MATLAB >> Simple RC circuits in sinulink

by Michael Chan » Wed, 17 Nov 2004 11:27:59 GMT

Hi all,

Is it possible to model the behaviour of an RC circuit in simulink? I know
that I can simply represent a circuit as a transfer function, but this
doesn't work if I cascade two circuits together, since the transfer function
isn't simply the product of the individual circuit transfer functions.

It is my opinion that simulink cannot handle this (although I don't know
alot about simulink). To simulate blocks of circuits, a block needs to know
the loading effect of the block in front of it, and the output impedance of
the block behind it; something simulinks simulator cannot easily account

Please let me know if I'm wrong, or how I might accomplish the above. Also,
if anyone knows of a simulation program that is like simulink, but accounts
for loading effects, I would be very interested.


Michael Chan.

MATLAB >> Simple RC circuits in sinulink

by kludge » Wed, 17 Nov 2004 22:38:58 GMT

Why not use SPICE? It would seem a much more elegant way of doing what
you describe, with a lot less work.

"C'est un Nagra. C'est suisse, et tres, tres precis."

MATLAB >> Simple RC circuits in sinulink

by Steve Amphlett » Wed, 17 Nov 2004 22:48:39 GMT

The problem with using Simulink is that it is causal. Information
flows from one block to the next in a pre-described manner. So you
really need to know all the interaction effects to model the whole

You might want to take a look at dymola from dynasim ( < >
). It uses an acausal model description language (modelica) to
describe systems, completely eliminating the problem you describe.

MATLAB >> Simple RC circuits in sinulink

by No one » Thu, 18 Nov 2004 00:04:21 GMT

"possible"? yes. "do you really want to try"? depends.

why would that be true? if you were to implement a two-port block, say
with 'gain' relationships from port 1 to port 2 and port 2 to port 1,
would that, somehow, not account for 'input' and 'output' loading?

Hspice, Pspice, some variants, are 'like' simulink in a fundamental
sense, but are intended for a different level of work.
'No one'

MATLAB >> Simple RC circuits in sinulink

by Per Sundqvist » Thu, 18 Nov 2004 01:23:32 GMT

Yes I got it to work, and I have right now 15 min. experience in

It's a feedback system (serial RC circuit was assumed) with the
a) Uc=U(t)-i*R
b) i=C*dUc(t)/dt

Uc from a) goes into b) which deliver back i to a). As U(t) I used a
sinewave (the second input of a).
2 gain (one has gain=-R, and the other gain=C), 1 sum, 1
derivative, 1 mux, 1 sinewave and 1 scope. Thats it.

The result seemes resonable with 90deg diffrence between Uc and i.


MATLAB >> Simple RC circuits in sinulink

by Per Sundqvist » Thu, 18 Nov 2004 07:18:23 GMT


Hi again

I found it possible to build a feed-back based resistor-block, with
inputs R, U2 and "output" U1. The current i=(U2-U1)/R, but is only
"seen" inside the block. The trick is an infinitisemal time delay dt
(not nesseary to invoke explicitly -it works anyway), so the next
"independent" equation is U1=U2-R*i. The I-V for my model agrees
perfectly with Ohm's law.

The value of U1 between different resistors are unknown, until the
last restistor is grounded. we get then a sort of coupled system with
unknown U1 for each R, but we leave to simulink to solve that.

Next to build blocks for C and L etc, would be similar with feedback
system. Note: in real components there is a time-delay due to the
limit of the speed of light, which is clear if we solve Maxwells
equations instead of a "discrete" approximation such as a RC-net for
example. Therefore the "independency" a)i=(U2-U1)/R and b)U1=U2-R*i
is not wrong.

PS. Is it possible to get a .m file with some sort of "command line"
version of the simulink model? I use it in femlab, but don't know how
to do it with simulink. DS


MATLAB >> Simple RC circuits in sinulink

by Michael Chan » Thu, 18 Nov 2004 08:52:18 GMT

SPICE will work, but SPICE is slow, and is quite low level. With simulink,
prototyping systems is quite fast and the GUI makes it easy. A behavioural
level simulator also allows one to model a systems behaviour, without
needing to be clear on an implementation.

MATLAB >> Simple RC circuits in sinulink

by Michael Chan » Thu, 18 Nov 2004 08:56:08 GMT


I'm not sure I follow you. I have considered a scenario where each block
has an output port that feeds back loading information to the block
preceeding it, and an output port that feeds forward information to the
block in front of it, but this gets very messy very quickly, and is far from

MATLAB >> Simple RC circuits in sinulink

by No one » Thu, 18 Nov 2004 11:53:16 GMT

[behavioral model of r-c network]

true, hence the suggestion to use Hspice (or equivalent) mentioned by
someone else. but you want to build a behavioral model that includes
this, so you can model higher-level systems (i imagine...), in short
simulation time, and you don't want to ignore loading.

a two-port can be described by a 2 x 2 matrix: 1,1 is the input; 2,2 is
the output; 1,2 is forward transfer; 2,1 is reverse transfer. want
something simpler? Per Sundqvist has one suggestion; J. M. Miller gave
us all a way to approximate port loading, back in 1919.

so... will any of that work well in Matlab? depends... on how far you
want to go, on how much 'reality' you want to include.
'No one'

MATLAB >> Simple RC circuits in sinulink

by Michael Chan » Thu, 18 Nov 2004 13:49:02 GMT

Yeah, that's what I figured. Working around this is probably too much
effort in simulink. Thanks for the tip about dynasim. This looks like the
sort of program I was thinking of.


< >

MATLAB >> Simple RC circuits in sinulink

by DJL » Thu, 18 Nov 2004 14:49:13 GMT

wants to do circuit simulation and should use PLECS available from

There is a freeware student version.

See for details: ~jost/peds99.pdf

MATLAB >> Simple RC circuits in sinulink

by Steve Amphlett » Thu, 18 Nov 2004 17:20:49 GMT


I gave you a bad link - sorry. It should be:

< > (.se as in Sweden)

MATLAB >> Simple RC circuits in sinulink

by Robert Zeng » Thu, 18 Nov 2004 23:22:53 GMT

Look at two new RF products from MathWorks:

I believe these two products are what you are looking for.

Robert Zeng


MATLAB >> Simple RC circuits in sinulink

by Michael Chan » Fri, 19 Nov 2004 11:38:18 GMT

Thanks, I'll look into these when I get a chance.

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