at all voltage values. (Image: Electrical Technology)

Q: What about a real current source?
A: It has very high but not infinite resistance, modeled in parallel with the source (Figure 3). The current output level of a non-ideal current source will “droop” as the voltage drop across the load it is driving increases.

Q: As with voltage sources, how much “imperfection” is tolerable?
A: It depends on the situation, of course, but most designers like the current source to have an internal resistance at least one hundred times the resistance of the load it is driving.

Figure 3. The non-ideal, real current source has a high but non-infinite impedance, so the current decreases as the load resistance increases, and thus, the voltage across the load also increases. (Image: Electrical Technology)

Q: With all this discussion of a current source, what’s a current sink?
A: It is the same as a source, but instead of delivering a known current to a load, it absorbs a known current from the load. In terms of circuity, if the source circuit is ungrounded, the same circuit or device can be used for either mode by simply “flipping” it; if it is grounded, standard current source and current sink devices are available.

Q: What is Norton’s Theorem concerning current sources?
A: Norton’s theorem (named after Bell Labs engineer Edward Lawry Norton, 1898–1983) is the current-source analog to Thévenin’s theorem. It shows a way to simplify a circuit for current/voltage analysis and represent it with only two components: a current source and an equivalent resistor in parallel (Figure 4). This contrasts Thévenin’s theorem, which used a voltage source and a resistor in series.

Figure 4. Norton’s theorem simplifies the voltage and current relationship by representing the source with a single resistor R_N in parallel with the ideal current source and the load R_LOAD. (Image: Circuit Bread)

Q: Besides the noticeable difference between voltage and current sources, what else differs between the two types of sources?
A: The current source can only function and have meaning in a completely closed circuit because there is no current flow in an open circuit. In contrast, a voltage source can exist as an open-terminal device.

Q: What applications need a current source?
A: There’s a surprisingly interesting and long list. Perhaps the best known is the LED, specified for brightness and color rendition at defined current levels. Therefore, you want to ensure the desired current value is sourced to the LED.

Also, many sensors (transducers), such as PT100 RTDs, need a known amount of current to function properly, and they return a voltage signal in response to that current. Finally, magnetic elements such as the coils of motors and solenoids are current-driven elements, with performance determined by the current passing through them, not the voltage driving that current.

Q: What are some “natural” current sources, meaning components which provide current output?
A: Photovoltaic cells and devices such as photomultiplier tubes (PMTs) are sources. Other transducers that transform mechanical or other energy into electrical energy, such as generators, are natural current sources.

Q: How do you build a current source?
A: The easiest way to build a “crude” current source is to use a voltage source with a series resistor between the source and load, such as an LED. The resistor defines the current at the desired level (Figure 5).

Figure 5. A voltage source and a current-defining resistor form a crude but lightly regulated current source for loads such as an LED. (Image: Circuit Digest)

However, this method yields the current parallel to an unregulated voltage source, as the current going through the LED is a function of the voltage, the resistor value (and its tolerance and drift), and the vagaries of the LEDs used. It also wastes power due to I²R dissipation in the resistor. Therefore, it is only used when “on target” performance is not critical, such as when a basic indicator LED is on a panel.

The final part further explores details of current sources and related issues.