The Voltage Controlled Current Source (VCCS):
What is it?
A current source causes charge to flow like water flowing in a pipe. The amount of water passing through the
pipe might me measured in gallons. The amount of charge flowing in the current source is measured in coulombs.
In a water pipe the flow rate might be measured in gallons per second. In the current source the flow rate
is measured in coulombs per second. A coulomb per second is also referred to as an amp of current.
The VCCS has a valve that controls the amount of charge flow. The voltage applied to the VCCS controls the
charge flow rate.
What is it good for?
The VCCS is useful for adding or removing charge on a capacitor. Using the water analogy, the VCCS is like
the water source that fills a bath tub. In this analogy, the bath tub is the capacitor. In the analogy, a
VCCS can also drain the tub.
If we wished to model a bank savings account where we add $1K ($1,000) each month, we could do this by using a VCCS and a capacitor.
We simply use the VCCS to charge up the capacitor.
In this case we could use one amp of current flow to model the addition of $1K each month to a savings account. Each
coulomb of charge represents a thousand dollars and each second in the simulation can be used to represent one month of real time.
By monitoring the voltage change on the capacitor over time, we can know how much money we have in our savings account.
Tell me more!
At the left is shown the electrical symbol for a Voltage Controlled Current Source (VCCS) named G1.
The current in the VCCS flows in the direction of the arrow from circuit connection at N2 through the current source
to the circuit connection N1. The voltage applied between the control terminals at C2 and C1 determines the amount of current, I,
flowing in the VCCS. This relation of control voltage to current flow can be represented mathematically as:
I=(V(C2)-V(C1))*GAIN
OR
Current=(voltage at C2 minus voltage at C1)times GAIN
If the voltage at C1 is greater than the voltage at C2 then the current will switch direction and flow
opposite to the direction of the arrow.
But how do we set the voltage at the control terminal to set the current flow? This is done by using another
electrical component called a PieceWise Linear (PWL) Voltage Source. The function of this electrical component
is explained in the next section.
SPICE Description Format for the VCCS:
A text editor can be used to make a SPICE input file that describes the circuit model used to do an economic simulation.
The SPICE program will read this circuit description file and then simulate the behavior of the circuit model. The
SPICE format used to describe the VCCS pictured at the top left of this page is shown below:
G1 N2 N1 C2 C1 GAIN
The first character in this description must be a "G" so that the SPICE program knows that this circuit element is a
current source. G1 is the name of this particular VCCS. All element names in the SPICE circuit description must be
unique. The name used for a second VCCS element in our circuit description could be G2 or G_assets etc.. The N2 and N1
are the current flow connections and must be listed before the control connections C2 and C1. The current flows from the
first listed current connection (N2) through the body of the current source to the second listed current connection (N1).
The first listed control connection (C2) is
the positive connection and the second control connection (C1) is the negative connection.
The GAIN term in the above description must be a number such as 1 or 4.5.
We will later use the VCCS along with the capacitor and the PieceWise Linear (PWL) components to solve several financial
problems.
Back to the components page