# Does order ever matter in a series circuit?

Discussion in 'General Electronics Discussion' started by renzokuken, Nov 6, 2012.

1. ### renzokuken

2
0
Apr 26, 2011
My professor emphasizes that we must power a LED with a resistor "before" it and he draws the example as follows,
5vcc ----/\/\/\/\/----|>----GND

He explicitly said that the LED cannot be before the resistor or else it would not work and draws,
5vcc ----|>----/\/\/\/\/----GND

I was hesitant to ask him why because he expected the class to all know why and made it seem like it was trivial.

From my previous understanding, I thought the order did not matter in a series circuit.
The LED will receive the same voltage drop and current in both configurations.
I want to simply dismiss what my professor emphasized about the importance of the resistor needing to be placed before the resistor.
But I need to be sure, is there ever a time where order matters in a series circuit?

The class I'm taking is a combinational and sequential logic class if it makes any difference.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,449
2,809
Jan 21, 2010
Either your professor is wrong, or you are wrong in the way you draw the circuit.

For the circuit you have given it doesn't matter at all.

Does it ever make a difference, yes. But in those cases there is some connection to the node between them or a third (possibly controlling) electrode to one of them.

3. ### BobK

7,682
1,686
Jan 5, 2010
Hand your professor a sealed box with an LED and resistor in series and a + and - lead coming out and ask him to determine whether the + lead is connected to the LED or to the resistor.

Bob

4. ### Laplace

1,252
184
Apr 4, 2010
Maybe your professor was fixated on an LED array like a 7-segment display in a common cathode configuration. The common cathode would be grounded and each anode would be connected to its individual resistor/driver. But if he was thinking common cathode, wouldn't he also realize there are common anode configurations?

5. ### CocaCola

3,635
5
Apr 7, 2012
Not a rule there either, if you are multiplexing and rotating though each segment one at at time it doesn't matter...

I contribute this to faulty teaching practices where teachers teach their personal 'preferences' as rules or laws... When I was in high school I used to poke at those teachers on purpose to add some excitement to the mundane... I had a teacher comment on my spelling of gray when I spelling it using the UK standard grey format... From that point on with each assignment I purposely used UK standard spellings in anything I did for that teacher She never commented on it again, but I suspect it rubbed her just the same...