Why is the following circuit an adjustable current limiter? It looks like a adjustable current source to me.

Power supply with current limiter and stand alone current limiter

EDIT: I know the difference between current limiters and current sources. Lets analyze the circuit the clear up some misunderstandings.

From the datasheet: "In operation, the LM150 develops a nominal 1.25V reference voltage, VREF, between the output and adjustment terminal.".

I think that R2 in combination with the diodes and jfet is used to superimpose a voltage on top of the adjust terminal so that the voltage over R1 decreases, which in turn limits the current that can flow through R1. This sets an upper limit for the current, but what happens if output current is less than current that we can draw? Since the difference between output and adjustment terminal is set to 1.25V and a voltage forced on top of the adjust terminal (and thus over R1), what is left to change to downregulate the current? The only thing I can come up with is that the excess current needs to be sinked, but where?


2 Answers 2


A current limiter is a current source with a limited voltage compliance. All practical current sources have this limitation, so they are current limiters.

Consider the humble current-limited lab supply. Set its current limit to 20 mA, and voltage at 5 V, and now it's a constant-current source for testing LEDs.

enter image description here

(answers with pictures get upvotes!)

  • \$\begingroup\$ Be careful with that strategy.. if there's a big cap on the output, and connect the LED after the supply is turned on, you may get 20mA constant current only after the LED becomes a DED. \$\endgroup\$ Mar 28, 2014 at 23:04
  • \$\begingroup\$ What about the case that the current limiter is set to 1A and output voltage 5V and build a circuit with a LED in series with a 330 Ohm. What will happen with the 'excess' current (1A-20mA)? Because in the circuit I get that when the current is too high that it will lower the voltage, but I don't see where the excess current goes when you draw less than the limit. The JFET draws 5mA in that configuration. \$\endgroup\$
    – Henk
    Mar 28, 2014 at 23:20
  • \$\begingroup\$ @SpehroPefhany there is not a large post-regulator capacitor on bench supplies for this very reason. And LEDs can handle moderate overload for brief periods. Still, I wouldn't crank up to 30 V and try it. \$\endgroup\$
    – markrages
    Mar 29, 2014 at 2:22
  • \$\begingroup\$ @Henk then the supply will output the voltage (5 V in your example). There is no 'excess' current because the voltage limiter will limit the voltage to 5V. The LED + resistor will only draw 10 mA or so at 5V. \$\endgroup\$
    – markrages
    Mar 29, 2014 at 2:26

A current source varies the voltage both up and down as to force a set current through the circuit where as a current limiter has a set maximum voltage and can only lower the voltage to maintain a set limited maximum current through the circuit.


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