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Author Topic: Dont understand the meaning of series circuit  (Read 860 times)
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They are teaching me that in a seires circuit the current through each component is the same but that makes no sense to me.

If my power source can only source 200ma and  i hook up 10 LEDs each operating at , just for argument sake , 100mA , then I will not light more then 2 right?


And the picture below is a series circuit with CURRENT LIMITING RESISTORS , so does a resistor not LIMIT current hence the current after the resistor will be less? Like 240ohm resistor before an led so it doesnt ruin it...
I just dont understand how they can say the current is the same :/
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Your leds may be able to draw a certain amount of current but that doesn't mean they're going to get that much.
And then the little buggers ain't so bright any more!

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They are teaching me that in a seires circuit the current through each component is the same but that makes no sense to me.


 They are teaching you correctly. Think of current as water flowing in a series of connected hoses, and you have a flowmeter you can insert anywhere in the line, would it not measure the same water flow quantity no matter where you measure it ?  How could current be less in one spot but more then another spot in a simple series circuit. Recall the total current source leaves one terminal of the battery flows through the circuit and returns to the other terminal of the battery, you can't have more or less current leaving the battery then returns to it. The key is that current flowing through a component is not 'lost' or 'consumed'  in the component, the current flow just continues on it's way back to the voltage source.

 Perhaps reviewing the subject from different sources will make the light turn on in your head.


http://en.wikipedia.org/wiki/Series_and_parallel_circuits
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The series current limiting resistor limits the current to the whole circuit.

You could put the resistor after the LED and it would still protect it.
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yeah i got the picture , its like all the components are one big resistor in which case the current through one resistor can only be one.
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LEDs
If my power source can only source 200ma and  i hook up 10 LEDs each operating at , just for argument sake , 100mA , then I will not light more then 2 right?

You're taking a very simple concept and adding a non-simple device (LED).  Which is probably part of your confusion.

When LEDs have enough voltage dropped across them (forward voltage), they turn into a short circuit.  So LEDs don't "take 100mA" or any current for that matter. 

Resistors are linear devices.  When a given voltage is dropped across them, a circuit amount of current will flow through them.  LEDs are not linear in this sense.

Instead, how much current flows in a circuit with a 100ohm, 200ohm, and 300ohm resistor in series?  How much current flows through each resistor... 
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Another way of looking at this is as follows.  Lets say that you have a functioning circuit consisting an LED and a series resistor.  You would like to know the LED current but you do not have an ammeter.  You can measure the voltage across the resistor and also knowing the resistance you can use Ohm's law to calculate the current through the resistor.  Since this is a series circuit you know that the same current is flowing through the LED. 

Add another LED in series and the technique is the same.  Measure the voltage across the resistor, calculate the current through the resistor, and this is also the value of the current through each of the LEDs.

If you have two LEDs and two resistors all in series (in any order) measure the voltage across either resistor, divide by the resistance of that resistor, and this will be the current through each of the four components.


Don
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measure the voltage across either resistor, divide by the resistance of that resistor, and this will be the current through each of the four components.

Don

The how-to part. This should work with any number of parts in serial, yes?
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The how-to part. This should work with any number of parts in serial, yes?

Depends on what you mean by "parts". For any series wired resistors it will. However if there are LEDs in the series string their voltage drop remains constant regardless of the actual forward current flowing through them.

Lefty

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Is that also true for other diodes?
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http://en.wikipedia.org/wiki/File:Diode-IV-Curve.svg

That's a picture of the current-voltage graph of a diode - how much current passes through the diode for a specific amount of voltage (or the opposite - the voltage drop per amount of current passing through it!) Any diode (even an LED) has a graph that looks like that, but with different scales. For a resistor, the graph would look like a straight line. Ohm's law just states that this line is linear, and the slope varies with the resistance.

Diodes, on the other hand, are not linear. Anywhere from Vbr to Vd (on the picture) means pretty much no current is passing through (note: Vbr is usually a huge negative voltage, like 500V. That means the diode doesn't act one-way anymore at about 500V. Vd is around 3.3V for an LED and around 0.7V for a silicon diode). When your voltage gets above Vd, the LED "takes up" less than the "available" voltage and then the resistor uses up the rest.

The nearly vertical line after Vd is why you need the resistor when using an LED. If you fed it a voltage without any resistor, either very little current would pass through and not light it up (brightness depends on current times voltage), or a huge amount of current would pass through and heat it up enough to burn it out. It is *possible* to get the voltage exactly correct to have it light up perfectly, but manufacturing defects on the LED and slight voltage differences on the battery would make that far too risky.
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The how-to part. This should work with any number of parts in serial, yes?
Yes

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Depends on what you mean by "parts". For any series wired resistors it will. However if there are LEDs in the series string their voltage drop remains constant regardless of the actual forward current flowing through them.
It doesn't matter what is in series, there could be marshmallows (as long as they conduct electricity), if you determine the current through any of the resistors using Ohm's law for that resistor then you also know the current through the other parts, including the marshmallows.

Don
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It doesn't matter what is in series, there could be marshmallows (as long as they conduct electricity), if you determine the current through any of the resistors using Ohm's law for that resistor then you also know the current through the other parts, including the marshmallows.


No argument there, my point was you can't calculate the series circuit current by just measuring the voltage drop across a series LED and then using ohms law, you must use the voltage drop measured across a series resistor as it is a linear device while an LED is not.

Lefty

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Thanks for the lesson though, it should save work determining series current and voltage.


 
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