Beginner Complex Circuit analysis - Page 1
Forum section: electronics / beginners

author: JacobEdward
author_rank: Regular Contributor
timestamp: March 16, 2015, 10:12:23 pm
content: http://www.allaboutcircuits.com/vol_1/chpt_7/2.htmlI'm wondering if someone could explain why you are suppose to use the total Voltage in order to determine the resistance of a parallel segment... I've taken a screenshot of the specific part of the book where I get confused... It's not intuitive since, with the equivalent circuit, there would be voltage drops before you got to the next segment... for example, if there was a resistor between those two parallel segments, there would be a resistance drop, why would you still use the total voltage to calculate the equivalent resistor...

author: mtdoc
author_rank: Super Contributor
timestamp: March 16, 2015, 10:17:01 pm
content: You don't use voltage to calculate equivalent resistance. Look again at your equations for calculating equivalent resistance for resistors in series or parallel.

author: JacobEdward
author_rank: Regular Contributor
timestamp: March 16, 2015, 10:27:19 pm
content: Quote from: mtdoc on March 16, 2015, 10:17:01 pmIf you look at the values the book came up with: 71.429 and 127.27... you get those values by using 24 volts for both segments... even though one of the segments would have had a voltage drop from the first at run time.

author: JacobEdward
author_rank: Regular Contributor
timestamp: March 16, 2015, 10:31:18 pm
content: Quote from: mtdoc on March 16, 2015, 10:17:01 pmI'm following this procedure which uses voltage...http://www.allaboutcircuits.com/vol_1/chpt_5/3.html

author: rs20
author_rank: Super Contributor
timestamp: March 16, 2015, 10:42:38 pm
content: Quote from: JacobEdward on March 16, 2015, 10:27:19 pmTry doing those calculations again using 12V, 1V, 1735923V... The voltage cancels out at the end of the day, you'll get exactly the same answer for the parallel resistance. Just use 1V, it's simpler to calculate. Or, use the real parallel formula that has voltage already cancelled out for you.

author: mtdoc
author_rank: Super Contributor
timestamp: March 16, 2015, 10:52:08 pm
content: It might help to think of it this way: Resistance is an inherent property of the resistor (or wire, or any circuit element) whose value is independent of the DC voltage across it. You can measure it even when it is not part of the circuit.

author: rolycat
author_rank: Super Contributor
timestamp: March 16, 2015, 11:00:36 pm
content: Quote from: mtdoc on March 16, 2015, 10:52:08 pmTrue for a simple resistor, but notanycircuit element. Not true of a varistor, for example.

author: Brutte
author_rank: Frequent Contributor
timestamp: March 16, 2015, 11:05:43 pm
content: The voltage applied is not the property of the circuit as such. It is the equivalent total resistance that is. However, once the whole resistor network is excited (be it with x or y volts), each sub-resistor shares some part of the electron flow (called current I) and squeezes out some energy from electrons flowing through (called voltage E). The task is to calculate these subvoltages and subcurrents (individual I+E for each resistor R1,R2,R3 and R4, plus fifth for hypothetical equivalent resistor, called "Total").The last column name is a little bit misleading. The last column/resistor should have been named "R_total" instead of "Total" IMHO.

author: mtdoc
author_rank: Super Contributor
timestamp: March 16, 2015, 11:20:47 pm
content: Quote from: rolycat on March 16, 2015, 11:00:36 pmOops! Correct you are! I should have said anylinearcircuit element. What others besides varistors? Any semiconductor I suppose..

author: KM4FER
author_rank: Regular Contributor
timestamp: March 16, 2015, 11:57:50 pm
content: I think the wording they used to describe the problem is what has you confused. After reading the text in the link you provided I don't get that they are telling you to use the total voltage to calculate the resistance. What they are saying is to use the formulas for resistances in parallel and series to calculate a single equivalent resistance for the circuit and then use the total voltage to compute the current through that resistance.Then they want you to work backwards towards the original circuit adding in more complexity until you can ultimately compute the voltage drop across and current flow through each individual resistor.

author: IanB
author_rank: Super Contributor
timestamp: March 17, 2015, 12:57:34 am
content: Quote from: JacobEdward on March 16, 2015, 10:12:23 pmYes, what KM4FER said. You are supposed to add more columns to the table until you have reduced the circuit to a single equivalent resistance. Here is the final picture where you use the total voltage to obtain the total current in the circuit:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2015, 02:41:41 am
content: When writing out all quantities (i.e., including node voltages and branch currents), the reason is because each resistor affects the total current, and therefore, potentially, the voltages between nodes.So you always have to start at the bottom (individual resistors) and work your way up (resistors in series and parallel; equivalents in series in parallel; etc.) to find the total voltage and current, then work back down to find the voltage drops and current flows through each series or parallel equivalent.Tim

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2015, 12:44:23 pm
content: Quote from: mtdoc on March 16, 2015, 11:20:47 pmOther examples of non-linear resistors: light bulbs, heating elements, vacuum-tube cathode heaters. Actually, Ohm's law is approximate, but usually good enough for "normal" resistors. For two-terminal resistors, the equation we all learned isV = I x R , where the voltage, current, and resistance are "scalars" and the resistance is constant. For a more general problem (such as a surface or bulk conductor) the linear equation is J = (sigma) x E , where the current density J (A/m2) and electric field E (V/m) are vectors and the conductivity (sigma) (1/A m) is a constant. For more complicated cases, the conductivity can be a tensor (matrix) if the material is not isotropic (conduction depends on direction).In non-linear resistor theory, a "resistor" is a device where the voltage across the device is a function of only the current through the device, while a "capacitor" is a device the voltage is a function of only the charge in the device, etc.(PS: how does one insert symbols into these posts? I tried copy/paste from Word, but it didn't work.)

author: rs20
author_rank: Super Contributor
timestamp: March 17, 2015, 01:04:44 pm
content: Quote from: mtdoc on March 16, 2015, 11:20:47 pmYou meanresistive. Linear circuit elements include capacitors and inductors, which are even less Ohmish than varistors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2015, 01:15:19 pm
content: Quote from: TimFox on March 17, 2015, 12:44:23 pmWe don't have symbols here; you'll have to format an equation and attach/link the picture.Something like JSLaTeX has been suggested several times, but... well, with the new forum assistance, maybe something will come of it?Tim

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2015, 01:25:00 pm
content: Thanks. Next time, I'll try Visio and export it to .png