LTSpice two transistor circuit simulation

[chipwitch]

For the past week, I've been trying to grasp how a "two transistor" circuit is used to limit current in a circuit, such as in a PSU to limit current.  I've had just enough training in electronics to have a firm understanding of Ohm's and Kirchoff's Laws. 

When I first learned of the existence of a "current limiting" circuit a couple weeks ago, my brain short circuited.  I was intrigued, but confused.  What little I understood of electronics was enough to tell me that current is a function of voltage and resistance and that limiting current was not possible in a DC circuit.  Of course, I was assuming that voltage and resistance were both remaining constant.  Experimenting in my lab, I came to realize that the V_O was being adjusted by the circuit to maintain the constant current.  Ah, that made sense! 

So, this morning I thought it might be a good idea to simulate my circuit in LTSpice where I could easily play with various values and manipulating the circuit in order to get a better understanding of the two transistor circuit.  I've been working on it all morning with no success.  Needless to say, I'm new at Spice too.

Is it possible to simulate a two transistor circuit such that the circuit V_in varies with respect to the R_load?  I got as far as the "H source" and thinking it could be used in conjunction with a fixed current source in series with the load.  I still think it may be the key, but I can't see how to apply it.  I tried various arrangements with no success. 

Am I trying to get LTSpice to do something it wasn't designed to do?  Would some other simulator serve the purpose?

[armandas]

Not sure if we're talking about the same circuit, but see the attached LTspice schematic (rename it to asc). It contains two versions of "two transistor" current limiter: one for sourcing and one for sinking current. Hope this helps.

[chipwitch]

Hi Armandas,

Thanks for the suggestion.  That's basically the right circuit, I think.  But the key point I'm after is that the Voltage source is not a fixed value.  In the second circuit, if you had a constant current of say, 15mA, the voltage would update to whatever voltage would be required to deliver the current.

That being said, I didn't understand the whole derivative thing... Your example cleared some of it up for me.  I may be able to hobble something together thanks to you.  Not sure though. 

Thanks, in either case

[c4757p]

Thanks for the suggestion.  That's basically the right circuit, I think.  But the key point I'm after is that the Voltage source is not a fixed value.  In the second circuit, if you had a constant current of say, 15mA, the voltage would update to whatever voltage would be required to deliver the current.

That's the whole point of the circuit... Unless I'm misunderstanding what you're saying, I think you are still misunderstanding what a CC circuit does.

Here's a slightly modified version, updated to your example of 15mA into a relay, but also changed so that it plots the current through the relay vs. the voltage source. Poke around with it a bit.

(Again, rename to CC.asc)

[chipwitch]

Your stepped voltages paint a clearer picture. 

Thanks

[c4757p]

The first one did the same thing, just differently. Instead of plotting vs. voltage like I did, it plotted vs. time, but had the voltage itself change over time.

[chipwitch]

You'd have to have been inside my head to know why the first circuit didn't look right... and believe me... I wouldn't wish THAT on anyone!!! Yours didn't show me what I was asking for either, but it did allow me to identify at least one flaw in my thinking.