Electricity: Help with basic flow question

by EricBrian on Tue Aug 07, 2012 6:30 pm

In this setup, there is a battery to resistor to LED to battery. Can I put the resistor after the LED? And, if so, does that generally hold true for other things? Can the resistor be placed on the ground wire instead of the hot wire?

Thanks.
Eric
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Re: Electricity: Help with basic flow question

by franklin97355 on Tue Aug 07, 2012 9:31 pm

Yes, but you are posting in the wrong forum. Check the forum rules above.

[Edit - moderator - thread moved to General Project Help]
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Re: Electricity: Help with basic flow question

by adafruit_support_mike on Wed Aug 08, 2012 3:11 pm

As above, the short answer is 'yes'. The more general answer takes a bit of vocabulary:

First, components fall into two broad categories called 'passive' and 'active'..

Passive components are things like resistors, capacitors, and inductors. They generally have two 'terminals' (places where you can connect them to something else), and 'what you get out' is a function of 'what you put in'. We often call one terminal the 'input' and the other the 'output' to make circuit descriptions flow better.

Active components are more complicated, and usually contain transistors. They generally have at least three terminals, and what happens between terminals A and B depends on what's happening at terminal C.

Second, there's a design abstraction called a 'node'.. It's how we describe connections between components. All component terminals connected to the same node see the same voltage, and the current going out of a node always equals the current coming in.

Third, circuits fall into the general categories of 'series' and 'parallel'..

In series circuits, only two component terminals connect to any node.. "Vcc-R1-N1 ; N1-R2-N2 ; N2-R3-GND" describes the connections in a series circuit (Vcc and GND count as nodes).

In parallel circuits, there's a node that connects three or more terminals.. "Vcc-R1-N1 ; N1-R2-GND ; N1-R3-GND" describes the connections in a parallel circuit.

Finally, there's another design abstraction called the 'load'. We often break designs into chunks, and arrange things so chunk 1 does something interesting at a specific node (call it Nout). The load is an imaginary circuit that has one terminal connected to Nout and the other terminal connected to GND. Understanding what Nout does when you connect it to different loads helps you understand the circuit.

Thing is, the load almost always sits in parallel with some other part of the circuit.

So.. with all that under your belt, you'll understand what I mean when I say:

Passive components in series can be rearranged in any order without changing the behavior at the final node of the chain. If you have:

- N.in-R1-N1 ; N1-R2-N2 ; N2-C1-N.out
- N.in-C1-N1 ; N1-R2-N2 ; N2-R1-N.out
- N.in-R2-N1 ; N1-C1-N2 ; N2-R1-N.out

you won't be able to tell them apart if all you can do is send signals into N.in and measure the response at N.out.

That only works for passives in series though. If you use active components, put components in parallel, or connect a load to a node somewhere in the middle of the chain, the order of the components starts to matter.

You can argue over whether diodes (including LEDs) are active or passive components in general, but 'resistor and LED in series' uses the LED like it was passive. So yes, you can put the resistor above the LED or the LED above the resistor and have things work.
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Re: Electricity: Help with basic flow question

by EricBrian on Thu Aug 09, 2012 7:48 am

Wow, thanks for your post mstone. Too bad this forum doesn't allow one to give others Karma points. You'd get them from me. :) Again, thank you.
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Re: Electricity: Help with basic flow question

by adafruit_support_mike on Thu Aug 09, 2012 10:36 am

Heh.. I'm a compulsive explainer. ;-) Thank you, and I'm glad the information helped.
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