Okay, that explains it. That reminds me of something I overheard one time.
"Put down that wheelbarrow. You don't know nothin' about machinery!"
Just to show how I routinely use LTSpice, look
at this recent forum thread.
I used my experience ( I've been around the block a few times) to come up with a tenative circuit design that solves the TS's problem. LTSpice didn't do that; it came out of my head. I have done similar things with the TL431, so had that in my bag of tricks...
First thing I used LTSpice for was to draw the beginnings of the schematic. I'll be the first to admit that LTS is not a great schematic drafting tool, but it is very simple and straightforward (Programmers hate it, but this Engineer likes it).
I had the idea of putting the LEDs into what looks like a bridge circuit, where the current through the inverse-parallel connected LEDs reverses when the TL431 switches on/off. LTS didn't suggest that... I had done it before.
I fleshed out the design as I was drafting the schematic, adding needed components to the schematic like current limiting resistors for the LEDs. Since the TS wanted two LEDs, based on previous experience, I extended the design to use the bi-color LED or two different color LEDs. LTS didn't do that; I did...
I knew that TL431 is essentially a voltage comparator (opamp) with a band-gap voltage reference connected to one of its inputs, and could be used to switch a LED as the input voltage changes by a few mV. This application is not what the TL431 "Shunt Regulator" is usually used for.
I knew it could be used this other way because I have done it before, many times. LTS doesn't know that so that it can't suggest that type of usage. It can only show how a circuit works after you have allread created a schematic...
So, as I got closer to having a complete schematic, I ran a couple of quick LTS simulations to see how my design was going to work. The first step was modeling the TS's existing circuit, and then to see what happens if his 28V supply stays fixed, but the load voltage varies.
LTS just does only what it is told, so my experience said that there will be stuff in the schematic that models his existing stuff, and my Voltage Detector circuit would be added on. Note how I used the LTS schematic to show which parts are existing; and which parts need to be added. LTS didn't do that, I did...
Since the purpose of the design is to demonstrate that the LEDs flip just as the load voltage V(load) crosses 10V, I set up a sweep of V(load) from 11V to 9V to see if the trip point is where it is supposed to be. LTS knows how to do lots of different types of simulations; I knew that the .DC simulation was the appropriate one for this test...
So I didn't guess the initial R2/R3 voltage divider ratio quite right, and the trip point wasn't right at 10V, so I did several simulations changing R2 and R3 by selecting standard 1% values that can be purchased and ended up pretty close to a 10V trip point. I did this much quicker than I could have solved two equations in two unknowns. I know that the LM431 switches when its Ref pin has 2.495V on it; LTS can model that, but it can't tell you that directly... You still have to read and understand the
TL431's data sheet.
Next, I plotted the current through D1 and D2, and noticed that based on my initial guesses for R1, R5 and R6, the currents were not well matched. A few more trial simulations with changing the values of R1, R5 and R6 brought the currents into balance... LTS made the simulations instantaneous, but my experience told me which values to change, and by how much.
Next, I asked LTS to tell me what power is being dissipated in the TL431 and the various resistors. I didn't post the plots of power, but I checked to make sure all the resistor powers where substantially less than 250mW, so 1/4W resistors can be used. The power dissipated in the TL431 is a tiny fraction of it maximum allowed values. Again, LTS didn't tell me that this should be done, but it makes asking the question and getting the answer almost effortless...
So, hopefully, from this posting, you have figured out that LTS doesn't turn you into a circuit designer if you aren't one already, but it makes a lot of the tasks that circuit designers do practically painless...
I find the "animated" components that
cowboybob talks about are worthless to me. In this project, for example, I wanted to see when the Red LED lights up vs when the Green LED lights up as a function of V(load) [not as a function of time], so I simply asked LTS to plot the current through the two LEDs as V(load) sweeps from 11V to 9V. It showed me exactly what I can expect from the circuit, including the little dead spot. Not likely that having animated LED's would have exposed that little detail...