Alice's Loan Payment:
Alice has a loan of $100K dollars at 5% interest rate to pay off.
She wants to pay $500 per month for the first 3 years and then
increase her payment to $550 per month. How long will it take
to pay off the loan? Use
Standard Simulation Conditions.
The circuit below is used to simulate Alice's debt. The capacitor
voltage is set to 100 at the start of the simulation to model the
starting loan debt. This is done by using the initial condition (I.C.)
component attached at the top of capacitor C1.
The Voltage Controlled Current Source (VCCS) named
G1 is used to model the loan interest rate of 5%. The VCCS named G2 models
Alice's payments. G1 adds charge to the capacitor while G2 removes charge
from the capacitor. When the voltage on the capacitor reaches zero, the
loan is paid off. The "PWLmodel" device at the G2 positive control terminal is used
to vary Alice's payment amount from $500/mo to $550/mo. Note that the gain term of
G2 is set to 0.001. This allows the PWL voltage terms to be entered directly in
the dollar amount rather than dividing the dollar amounts by one thousand. Recall that
one volt is used to represent one thousand dollars in the simulation. The TPv1
component attached at the top terminal of the capacitor is a probe point used to tell
the simulator to graph this terminal voltage.
After the above schematic is entered, a SPICE description format is automatically generated
and simulated. This particular simulation took about two seconds of computer time. The simulation result
is shown below.
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