Electrical Engineering - Circuit Theorems and Conversions - Discussion
Discussion Forum : Circuit Theorems and Conversions - General Questions (Q.No. 1)
1.
Find the Thevenin equivalent (VTH and RTH) between terminals A and B of the circuit given below.
4.16 V, 120 
41.6 V, 120
4.16 V, 70
41.67 V, 70
Discussion:
42 comments Page 1 of 5.
T.Ramachandran said:
9 years ago
Vth = the voltage across the 120 Ohms. this is calculated by determining the current in the loop by applying KVL.
The KVL in the loop is,
100 - 68I - 100I - 120I = 0
=> 100 = 68I + 100I + 120I
=> I = 100/288
=> I = 0.347A
and,
Vth = 120 * I
Vth = 120 * 0.347 = 41.67V
To find Rth,
68 and 100 in series and this series combination come in parallel with 120 Ohms.
So,
Rth = (168*120/168+120)
Rth = 58.33 Ohms.
The KVL in the loop is,
100 - 68I - 100I - 120I = 0
=> 100 = 68I + 100I + 120I
=> I = 100/288
=> I = 0.347A
and,
Vth = 120 * I
Vth = 120 * 0.347 = 41.67V
To find Rth,
68 and 100 in series and this series combination come in parallel with 120 Ohms.
So,
Rth = (168*120/168+120)
Rth = 58.33 Ohms.
(1)
K.Hanumantha said:
1 decade ago
Friends.
According to Thevenin theorem voltage should be short while calculating of resistor Rth, when we do that, R3 is parallel with (r1+r2). So equivalent, 168//100 = 70 ohm Rth.
Vth = r3*I. where I = Vs/(r1+r2+r3) = 100/(68+100+120) = 0.3472A.
We need Vth = I*r3 = 0.3472*120 = 41.67V.
According to Thevenin theorem voltage should be short while calculating of resistor Rth, when we do that, R3 is parallel with (r1+r2). So equivalent, 168//100 = 70 ohm Rth.
Vth = r3*I. where I = Vs/(r1+r2+r3) = 100/(68+100+120) = 0.3472A.
We need Vth = I*r3 = 0.3472*120 = 41.67V.
Keerthana said:
1 decade ago
1. Find Open Circuit Voltage(Vth):
Req = 68+100+120 = 288v.
Ieq = V/Req = 100/288 = .34722 A.
Voc =100-(68*.344722)-(100*.34722).
We get Vth = Voc = 41.667v.
Isc = 100/(68+100)= 0.5952A.
Rth = Vth/Isc.
Rth = 41.667/.5952 = 70 ohm.
Req = 68+100+120 = 288v.
Ieq = V/Req = 100/288 = .34722 A.
Voc =100-(68*.344722)-(100*.34722).
We get Vth = Voc = 41.667v.
Isc = 100/(68+100)= 0.5952A.
Rth = Vth/Isc.
Rth = 41.667/.5952 = 70 ohm.
(1)
Anirban Sengupta said:
1 decade ago
Sorry, but I am not convinced with the explanation. Whenever we will find Rth, then we have to remove the load resistance completely and then we will find the Rth. Load resistance will not have any impact in it. Then please explain how Rth becomes 70?
Aaron said:
10 years ago
In what world is R3 parallel to R1 and R2? they look in series to me. It is parallel to the load yet, but not with R1 and R2. The definition of parallel connection is that it shares two or more nodes, and this simply does not. I am confused.
Ayushi said:
1 decade ago
Actually your doubt is common one, which usually strikes the mind anirban but R3 is not the load resistance infact the load is on a b terminal, and the load is not shown at the terminals. So the answer given that is 70 is correct.
(1)
Prafull gadekar said:
1 decade ago
Total Resistance,
Rt=R1+R2+R3=120+68+100 = 288ohm.
I = V/R = 100/288 = 0.347A(flowing same current in all resistance).
Now, we want to find voltage across R3=120 ohm.
V = 120*0.347 = 41.67V
Now, Rth = R3//(R1+R2) = 70 ohm.
Rt=R1+R2+R3=120+68+100 = 288ohm.
I = V/R = 100/288 = 0.347A(flowing same current in all resistance).
Now, we want to find voltage across R3=120 ohm.
V = 120*0.347 = 41.67V
Now, Rth = R3//(R1+R2) = 70 ohm.
(1)
Khinya Ram Choudhary said:
1 decade ago
Let R = R1+R2;
Rth = R||R3;
According to Thevenin Theorem:
Rth = (168*120/(168+120)) = 70 Ohm.
Vth is Thevenin voltage Between A&B,
According to Voltage Divider Rule:
Vth = Vs*R3/ (R+R3).
= 41.67 V.
Rth = R||R3;
According to Thevenin Theorem:
Rth = (168*120/(168+120)) = 70 Ohm.
Vth is Thevenin voltage Between A&B,
According to Voltage Divider Rule:
Vth = Vs*R3/ (R+R3).
= 41.67 V.
Saleem shah said:
1 decade ago
Rth = R3(R1+R2)/(R1+R2+R3).
Rth = 120(68+100)/(68+100+120).
Rth = 120*168/288.
Rth = 20160/288.
Rth = 70 ohm.
Vth = Vs*R3/(R1+R2+R3).
Vth = 100*120/68+100+120.
Vth = 12000/288.
Vth = 41.6 v.
Rth = 120(68+100)/(68+100+120).
Rth = 120*168/288.
Rth = 20160/288.
Rth = 70 ohm.
Vth = Vs*R3/(R1+R2+R3).
Vth = 100*120/68+100+120.
Vth = 12000/288.
Vth = 41.6 v.
Nagesh said:
9 years ago
The resistances 68 and 100 are in series ; 68 + 100 = 168.
168 ohms is in parallel with 120 ohms. So,
Rth = (168 * 120) / (168 + 120) = 70 ohms.
Vth = R3 * (Vs/R1 + R2 + R3).
Vth = 41.67V.
168 ohms is in parallel with 120 ohms. So,
Rth = (168 * 120) / (168 + 120) = 70 ohms.
Vth = R3 * (Vs/R1 + R2 + R3).
Vth = 41.67V.
(11)
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