RLC circuit

Thread Starter

arthur442

Joined Jul 10, 2021
18
Hello everyone,

I hope you don't mind if I regularly post things here, as I'm trying to learn electronics
in my spare time all I have is a book and the internet to get info from.

My question:
I'm trying to find i(t) for t > 0 as you see below.
The answer is given right up in the corner,
I already tried to try to solve it but can't get to the right answer, do you know what I did wrong?

Thank you in advance.
1626513986031.png
1626514096618.png
 

MrAl

Joined Jun 17, 2014
8,562
Hello,

I verified the given answer was correct. What else is interesting is doing an analysis where the switch CLOSES at t=0 instead of OPENS at t=0. A good exercise too.
 

crutschow

Joined Mar 14, 2008
28,227
For your interest, here's an LTspice simulation of various voltages and currents in the circuit:
I(C1) is the same as i(t).

1626626542946.png
 

MrAl

Joined Jun 17, 2014
8,562
For your interest, here's an LTspice simulation of various voltages and currents in the circuit:
I(C1) is the same as i(t).

View attachment 243872
Hi, did you leave the closing parenthesis off the end of the model statement on purpose or it just got lost somehow.
I find some 'models' in LT Spice to be a little strange as they show spice lines in the parameter dialog but nothing in them sometimes even when we have a model statement in the schematic with several parameters listed.
 

crutschow

Joined Mar 14, 2008
28,227
did you leave the closing parenthesis off the end of the model statement on purpose or it just got lost somehow.
It was accidentally left off.
Apparently it accepts the model without the last parenthesis.
I simulated with the added end parenthesis and there was no change in the simulation outputs.
 
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MrAl

Joined Jun 17, 2014
8,562
It was accidentally left off.
Apparently it accepts the model without the last parenthesis.
I simulated with the added end parenthesis and there was no change in the simulation outputs.
Oh ok great. I thought it may have just been the graphic because that happens to me sometimes when i copy and paste a graphic image sometimes some parts dont get selected so i have to go back and do it again.
But it's good to know that there were no other parameters declared in that model statement. I also see that there is a hysteresis parameter that may come in handy too.
 

crutschow

Joined Mar 14, 2008
28,227
I also see that there is a hysteresis parameter that may come in handy too.
The switch hysteresis parameter in LTspice does not likely work the way you might think.
It actually determines the voltage difference it takes for the switch to linearly go from full on to full off (or vice versa).
 

MrAl

Joined Jun 17, 2014
8,562
The switch hysteresis parameter in LTspice does not likely work the way you might think.
It actually determines the voltage difference it takes for the switch to linearly go from full on to full off (or vice versa).
Oh really? I havent played around with the switch component in LT Spice much so i'll have to check this out. I mostly use MicroCap except when someone here wants to use LT Spice and so i have used the switch in that simulator many times but not much in LT Spice.
I also ran into an "Ideal Diode" i did not know LT Spice had until recently. In the older MicroCap i had to make my own (not sure if they have one in the newer version). An ideal diode helps when simulating a circuit when i want exact or nearly exact theoretical results so i'll have to look into that now too in LT Spice.
I hope i can get the switch and the "Ideal Diode" to work the way i want. I'll check these out later today.
Thanks for the heads up.

@crutschow Ok i tried it and it seems to work almost the way i would expect.
When i set the threshold to 0.5v and the hysteresis to 0.4v the switch turns on at 0.9v and off at 0.1v and that's usually how hysteresis in a switch works. So i guess i dont understand what you would expect it to do. Did you think it would center that 0.4v about the 0.5v threshold? That would make it switch on at 0.7v and off at 0.3 volts and to me that would be reasonable also in fact may be more reasonable because that is how hysteresis is usually specified (the range not the upper and lower bounds). Either way i dont mind it though.
Now for the "Ideal Diode".

LATER:
I set up the "Ideal Diode" as follows:
.model D1 D(Ron=1 Roff=10000k Vfwd=0.7)
but it didnt work exactly like that would suggest.
With a 5.7v source i would expect it to drop 0.7v when in series with a resistor so i should measure 5.0v at the cathode, but it's more like something around 5.1v so a 0.6v drop, and when conducting it drops a little more but it does not appear to follow that Ron value.
I guess more experimentation will be needed for this perhaps the other params have to be set also.
 
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