A way to find total resistance when no resistors are in series or parallel

Thread Starter

johngradycole

Joined Sep 24, 2019
2
Is there any way to find the total resistance in a circuit when no resistors are in series or in parallel with each other?

This is the question from my homework:
In the bridge circuit of Figure 7, obtain expressions for the Thévenin equivalent voltage source and resistance of the part of the network connected to the load resistor RL. If the bridge resistors R1, R2, R3 and R4 have resistances of 1k-ohm, 2k-ohm, 4k-ohm, and 6k-ohm respectively. Calculate the current through the load resistor when RL= 500 . [Answer: 0.188 mA]
upload_2019-9-28_13-42-2.png


I'm not totally confident with Thevenin's Theorem yet so maybe I'm doing something wrong but this is as far as I got:
 

Alec_t

Joined Sep 17, 2013
10,384
Have you fully understood what 'in series' and 'in parallel' means?
Do you think that the circuit in post #1 has no resistors in series or parallel?
 

Papabravo

Joined Feb 24, 2006
12,407
I'm not asking you to do my homework, I'm saying never mind with helping me because it won't let me upload the working I've done so far
You are in the sense that you have not explained what you have done to solve the problem. If we don't know where or why you are stuck we can't help you get to the right place. You don't need to upload an image for that to happen.
 

bertus

Joined Apr 5, 2008
20,061
Hello,

I am just a forum administrator.
The shown isssue is probably more a thing for @jrap , the technical administrator for the whole site.

You could try to disable the flash uploader in your profile in the preferences page:

Browsing Preferences All About Circuits.png

Bertus
 

MrAl

Joined Jun 17, 2014
6,510
Hi,

What you do is use Thevenin and Norton equivalents. What happens then is some resistors end up being in parallel.
For example, R1 ends up in parallel with R2, then R3 in parallel with R4. This reduces the circuit to two voltage sources and three resistors in series, from which Va and Vb are easy to calculate and thus the voltage across RL is Vb-Va or alternately Va-Vb and so the current is easy to calculate.
I dont want to give the whole answer though, but first you convert the voltage source into two current sources and then those resistors appear in parallel, then you convert them back to two voltage sources and you end up with one series string.

See if you can figure out how to do this. If not i'll provide more info but here you are supposed to make your first attempt to solve first.
 

MrAl

Joined Jun 17, 2014
6,510
Hello again,

Here is an illustration of how to go about this using source transformations.

You'll need to calculate I1 and I2 based on the original circuit to get the top circuit, then
calculate V1, V2, R6 and R7 to get to the bottom circuit.
The bottom circuit is then very easy to solve for the voltage across RL the 500 ohm resistor or just solve for the current.



SourceTransformations_20191002_013829.gif
 
Last edited:

ci139

Joined Jul 11, 2016
725
just solve for two bridge points (below)
(also : a random Google search on the subject)
____________________
below -- is given formula derivation for , at a ↑↑ fuzzy continuation point ↓↓ , the Ux = (U₀R₁R₂+U₁R₀R₂+U₂R₀R₁)/(R₁R₂+R₀R₂+R₀R₁)
mind the arrowed directions for the currents -- I₀ I₁ I₂ -- /!\ /!\ /!\
 

MrAl

Joined Jun 17, 2014
6,510
just solve for two bridge points (below)
(also : a random Google search on the subject)
____________________
below -- is given formula derivation for , at a ↑↑ fuzzy continuation point ↓↓ , the Ux = (U₀R₁R₂+U₁R₀R₂+U₂R₀R₁)/(R₁R₂+R₀R₂+R₀R₁)
mind the arrowed directions for the currents -- I₀ I₁ I₂ -- /!\ /!\ /!\
Hi,

Did yo really use the word "just" with 20 lines of math text? :)
 

ci139

Joined Jul 11, 2016
725
Did yo really use the word
? now that you asked it looks so :confused:

besides i got used to lengthy formulas while digging into the subject (math.) in depth decades ago at university times -- this one is just one page of relatively large script . . . there were over a 4pg. ones . . . if the sign error sneaked in -- :eek: it was "fun" to track one down

you need to save your work with Word . . . regularly -- because the Microsoft (after all these years) haven't got the memory management right
. . . well there has been some progress but not much signifficant
 

MrAl

Joined Jun 17, 2014
6,510
? now that you asked it looks so :confused:

besides i got used to lengthy formulas while digging into the subject (math.) in depth decades ago at university times -- this one is just one page of relatively large script . . . there were over a 4pg. ones . . . if the sign error sneaked in -- :eek: it was "fun" to track one down

you need to save your work with Word . . . regularly -- because the Microsoft (after all these years) haven't got the memory management right
. . . well there has been some progress but not much signifficant
Hi,

Yeah i hear 'dhat :)

I did about 100 lines of math and code this morn so i could 'easily' tabulate values for ripple in a full wave rectifier. Fun :)
 

ci139

Joined Jul 11, 2016
725
tabulate values for ripple
o_O why you need ripple stats. ?

i did a local weather trends for couple of years -- only to find out that the local meteorological department can not convert TIME properly from UTC to local daylight saving one -- the time (registered by automated stations) was adjusted later (in data server) so parts of the stream had gaps or overlapping -- even up to several days
?? why to provide a data at public server for what the exact time can't be uniquely determined (a state or EU money ... who cares :confused:)
 
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