LTSpice two transistor circuit simulation  - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 14, 2013, 06:33:25 pm
content: 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 VOwas 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 Vinvaries with respect to the Rload? 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?

author: armandas
author_rank: Frequent Contributor
timestamp: April 14, 2013, 08:42:10 pm
content: 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.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 14, 2013, 09:50:17 pm
content: 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

author: c4757p
author_rank: Super Contributor
timestamp: April 14, 2013, 10:27:51 pm
content: Quote from: chipwitch on April 14, 2013, 09:50:17 pmThat'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)

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 15, 2013, 03:23:15 am
content: Your stepped voltages paint a clearer picture.Thanks

author: c4757p
author_rank: Super Contributor
timestamp: April 15, 2013, 03:29:57 am
content: 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.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 15, 2013, 04:52:52 pm
content: 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.