I have been experimenting with simulating current limiting circuits. I am trying to limit current to ~500mA given a fixed 4.8V source. I started a question based on a conventional 2-transistor current limiting circuit over here based on the following schematic . . .

enter image description here

Rather than make that question more convoluted, I thought I would ask a separate question to determine what the difference is between the above circuit, and another variety that I found commonly using google image search that involve use of diodes. The 1-transistor variant of these looks like the following . . .

1 Transistor

enter image description here

Then there is a circuit with another transistor added

2 Transistor

enter image description here

... and finally there is a circuit that looks like a similar configuration using 3 transistors . .

3 Transistor

enter image description here

What are the differences between these diode based circuits and what are their advantages/disadvantages compared to the current limiter shown at the top of the page?

  • \$\begingroup\$ Your one transistor and two transistor circuits are identical. Wrong picture upload? \$\endgroup\$
    – stevenvh
    Commented Sep 4, 2012 at 10:30
  • \$\begingroup\$ Ooops . . . fixed :) \$\endgroup\$
    – learnvst
    Commented Sep 4, 2012 at 10:36
  • \$\begingroup\$ The three new circuits are voltage regulators with a current-limiting feature. Note that if you eliminate the lower transistor and the reference diode from the third circuit, it's essentially identical to your original circuit. \$\endgroup\$
    – Dave Tweed
    Commented Sep 4, 2012 at 10:49
  • \$\begingroup\$ So what would the voltage regulation do if you know that you have a steady 4.8V supply? \$\endgroup\$
    – learnvst
    Commented Sep 4, 2012 at 10:52
  • \$\begingroup\$ @learnvst - it would drop the voltage to whatever it's regulated level is set to (the zeners voltage minus ~0.6V base-emitter drop) It won't help for what you are trying to do, as if your voltage source is fixed at what you want it to be, then all you need is the current limit. \$\endgroup\$
    – Oli Glaser
    Commented Sep 4, 2012 at 11:00

1 Answer 1


The original circuit is a "low-side" current sense. The first variant is a voltage source with a high-side current sense. The second variant is the same but is now adjustable (tweak trip current). The third variant replaces the two diodes with a transistor to make it a little more precise. All these circuits are not going to be very precise as the Vbe junction of any transistor is sloppy and changes a lot of temperature. The low side vs high side just depends on your application (maybe you don't have access to the high-side).

The three transistor version is going to be the most precise and give you adjustments to trim some error and tune the current trip level.


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