This article is part of a guided learning series exploring the theoretical and practical aspects of resistors in parallel.
Canonical Article: Current Divider Formula and Derivation
Learning Companion (Q&A): Explore All Questions
You are reading: Question 4
What are the properties of an ideal constant current source?
This post also answers these closely related questions:
- Can a constant current source exist in real life?
- Why does the current divider require a constant current source?
It will provide a constant current
A constant current source will provide a constant current with zero stipulation on the load. Stated another way, the internally generated voltage will do whatever is necessary to maintain the rated current.
For instance, let’s assume a 10 A constant source. It will provide:
- 10 A in a short
- 10 A into a 1 Ω load
- 10 A when connected across a -10 VDC power supply - same for a 10 VAC supply
- 10 A when connected in parallel with a 100 A constant current source
- Undefined when connected in series with anything other than an ideal 10 A source
- 10 A when connected to an open circuit
That last example is a bit of a non sequitur on paper as there is no way to pass current through an open circuit. In practice the voltage will increase sufficiently high to cause a flashover allowing the 10 A to flow though the resulting plasma arc.
Tech Tip: This flashover is closely related to the inductive kick (flyback voltage) associated with relays As counterintuitive as it may seem, an inductor appears to be a constant current source the moment it is turned off.
Job Interview Pro Tip: The relay as a constant current source is a golden job interview challenge. Chances are very high that your interview panel will ask about relay flyback and how to solve inductive kick. Impress them with a cherry on top by mentioning that the relay acts as a constant current source at t = 0. This causes a high voltage spike that could destroy sensitive circuitry it not handled correctly.
The output power varies with the load resistance.
In the previous section we claimed that the constant current source remains viable when connected to a variety of circuits. Let’s explore the power associated with our 10 A source:
- 10 A into a short yields 0 W (no voltage yields no power)
- 10 A into a 1 Ω load yields 100 W (calculated as P = I^2R)
- 10 A connected to a -10 VDC (ideal) voltage source yields 100 W (calculated as P = IE)
- 10 A connected parallel with a 100 A constant current source is undefined without knowledge of the load resistance
- 10 A in series for other than 10 A (ideal) is undefined as we cannot determine the current
- 10 A into an infinite load resistance implies infinite voltage with infinite power (no possible)
The first three examples clearly demonstrate a load dependent output power. The other examples do not compute, strongly suggesting that a constant current source is a mathematic construct.
Infinite resistance of the constant current source
The constant current source has an infinite resistance (impedance). This is a challenging concept best understood by comparing it to the constant voltage source. We then make a mental leap and assume that the constant current source is the opposite of the constant voltage source.
As a thought experiment, suppose we try to change the voltage as measured across a constant voltage source. By definition, this is impossible. On the other hand, we can place the measured voltage of a constant current source to be anything we please by simply changing the load.
With that said, we define output impedance of a source based on the voltage drop when a load is added to the source:
- Constant voltage source (ideal): no change under any conditions. This implies a zero output resistance impedance.
- Constant current source (ideal): free to change voltage depending on the load conditions. This implies infinite output impedance.
Can a constant current source exist in the real world?
No, at least not as an ideal construct. However, we can construct current sources compliant over a limited range.
Physical impossibility
The constant current source is a thought experiment used for circuit analysis. There are no real-world circuit that come even close to the ideal constant current source. The range of power as calculated in the previous section is enough to convince us of this fact. For example, a system that provides infinite voltage with infinite power across an open circuit is a bit silly.
It would take all the power in the universe to construct the ideal constant current source!
Practical real-world examples
There are many examples of constant current sources in electronics. Some, like the previously mentioned relay, have fleeting constant current properties, while other are steady-state. Here are a few for your consideration:
LAB DC power supply
The DC supply used in your school’s lab is likely a dual mode device with both constant voltage and constant current operating capabilities. In constant current mode the supply can:
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Supply current in one direction
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Supply its set current up until the required voltage attempts to reverse polarity or until it attempts to exceed the internal voltage rail of the power supply.
In both cases our “voltage will do whatever is necessary” stipulation is broken. A representative Teledyne LeCroy T3PS3000 power supply is shown in Figure 1.
Figure 1: The T2PS3000 may operate in constant current or constant voltage modes.
LED driver
LED lighting often uses a power supply featuring a constant current power supply. This driving method is preferred as it removes the LED voltage drop variability from the equation. A representative Mean Well LDD-700LW example is shown in Figure 2.
Figure 2: Image of a Mean Well LDD-700LW constant current LED driver.
Transistor and op amps
Discrete transistor amplifiers (think audio) and operational amplifiers often feature a constant current source as active loads. Without getting too involved in theory, we recognize that amplifier gain is directly related to the value of the collector resistor - the higher the resistance, the larger the gain.
This is a perfect application for the constant current source with its theoretical infinite resistance (impedance). This allows circuit designers to achieve gain values that would otherwise be difficult to achieve.
Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey. Author bio.