I am trying to design a current limiter that limits current through R4 based on a typical transistor-MOSFET topology like the one in the picture:

enter image description here

This circuit works fine. When the voltage accross R3 is Veb (0.65) current begins to be limited (at 0.25 amps) by means of the MOSFET.

The problem is that I want to limit the current from a negative voltage source. For example, if VS1 would be -5V or -10V. Of course, then the current flows in the opposite direcction and the circuit does not work. I've tried changing both transistors (to npn and Nchannel) without success. Also I tried changing the arrangement of components (bipolar circuit to the right and MOSFET to the left), also without success.

Could you hint me how to arrange components to obtain my goal? Thanks

PD:Other topologies are also accepted. PD2: No ICs if possible

UPDATE: Should this topology work (note the minus sign in the source) for limiting current through RLOAD? Or should I keep the old trt and mosfet? enter image description here Thank you very much

  • \$\begingroup\$ Sure it will work if you reverse everything correctly. \$\endgroup\$ – Andy aka May 19 '16 at 11:37
  • \$\begingroup\$ Hmmm. I thought the 2n6804 was a P channel MOSFET. \$\endgroup\$ – JIm Dearden May 19 '16 at 12:17
  • \$\begingroup\$ @JImDearden Yes, my mistake. Corrected. Also new circuit added \$\endgroup\$ – Anonworriedworker May 19 '16 at 12:30

It should be symmetrical to the original but not like that. The circuit you need is following:


simulate this circuit – Schematic created using CircuitLab

Decreasing voltage of V1 will make BE junction constant at cca 0.6V, what means BJT will be in active region. As \$I_B\$ increases so do the gate voltage and source voltage, therefore \$V_{GS}\$ will be constant as well as your drain current.


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