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Voltage distrubution in a circuit 
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#1
Apr2512, 02:33 AM

P: 645

Hi guys, I came across a pretty weird question regarding electricity in a parallel circuit. There are two branches of the circuit with a resistor and a light dependant resistor on each branch. The weird part is there a wire is connected in between the resistor and LDR for both branches which is then connected to a logic box. So the logic box has 2 input wires but only one output wire which is also connected to a bulb, lastly it is connected to the neutral wire leading back to the power source. So I'm not sure how the voltage will be distributed amongst the two resistor, LDRs, logic box and bulb. I'm not sure how will the voltage will be divided once the current in the two branches meet the wire with the logic gate. Thanks for the help guys!



#2
Apr2512, 05:34 AM

P: 10

I hope I managed to follow you. Maybe a link with the circuit would help. If I understand correctly than you're asking why there's another resistor connected to the LDR.
The reason the LDR is connected in series with a resistor is to limit the current that reaches the input. The distibution of voltage depends on the situation and the LDR's resistance at the time. The voltage in parallel is equal and the sum of the currents in parallel should equal the initial current. Without the parameters you can't predict the initial voltage distribution. 


#3
Apr2512, 06:11 AM

P: 645

diagram: http://postimage.org/image/xg3p2s8xn/ (see the picture before reading this) That being said, what will happen if i connect two cells in parallel but one cell has 10V and the other 5V? What will happen if something like this happens? Thanks for the help! 


#4
Apr2512, 06:43 AM

P: 10

Voltage distrubution in a circuit
Hi sgstudent,
The voltage on a resistor or LDR can change. In other words  the distribution doesn't have to be the same for the resistors in parallel. If you add an identical resistor  it won't have the same voltage distribution as the others. The current through the resistors doesn't have to be the same. Their sum must equal the original current. The logic box can decide what voltage it requires for a logical True or false. The cutoff is usually 1V, 3V or 5V. The logic box would decide what combination of True and Falses will light the bulb. It's kind of hard to imagine a state where one LDR would receive more light therefore altering the resistance ratio with the resistor but it exists. Some appliances use this fact to shift the angle. The difference in ratio would result in a difference of logical inputs. Lets look at the case where one LDR receives an extreme amount of light while the other none. Its resistance would turn to a small fraction of the orginal resistance. The series' resistance would change to R + frac of LDR. Most of the current would choose to pass through this path. The potential at the end of the path would be a logical one. There are a few circuit simulation sites that could be useful. I've tried a few and I think they will help you out. Just try out a few combinations and see what happens. Good Luck :) 


#5
Apr2512, 07:15 AM

P: 645

thanks for the help! 


#6
Apr2512, 06:00 PM

P: 645




#7
Apr2612, 07:52 AM

HW Helper
Thanks
P: 5,142

It is not possible to say what the voltage measured at the terminals of the batteries would be, as the calculations require details of the electrical characteristic of the cells when experiencing high currents and in the directions as would arise here. Though we can be sure 5 < V_{T}<10V 


#8
Apr2612, 08:07 PM

P: 645




#9
Apr2712, 02:37 AM

P: 10

I think that when you connect to a logic box  you don't care what voltage you get exactly. What you care about is getting a logical one or logical zero. BTW, what kind of bulb is connected to the logic box? Is there a separate VCC connected to it allowing it to turn on the light? I think the logic box supplies a different VCC for the bulb because without an amplifier, it would be difficult to light the bulb.



#10
Apr2712, 03:05 AM

P: 645




#11
Apr2712, 03:24 AM

P: 10

Yeah, I understood what you meant but I think there's more to the logic box than logic. A simple LED might turn it into a simpler circuit.
http://www.allaboutcircuits.com/vol_1/chpt_5/3.html This site explains a bit about circuit in series and in parallel. After you read that  you should read a bit about changeable resistance. 


#12
Apr2712, 03:35 AM

P: 645




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