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Dennis Feucht

Seemingly Simple Circuits, User-Proof External Supplies: Circuit 3: The Two-BJT Current-Limited Supply

Test Circuit
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D Feucht
6/22/2016 1:02:31 AM
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Re: A cliff, so to speak?
CC,

The purpose of thecircuit is to set a maximum current that can be drawn from the user terminals. That occurs when the terminals are shorted. At less current, the goal of the dsesign is to maintain a terminal voltage as close to the supply as possible, and the saturating pass transistor largely accomplishes this. By varying the resistor values, it is possible to achieve as sharp of a knee in the v-i curve as possible.

The foldback current-limiting circuit is developed in my book Designing Waveform-Processing Circuits, chapter 1.

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CC VanDorne
6/21/2016 11:09:28 PM
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Re: A cliff, so to speak?
Dennis,

From my recent work on the astable saw-tooth genorator, or whatever it shall be called, I am still in Breadboard-Dennis'-Two-Transistor-Circuits-Mode, and I decided to go back to this briliant current limitor.  I validated your results (woohoo!) and then tweaked both the emmitor resistor values to learn the behavior of the circuit.  It turned out to be a good learning experience about this circuits behavior in general, but it also made me doubt its usefulness as a current limiter.  In other words, it does operate predictably as a current limitor but that comes at the massive expense of voltage.  To stay at or near 5V on the curve you'd better not need much more than 5mA.  As such, I'd think of this as more a protection circuit, or a shut-down circuit.

Oh boy, there I go with the names again, right?  Maybe not.  I'm open to the possibility that I am missing this circuits full potential.  Perhaps if the pass transistor would be beefier and the Re1 were much smaller it would work well sourcing more current?  Are there some other real-world examples out there that you can direct me to?  Thanks.

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D Feucht
3/19/2016 12:30:49 AM
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Re: A cliff, so to speak?
CC,

A "cliff" is exactly the right idea for this circuit, and this third circuit accomplishes it better than the previous two. The second BJT helps to increase the loop gain of the current-limit response. I did not explore this too far in the articles but the equations are there for analyzing voltage fall-off after a given threshold current.

Analog circuit analysis is hard at first but it is a skill that develops over time, and in the long run, is well worth it. Even with powerful circuit simulators, some calculator-level analysis confirms that what is produced from SPICE is the right response. With algebraic equations, it is easier than in SPICE to see what variations in voltages or parts values will do to circuit behavior. This is important for optimal design.

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CC VanDorne
3/18/2016 6:10:59 PM
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A cliff, so to speak?
Dennis,

Transistor study has always fascinated me and your analysis has reminded me of how fascinating and agravating it can be.  man, that's a lot of work for a couple of three pin devices!  I think you nailed with the word "seemingly", because it's never simple once the equations start stacking up.

But ironically this last circuit, the most complex of the three, is the only topology that's actually intuitive to me.  Anyway, I have a question about your test results and would be design goals.

I'd think that the goal would be to hold the output voltage as close to 5V, in this case, as possible until current limiting kicks in.  A "cliff", so to speak.  Instead, with this circuit we see a gradual slide.  Would value tweaks get us there or would we need a new topology?

Thanks,
CC

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