All About Circuits Forum Thevenin Equivalent of a Voltage Divider circuit
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
01-31-2010, 04:02 PM
 Fraser_Integration Senior Member Join Date: Nov 2009 Location: Rochester, Kent, England Posts: 142
Thevenin Equivalent of a Voltage Divider circuit

Hi there.

The attached is probably really simple, but I've never thought about Thevenin being used for something like a voltage divider, would just like to confirm my steps:

I would think V(th) would be the voltage at the output terminals found by usual voltage divider equation: 10V, and for R(th) I would think that if you shorted the voltage source (that isn't there I know, but bare with me) then the two resistors would be in parallel, so effective resistance of: 22k ohms?

Thanks for looking.
Fraser
Attached Images
 Untitled.jpg (10.2 KB, 71 views)
#2
01-31-2010, 04:11 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

You are correct.

hgmjr
#3
01-31-2010, 07:52 PM
 Fraser_Integration Senior Member Join Date: Nov 2009 Location: Rochester, Kent, England Posts: 142

thanks a lot.

you can delete the thread to de-clutter if you like.
#4
01-31-2010, 08:44 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

No problem. We can let it stand. Other members/guests may benefit from it.

Knowing how to solve for Thevenin's Voltage and Resistance is the first step. To really understand Thevenin's Theorem, it is important to realize how to apply it analyzing a circuit.

hgmjr
#5
01-31-2010, 09:08 PM
 thyristor Member Join Date: Dec 2009 Posts: 94

Quote:
 Originally Posted by Fraser_Integration Hi there. I would think V(th) would be the voltage at the output terminals found by usual voltage divider equation: 10V, and for R(th) I would think that if you shorted the voltage source (that isn't there I know, but bare with me) then the two resistors would be in parallel, so effective resistance of: 22k ohms? Thanks for looking. Fraser
Possibly a better way of understanding the Thevenin equivalent is to consider that the circuit is inside a closed box so that you cannot see the components and that all you have access to are the Vout terminals.

All you can then measure is the open circuit voltage and the short circuit current. If you could then construct a circuit that gave the identical open circuit voltage and the identical short circuit current, then you would have built an equivalent circuit.

So the open circuit voltage is (as you correctly stated)
15 x R2/(R1 + R2) = 15 x 66/99 = 10v

If we short the output terminals, then the short circuit current is 15/R1 = 15/33 = 0.45A

Therefore there appears to be a resistor inside the box of 10/0.45 = 22K fed by a 10v source.

In algebra, the o/c voltage is Vcc x R2/(R1 + R2) and the s/c current is
Vcc/R1

Therefore the equivalent resistance is
Voc/Isc = Vcc x R1R2 / Vcc(R1 + R2)

which is R1R2/(R1 + R2) or the two resistors in parallel.

So the closed box is equivalent to a 10v source with a 22k series resistor
#6
01-31-2010, 11:25 PM
 Fraser_Integration Senior Member Join Date: Nov 2009 Location: Rochester, Kent, England Posts: 142

Thank you thyristor, that explanation would read better than my made up voltage source.

 Tags circuit, divider, equivalent, thévenin, voltage

 Related Site Pages Section Title Textbook Complex circuits : Rc And L/r Time Constants Worksheet Performance-based assessments for network analysis competencies Worksheet Th'evenin's, Norton's, and Maximum Power Transfer theorems Worksheet Voltage divider circuits Textbook The Unijunction Transistor (UJT) : Thyristors Textbook Field-effect-controlled thyristors : Thyristors Textbook Biasing calculations : Bipolar Junction Transistors Textbook -Y and Y- conversions : Dc Network Analysis Textbook Thevenin-Norton equivalencies : Dc Network Analysis Textbook Thevenin's Theorem : Dc Network Analysis

 Similar Threads Thread Thread Starter Forum Replies Last Post Mike33 General Electronics Chat 9 12-29-2009 11:24 AM Ynot1980 Off-Topic 1 12-16-2009 11:32 PM relicmarks General Electronics Chat 5 11-03-2008 12:32 AM Tammy1000 General Electronics Chat 3 10-06-2008 05:33 PM m.majid The Projects Forum 3 09-28-2008 09:12 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 06:32 AM.