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I have got a simple circuit, where a relay is used to decide which LED lightens up (and I did a small mistake in the image: The power source is actually 12V, not 1V):

schematic

simulate this circuit – Schematic created using CircuitLab

Now, my question is: Would it be feasible to have two resistors instead of one, if you would link the wires behind the resistors instead of before them? Does this make any difference?

schematic

simulate this circuit

(Of course it does not make sense to use two resistors if it also works with one, but I'm curious ;-))

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  • \$\begingroup\$ "... if you would link the wires behind the resistors instead of before them?" I'm not sure I understand what that means. Can you describe it in other words or draw a second schematic? \$\endgroup\$ – Dan Laks Mar 6 '15 at 8:08
  • \$\begingroup\$ I've updated my question. \$\endgroup\$ – Golo Roden Mar 6 '15 at 8:12
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    \$\begingroup\$ The answer is yes. Both schematics would work equally well. \$\endgroup\$ – Dan Laks Mar 6 '15 at 8:17
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    \$\begingroup\$ I don't see a reason to use a second resistor, at least with the given info. If you have two leds (with identical specs) and only one can be turned on at any given moment then a second resistor doesn't make a difference. On the other hand If your leds are not the same (e.g different color) then a separate resistor makes sense. \$\endgroup\$ – alexan_e Mar 6 '15 at 8:17
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    \$\begingroup\$ Where it does matter is if two LEDs are on at once. Two LEDs in parallel sharing one common series resistor will hav the two LEDs driven unequally as the voltage across each is identical but the CURRENT will divide unequally. How unequally deoends on LED matching and it can be vey poorin some cases. \$\endgroup\$ – Russell McMahon Mar 6 '15 at 10:01
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Both schematics would work equally well. In either position of the relay in both schematics, the current will find one LED and one resistor in series, which will light the LEDs as desired.

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As Dan Laks said, both of your circuits will behave the same, since there will only be one LED on at a time.

Imagine, however, that you had one relay per LED. (or, better yet, a microcontroller that could turn each one on or off independently).

In this case, you'd need two resistors. Otherwise, if you turned on both LEDs at the same time, they'd each be dimmer than when lit individually.

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