Serial Resistor solution to finding individual voltage not adding up

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Discussion Overview

The discussion revolves around calculating the voltage drop across individual resistors in a series circuit with a 20-volt power supply. Participants explore the application of Ohm's Law and the impact of resistor tolerances on measurements, while addressing potential errors in calculations and the understanding of basic electrical concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion over the voltage drops across resistors in a series circuit, noting a discrepancy in expected versus calculated voltage.
  • Another participant highlights the importance of measuring actual resistor values due to tolerance, suggesting that small errors are acceptable within a 5% range.
  • There is a question about whether theoretical calculations differ from practical measurements when using real components in a circuit.
  • Participants discuss the total resistance of the circuit and the correct calculation of current, with one asserting that the current should be 0.0125 amps instead of 0.0123 milliamps.
  • Concerns are raised about potential calculation errors and rounding issues affecting the results.
  • Some participants inquire about the actual measurements of voltage drops and resistor values, emphasizing the need for practical verification.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the source of the discrepancy in voltage calculations. Some agree on the importance of measuring actual resistor values, while others question the accuracy of the initial calculations. The discussion remains unresolved regarding the exact cause of the differences observed.

Contextual Notes

Participants mention potential measurement errors due to resistor tolerances and rounding issues in calculations. There is also uncertainty about whether the values used in calculations are based on actual components or theoretical examples.

Who May Find This Useful

This discussion may be useful for individuals learning about basic circuit theory, Ohm's Law, and the practical implications of resistor tolerances in electrical engineering or electronics.

perpc
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Ok. I am adding this up time and time again and its just not making no sense. I am sitting here trying to brush up on technical math skills this week before i get my soldering iron so i don't mess nothing up. and here i am i can't understand the basics.

I have a 20volt power supply and 4 resistors: R1, R2, R3 and R4. R1 = 200Ω R2 = 700
R3 = 300Ω R4 = 400 Ok what i did was use ohms law and all the other crap. so anyways in order to determine how much voltage is going through each individual resistor i need to multiply current by resistance. I did that to each one and when i get done i keep coming up with 19.68 rather than 20. Can someone please help me figure this out before i go and blow a circuit board or something one day not knowing basic stuff? OH yeah and the current running through the circuit is 0.0123 milliamps
 
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IM SORRY, even though its the subject itself, i forgot to mention they are in serial, not parallel. :)
 
You specified the banded resistance of your resistors, did you actually measure them? Remember they have a tolerance and can be off the banded number by up to 5%. There will always be some measurement error so do not expect to be perfect. It is good enough if you measure inside of 5%. The way I compute it you have about 1.6% error... Looks good to me.
 
thanks a lot. i just realized that i should of posted this under elec. tech what you say makes sense. does this still apply though if its just a simple problem though? I mean I am just makin up problems and random to make sure i got the hang of it. I mean should i still have a difference in potential energy when doing it on paper versus actually having resistors on a circuit?
 
perpc said:
I have a 20volt power supply and 4 resistors: R1, R2, R3 and R4. R1 = 200Ω R2 = 700 R3 = 300Ω R4 = 400
Are these actual resistors, or are they just values you invented on paper to use in sample calculations?
OH yeah and the current running through the circuit is 0.0123 milliamps
The theoretical value for the exact resistance values you listed is 0.0125A, so there you have a 1.6% departure from theory to start with.

It is not clear to me whether there is any electronic hardware involved. Integral is apparently privy to more information than is available here:
Integral said:
You specified the banded resistance of your resistors, did you actually measure them?
Those are definitely not run-of-the-mill preferred resistor values. :wink:
 
perpc said:
thanks a lot. i just realized that i should of posted this under elec. tech what you say makes sense. does this still apply though if its just a simple problem though? I mean I am just makin up problems and random to make sure i got the hang of it. I mean should i still have a difference in potential energy when doing it on paper versus actually having resistors on a circuit?

It applies if you are using real resistors in a real circuit and you are using a real meter to make measurements. Have you measured the actual value of your resistors?

To N.O.
Keep in mind that he is not a engineer working in a multinational environment designing circuits for commercial value. To a high school student preferred resistor values are usually multiples of 100 for easier computation.
 
The total resistance is 1600 ohms. (200 + 700 + 300 + 400 = 1600)

The current is 0.0125 amps, not 0.0123 milliamps (20 volts / 1600 ohms = 0.0125 amps)

The voltages are 2.5, 8.75, 3.75 and 5 volts which add up to 20 volts.

So, you must be making a calculator error somewhere.
 
thanks. maybe i just need to start doing all my math on a calculator rather than adding it up on paper. Someone told me that I am not rounding right or something. Whew, i knew i should have never tried to refresh upon my math skills. At 30 I am having just as much problems graspin basic mathematical concepts as a 70 year old.
 
What did you measure for the voltage drop across each resistor? What did you measure for the actual resistance?
 
  • #10
perpc said:
thanks. maybe i just need to start doing all my math on a calculator rather than adding it up on paper. Someone told me that I am not rounding right or something. Whew, i knew i should have never tried to refresh upon my math skills. At 30 I am having just as much problems grasping basic mathematical concepts as a 70 year old.

It was just one calculation that was wrong. The current should have been 0.0125 amps and after that everything works out OK.

So, don't worry about it.
 

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