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Circuit Theorems and Conversions - Engineering

Q1:

Referring to the given circuit, determine VTH and RTH if a 68 resistor is connected in parallel across R2 and R3.

A 3.3 V, 148 

B 330 mV, 148 

C 3.3 V, 68 

D 330 mV, 68 

ANS:A - 3.3 V, 148 

In order to solve this problem you just need to imagine there is an 68-ohm resistor in parallel in the current 20-ohm and 68-ohm resistors that is in series.

If we solve for rth its going to be:
RTH = 68//(68+20)//100 + 120 = 148 ohms.

To solve for vth you need to use the current divider rule:
VTH = 12 [(12)*(68//88)/(68//88 + 100)] = 3.3 volts.