After reading the answers of two questions regarding the LM317, the answers made me think about whether I realy understand current source and current limiter.

In this answer it says:

The LM317 with the single series resistor between output and adjust input is actually a fixed current source, not a current limiter.

In another answer the LM317 is used as a "Precision Current Limiter". It is taken from the datasheet, so the wording should be right(?)

Now when I compare the "Precision Current Limiter" on page 17 and the "1A Current Regulator" on page 16, both have a "single series resistor between output and adjust input". What is the difference here?

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From a user perspective my understanding up til now was:

  • Current limiter: does never supply more current then the limit, but can provide less.
  • Constant current source: the current sourced(/sinked) stays always the same (at least the regulator tries to do so)

Is my understanding right?


Yes, you're right, but the figures you posted are the same configuration: they regulate the current over the output resistor measuring its voltage drop.

The basic difference between a voltage source and a current source, is that the first has a low output resistance (ideally 0), while the current source has a high output resistance (ideally infinite).

The voltage source with current limiting is made to provide a constant voltage in its operating range, but drops the output voltage as protection mechanism to prevent damaging the load and the source itself. Note that there are different methods of current limiting, one of which brings the current below the limit to prevent overheating.

In practice you can use a current limiting source to generate a specific current, but while a supply can handle it without problems, for an integrated devices is not a standard operating mode, and can result in wrong behavior.

  • \$\begingroup\$ "... while the current source has a high output current (ideally infinite)" - typo? I assume you mean ideally infinite output impedance? \$\endgroup\$ – exscape Mar 3 '12 at 14:19
  • \$\begingroup\$ dehdeh yes sorry...but actually it's resistance, I'm not considering reactive effects \$\endgroup\$ – clabacchio Mar 3 '12 at 14:38

There is no firm dividing line between a "source" and a "limit" (either in voltage or current). Hence the fact that your circuit is alternatively labeled as both.

Consider that when a current limiting circuit is actively limiting it's behaving exactly like a source.

The difference may be in the expectations. A "limit" may have relaxed performance/accuracy requirements and/or may not be expected to operate all the time.


You are forgetting one very important aspect of the circuit! The Load. The load makes all the difference. If the load connected to the circuit is of high resistance then the current limit is never reached. However, if the load is of low resistance then the current limiting feature of the circuit kicks in to limit the current to the set maximum.


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