Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

How to calculate the total current in a particular circuit ?

Status
Not open for further replies.

munzir

Full Member level 6
Joined
Oct 11, 2011
Messages
365
Helped
22
Reputation
44
Reaction score
21
Trophy points
1,298
Location
Pak
Activity points
3,795
How to calculate the total current in a particular circuit ?

Do we have to sum up all the currents of individual components present in the circuit ?

thanks
 

nxt_

Member level 4
Joined
Aug 16, 2010
Messages
69
Helped
4
Reputation
8
Reaction score
4
Trophy points
1,288
Location
Egypt
Activity points
1,834
measure how much current does each components draws (Do not depend on data sheets only) it is good to measure the current of a component
by yourself and yes you can sum up all the currents of all your components.
 

LvW

Advanced Member level 5
Joined
May 7, 2008
Messages
5,858
Helped
1,746
Reputation
3,496
Reaction score
1,346
Trophy points
1,393
Location
Germany
Activity points
39,552
How to calculate the total current in a particular circuit ?
Do we have to sum up all the currents of individual components present in the circuit ?
thanks

Typically, an electrical circuit consists of more than only one component. Therefore, you only can ask for a current flowing through a single component or within one single path, respectively. With other words: In a circuit there are several currents. So - what do really mean?
 
  • Like
Reactions: nxt_

    nxt_

    Points: 2
    Helpful Answer Positive Rating

kak111

Advanced Member level 4
Joined
Mar 30, 2011
Messages
1,366
Helped
933
Reputation
1,868
Reaction score
907
Trophy points
1,413
Location
Finland
Activity points
10,864
Kirchhoff's circuit laws

Kirchhoff's current law = Kirchhoff's First Law

At any node (junction) in an electrical circuit, the sum of currents flowing into that node
is equal to the sum of currents flowing out of that node.
or
The algebraic sum of currents in a network of conductors meeting at a point is zero.

**broken link removed**

Kirchhoff's voltage law = Kirchhoff's Second Law

The directed sum of the electrical potential differences (voltage) around any closed circuit is zero.
or
More simply, the sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop.
or
The algebraic sum of the products of the resistances of the conductors and the currents
in them in a closed loop is equal to the total emf available in that loop.

**broken link removed**

When to use Kirchoff's laws (simple examples)

Although Kirchoff's laws were used to derive the simple relations for adding resistors in series and parallel,
not all problems can be broken down into such simple pieces. Below are three diagrams which may need to be solved
(e.g. given the resistances and voltages of the batteries, find all the currents).

kirchoff3pic_001.gif

Solution
When to use Kirchoff's laws
 

nxt_

Member level 4
Joined
Aug 16, 2010
Messages
69
Helped
4
Reputation
8
Reaction score
4
Trophy points
1,288
Location
Egypt
Activity points
1,834
lvw i am guessing he is asking about knowing the total current his circuit needs so that he can pickup a suitable power supply for it.
 

Evalyn27

Newbie level 2
Joined
Oct 19, 2011
Messages
2
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,283
Activity points
1,367
Let's start with a series circuit consisting of three resistors and a single battery:



The first principle to understand about series circuits is that the amount of current is the same through any component in the circuit. This is because there is only one path for electrons to flow in a series circuit, and because free electrons flow through conductors like marbles in a tube, the rate of flow (marble speed) at any point in the circuit (tube) at any specific point in time must be equal.

From the way that the 9 volt battery is arranged, we can tell that the electrons in this circuit will flow in a counter-clockwise direction, from point 4 to 3 to 2 to 1 and back to 4. However, we have one source of voltage and three resistances. How do we use Ohm's Law here?

An important caveat to Ohm's Law is that all quantities (voltage, current, resistance, and power) must relate to each other in terms of the same two points in a circuit. For instance, with a single-battery, single-resistor circuit, we could easily calculate any quantity because they all applied to the same two points in the circuit:







Since points 1 and 2 are connected together with wire of negligible resistance, as are points 3 and 4, we can say that point 1 is electrically common to point 2, and that point 3 is electrically common to point 4. Since we know we have 9 volts of electromotive force between points 1 and 4 (directly across the battery), and since point 2 is common to point 1 and point 3 common to point 4, we must also have 9 volts between points 2 and 3 (directly across the resistor). Therefore, we can apply Ohm's Law (I = E/R) to the current through the resistor, because we know the voltage (E) across the resistor and the resistance (R) of that resistor. All terms (E, I, R) apply to the same two points in the circuit, to that same resistor, so we can use the Ohm's Law formula with no reservation.

However, in circuits containing more than one resistor, we must be careful in how we apply Ohm's Law. In the three-resistor example circuit below, we know that we have 9 volts between points 1 and 4, which is the amount of electromotive force trying to push electrons through the series combination of R1, R2, and R3. However, we cannot take the value of 9 volts and divide it by 3k, 10k or 5k Ω to try to find a current value, because we don't know how much voltage is across any one of those resistors, individually.



The figure of 9 volts is a total quantity for the whole circuit, whereas the figures of 3k, 10k, and 5k Ω are individual quantities for individual resistors. If we were to plug a figure for total voltage into an Ohm's Law equation with a figure for individual resistance, the result would not relate accurately to any quantity in the real circuit.

For R1, Ohm's Law will relate the amount of voltage across R1 with the current through R1, given R1's resistance, 3kΩ:



But, since we don't know the voltage across R1 (only the total voltage supplied by the battery across the three-resistor series combination) and we don't know the current through R1, we can't do any calculations with either formula. The same goes for R2 and R3: we can apply the Ohm's Law equations if and only if all terms are representative of their respective quantities between the same two points in the circuit.

So what can we do? We know the voltage of the source (9 volts) applied across the series combination of R1, R2, and R3, and we know the resistances of each resistor, but since those quantities aren't in the same context, we can't use Ohm's Law to determine the circuit current. If only we knew what the total resistance was for the circuit: then we could calculate total current with our figure for total voltage (I=E/R).

This brings us to the second principle of series circuits: the total resistance of any series circuit is equal to the sum of the individual resistances. This should make intuitive sense: the more resistors in series that the electrons must flow through, the more difficult it will be for those electrons to flow. In the example problem, we had a 3 kΩ, 10 kΩ, and 5 kΩ resistor in series, giving us a total resistance of 18 kΩ:



In essence, we've calculated the equivalent resistance of R1, R2, and R3 combined. Knowing this, we could re-draw the circuit with a single equivalent resistor representing the series combination of R1, R2, and R3:



Now we have all the necessary information to calculate circuit current, because we have the voltage between points 1 and 4 (9 volts) and the resistance between points 1 and 4 (18 kΩ):



Knowing that current is equal through all components of a series circuit (and we just determined the current through the battery), we can go back to our original circuit schematic and note the current through each component:



Now that we know the amount of current through each resistor, we can use Ohm's Law to determine the voltage drop across each one (applying Ohm's Law in its proper context):



Notice the voltage drops across each resistor, and how the sum of the voltage drops (1.5 + 5 + 2.5) is equal to the battery (supply) voltage: 9 volts. This is the third principle of series circuits: that the supply voltage is equal to the sum of the individual voltage drops.

However, the method we just used to analyze this simple series circuit can be streamlined for better understanding. By using a table to list all voltages, currents, and resistances in the circuit, it becomes very easy to see which of those quantities can be properly related in any Ohm's Law equation:



The rule with such a table is to apply Ohm's Law only to the values within each vertical column. For instance, ER1 only with IR1 and R1; ER2 only with IR2 and R2; etc. You begin your analysis by filling in those elements of the table that are given to you from the beginning:



As you can see from the arrangement of the data, we can't apply the 9 volts of ET (total voltage) to any of the resistances (R1, R2, or R3) in any Ohm's Law formula because they're in different columns. The 9 volts of battery voltage is not applied directly across R1, R2, or R3. However, we can use our "rules" of series circuits to fill in blank spots on a horizontal row. In this case, we can use the series rule of resistances to determine a total resistance from the sum of individual resistances:



Now, with a value for total resistance inserted into the rightmost ("Total") column, we can apply Ohm's Law of I=E/R to total voltage and total resistance to arrive at a total current of 500 µA:



Then, knowing that the current is shared equally by all components of a series circuit (another "rule" of series circuits), we can fill in the currents for each resistor from the current figure just calculated:



Finally, we can use Ohm's Law to determine the voltage drop across each resistor, one column at a time:



Just for fun, we can use a computer to analyze this very same circuit automatically. It will be a good way to verify our calculations and also become more familiar with computer analysis. First, we have to describe the circuit to the computer in a format recognizable by the software. The SPICE program we'll be using requires that all electrically unique points in a circuit be numbered, and component placement is understood by which of those numbered points, or "nodes," they share. For clarity, I numbered the four corners of our example circuit 1 through 4. SPICE, however, demands that there be a node zero somewhere in the circuit, so I'll re-draw the circuit, changing the numbering scheme slightly:



All I've done here is re-numbered the lower-left corner of the circuit 0 instead of 4. Now, I can enter several lines of text into a computer file describing the circuit in terms SPICE will understand, complete with a couple of extra lines of code directing the program to display voltage and current data for our viewing pleasure. This computer file is known as the netlist in SPICE terminology:



series circuit
v1 1 0
r1 1 2 3k
r2 2 3 10k
r3 3 0 5k
.dc v1 9 9 1
.print dc v(1,2) v(2,3) v(3,0)
.end


Now, all I have to do is run the SPICE program to process the netlist and output the results:



v1 v(1,2) v(2,3) v(3) i(v1)
9.000E+00 1.500E+00 5.000E+00 2.500E+00 -5.000E-04


This printout is telling us the battery voltage is 9 volts, and the voltage drops across R1, R2, and R3 are 1.5 volts, 5 volts, and 2.5 volts, respectively. Voltage drops across any component in SPICE are referenced by the node numbers the component lies between, so v(1,2) is referencing the voltage between nodes 1 and 2 in the circuit, which are the points between which R1 is located. The order of node numbers is important: when SPICE outputs a figure for v(1,2), it regards the polarity the same way as if we were holding a voltmeter with the red test lead on node 1 and the black test lead on node 2.

We also have a display showing current (albeit with a negative value) at 0.5 milliamps, or 500 microamps. So our mathematical analysis has been vindicated by the computer. This figure appears as a negative number in the SPICE analysis, due to a quirk in the way SPICE handles current calculations.

In summary, a series circuit is defined as having only one path for electrons to flow. From this definition, three rules of series circuits follow: all components share the same current; resistances add to equal a larger, total resistance; and voltage drops add to equal a larger, total voltage. All of these rules find root in the definition of a series circuit. If you understand that definition fully, then the rules are nothing more than footnotes to the definition.

REVIEW:
Components in a series circuit share the same current: ITotal = I1 = I2 = . . . In
Total resistance in a series circuit is equal to the sum of the individual resistances: RTotal = R1 + R2 + . . . Rn
Total voltage in a series circuit is equal to the sum of the individual voltage drops: ETotal = E1 + E2 + . . . En
 
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top