Can I use a single resistor for multiple LED with different +ve sources?












1












$begingroup$


In the photo below I would like to use a single resistor after the LEDs, instead of having individual resistors before the LEDs. Is this problematic? Will it work?



enter image description here






arduino microcontroller led







share|improve this question









$endgroup$












  • $begingroup$
    No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
    $endgroup$
    – Unimportant
    40 mins ago






  • 1




    $begingroup$
    Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
    $endgroup$
    – Sunnyskyguy EE75
    32 mins ago
















1












$begingroup$


In the photo below I would like to use a single resistor after the LEDs, instead of having individual resistors before the LEDs. Is this problematic? Will it work?



enter image description here






arduino microcontroller led







share|improve this question









$endgroup$












  • $begingroup$
    No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
    $endgroup$
    – Unimportant
    40 mins ago






  • 1




    $begingroup$
    Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
    $endgroup$
    – Sunnyskyguy EE75
    32 mins ago














1












1








1





$begingroup$


In the photo below I would like to use a single resistor after the LEDs, instead of having individual resistors before the LEDs. Is this problematic? Will it work?



enter image description here






arduino microcontroller led







share|improve this question









$endgroup$




In the photo below I would like to use a single resistor after the LEDs, instead of having individual resistors before the LEDs. Is this problematic? Will it work?



enter image description here






arduino microcontroller led




arduino microcontroller led






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 43 mins ago









DonPDonP

257




257












  • $begingroup$
    No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
    $endgroup$
    – Unimportant
    40 mins ago






  • 1




    $begingroup$
    Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
    $endgroup$
    – Sunnyskyguy EE75
    32 mins ago


















  • $begingroup$
    No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
    $endgroup$
    – Unimportant
    40 mins ago






  • 1




    $begingroup$
    Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
    $endgroup$
    – Sunnyskyguy EE75
    32 mins ago
















$begingroup$
No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
$endgroup$
– Unimportant
40 mins ago




$begingroup$
No, there are tolerances inbetween one LED and another. The LED with the lowest Vf will carry a lot more current then the others.
$endgroup$
– Unimportant
40 mins ago




1




1




$begingroup$
Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
$endgroup$
– Sunnyskyguy EE75
32 mins ago




$begingroup$
Looks like Diode OR logic ;) but problematic due to sharing current and voltage drop.
$endgroup$
– Sunnyskyguy EE75
32 mins ago










2 Answers
2






active

oldest

votes


















4












$begingroup$

You can do that safely if:




  • Only one LED is on at a time.

  • The voltage of the circuit is less than the maximum reverse voltage of the LEDs. This often isn't specified by 5 V is generally quite safe.


Otherwise switching two or more on at a time connects them in parallel. In this situation the one with the lowest forward voltage will pass the most current. If they are different colour LEDs then the one with the lowest forward voltage, Vf, will be quite bright and the others relatively dim.



enter image description here



Figure 1. Variations in Vf with different colours of LEDs. Source: LED I-V curves.



enter image description here



Figure 2. Variations in Vf for the same type of LED due to variations in manufacture. Source: Variations in Vf and binning.




As you reminded me in the comments, I forgot to cover the current sharing aspect. The voltage at the top of R1 will be reasonably constant (because the voltage drop across the LEDs would be reasonably similar) so the current through R1 will be constant if one or more LEDs is on. That means that the current will be shared between the LEDs - fairly evenly if they're all the same colour but not so evenly if not.






share|improve this answer











$endgroup$













  • $begingroup$
    So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
    $endgroup$
    – DonP
    19 mins ago










  • $begingroup$
    Correct, and I completely forgot to cover that in the answer. I'll add it in.
    $endgroup$
    – Transistor
    17 mins ago












  • $begingroup$
    OK. Will mark as answer. THanks for the detailed response.
    $endgroup$
    – DonP
    16 mins ago



















0












$begingroup$

It's problematic because nothing is ever identical so the LED with the lowest voltage drop will cause all the current to flow through it while the other two LEDs never turn on because the voltage drop across them isn't high enough.






share|improve this answer









$endgroup$













  • $begingroup$
    even though they have different +ve signal source?
    $endgroup$
    – DonP
    24 mins ago










  • $begingroup$
    As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
    $endgroup$
    – Toor
    17 mins ago













Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["\$", "\$"]]);
});
});
}, "mathjax-editing");

StackExchange.ifUsing("editor", function () {
return StackExchange.using("schematics", function () {
StackExchange.schematics.init();
});
}, "cicuitlab");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "135"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f424033%2fcan-i-use-a-single-resistor-for-multiple-led-with-different-ve-sources%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









4












$begingroup$

You can do that safely if:




  • Only one LED is on at a time.

  • The voltage of the circuit is less than the maximum reverse voltage of the LEDs. This often isn't specified by 5 V is generally quite safe.


Otherwise switching two or more on at a time connects them in parallel. In this situation the one with the lowest forward voltage will pass the most current. If they are different colour LEDs then the one with the lowest forward voltage, Vf, will be quite bright and the others relatively dim.



enter image description here



Figure 1. Variations in Vf with different colours of LEDs. Source: LED I-V curves.



enter image description here



Figure 2. Variations in Vf for the same type of LED due to variations in manufacture. Source: Variations in Vf and binning.




As you reminded me in the comments, I forgot to cover the current sharing aspect. The voltage at the top of R1 will be reasonably constant (because the voltage drop across the LEDs would be reasonably similar) so the current through R1 will be constant if one or more LEDs is on. That means that the current will be shared between the LEDs - fairly evenly if they're all the same colour but not so evenly if not.






share|improve this answer











$endgroup$













  • $begingroup$
    So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
    $endgroup$
    – DonP
    19 mins ago










  • $begingroup$
    Correct, and I completely forgot to cover that in the answer. I'll add it in.
    $endgroup$
    – Transistor
    17 mins ago












  • $begingroup$
    OK. Will mark as answer. THanks for the detailed response.
    $endgroup$
    – DonP
    16 mins ago
















4












$begingroup$

You can do that safely if:




  • Only one LED is on at a time.

  • The voltage of the circuit is less than the maximum reverse voltage of the LEDs. This often isn't specified by 5 V is generally quite safe.


Otherwise switching two or more on at a time connects them in parallel. In this situation the one with the lowest forward voltage will pass the most current. If they are different colour LEDs then the one with the lowest forward voltage, Vf, will be quite bright and the others relatively dim.



enter image description here



Figure 1. Variations in Vf with different colours of LEDs. Source: LED I-V curves.



enter image description here



Figure 2. Variations in Vf for the same type of LED due to variations in manufacture. Source: Variations in Vf and binning.




As you reminded me in the comments, I forgot to cover the current sharing aspect. The voltage at the top of R1 will be reasonably constant (because the voltage drop across the LEDs would be reasonably similar) so the current through R1 will be constant if one or more LEDs is on. That means that the current will be shared between the LEDs - fairly evenly if they're all the same colour but not so evenly if not.






share|improve this answer











$endgroup$













  • $begingroup$
    So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
    $endgroup$
    – DonP
    19 mins ago










  • $begingroup$
    Correct, and I completely forgot to cover that in the answer. I'll add it in.
    $endgroup$
    – Transistor
    17 mins ago












  • $begingroup$
    OK. Will mark as answer. THanks for the detailed response.
    $endgroup$
    – DonP
    16 mins ago














4












4








4





$begingroup$

You can do that safely if:




  • Only one LED is on at a time.

  • The voltage of the circuit is less than the maximum reverse voltage of the LEDs. This often isn't specified by 5 V is generally quite safe.


Otherwise switching two or more on at a time connects them in parallel. In this situation the one with the lowest forward voltage will pass the most current. If they are different colour LEDs then the one with the lowest forward voltage, Vf, will be quite bright and the others relatively dim.



enter image description here



Figure 1. Variations in Vf with different colours of LEDs. Source: LED I-V curves.



enter image description here



Figure 2. Variations in Vf for the same type of LED due to variations in manufacture. Source: Variations in Vf and binning.




As you reminded me in the comments, I forgot to cover the current sharing aspect. The voltage at the top of R1 will be reasonably constant (because the voltage drop across the LEDs would be reasonably similar) so the current through R1 will be constant if one or more LEDs is on. That means that the current will be shared between the LEDs - fairly evenly if they're all the same colour but not so evenly if not.






share|improve this answer











$endgroup$



You can do that safely if:




  • Only one LED is on at a time.

  • The voltage of the circuit is less than the maximum reverse voltage of the LEDs. This often isn't specified by 5 V is generally quite safe.


Otherwise switching two or more on at a time connects them in parallel. In this situation the one with the lowest forward voltage will pass the most current. If they are different colour LEDs then the one with the lowest forward voltage, Vf, will be quite bright and the others relatively dim.



enter image description here



Figure 1. Variations in Vf with different colours of LEDs. Source: LED I-V curves.



enter image description here



Figure 2. Variations in Vf for the same type of LED due to variations in manufacture. Source: Variations in Vf and binning.




As you reminded me in the comments, I forgot to cover the current sharing aspect. The voltage at the top of R1 will be reasonably constant (because the voltage drop across the LEDs would be reasonably similar) so the current through R1 will be constant if one or more LEDs is on. That means that the current will be shared between the LEDs - fairly evenly if they're all the same colour but not so evenly if not.







share|improve this answer














share|improve this answer



share|improve this answer








edited 11 mins ago

























answered 39 mins ago









TransistorTransistor

85.1k784181




85.1k784181












  • $begingroup$
    So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
    $endgroup$
    – DonP
    19 mins ago










  • $begingroup$
    Correct, and I completely forgot to cover that in the answer. I'll add it in.
    $endgroup$
    – Transistor
    17 mins ago












  • $begingroup$
    OK. Will mark as answer. THanks for the detailed response.
    $endgroup$
    – DonP
    16 mins ago


















  • $begingroup$
    So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
    $endgroup$
    – DonP
    19 mins ago










  • $begingroup$
    Correct, and I completely forgot to cover that in the answer. I'll add it in.
    $endgroup$
    – Transistor
    17 mins ago












  • $begingroup$
    OK. Will mark as answer. THanks for the detailed response.
    $endgroup$
    – DonP
    16 mins ago
















$begingroup$
So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
$endgroup$
– DonP
19 mins ago




$begingroup$
So if switching 2 on at once is connecting them in parallel, this would mean the current drop would roughly double? And the resistor if limiting to 15ma would be shared between the 2 LED and would only see 7.5ma (roughly)?
$endgroup$
– DonP
19 mins ago












$begingroup$
Correct, and I completely forgot to cover that in the answer. I'll add it in.
$endgroup$
– Transistor
17 mins ago






$begingroup$
Correct, and I completely forgot to cover that in the answer. I'll add it in.
$endgroup$
– Transistor
17 mins ago














$begingroup$
OK. Will mark as answer. THanks for the detailed response.
$endgroup$
– DonP
16 mins ago




$begingroup$
OK. Will mark as answer. THanks for the detailed response.
$endgroup$
– DonP
16 mins ago













0












$begingroup$

It's problematic because nothing is ever identical so the LED with the lowest voltage drop will cause all the current to flow through it while the other two LEDs never turn on because the voltage drop across them isn't high enough.






share|improve this answer









$endgroup$













  • $begingroup$
    even though they have different +ve signal source?
    $endgroup$
    – DonP
    24 mins ago










  • $begingroup$
    As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
    $endgroup$
    – Toor
    17 mins ago


















0












$begingroup$

It's problematic because nothing is ever identical so the LED with the lowest voltage drop will cause all the current to flow through it while the other two LEDs never turn on because the voltage drop across them isn't high enough.






share|improve this answer









$endgroup$













  • $begingroup$
    even though they have different +ve signal source?
    $endgroup$
    – DonP
    24 mins ago










  • $begingroup$
    As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
    $endgroup$
    – Toor
    17 mins ago
















0












0








0





$begingroup$

It's problematic because nothing is ever identical so the LED with the lowest voltage drop will cause all the current to flow through it while the other two LEDs never turn on because the voltage drop across them isn't high enough.






share|improve this answer









$endgroup$



It's problematic because nothing is ever identical so the LED with the lowest voltage drop will cause all the current to flow through it while the other two LEDs never turn on because the voltage drop across them isn't high enough.







share|improve this answer












share|improve this answer



share|improve this answer










answered 40 mins ago









ToorToor

72219




72219












  • $begingroup$
    even though they have different +ve signal source?
    $endgroup$
    – DonP
    24 mins ago










  • $begingroup$
    As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
    $endgroup$
    – Toor
    17 mins ago




















  • $begingroup$
    even though they have different +ve signal source?
    $endgroup$
    – DonP
    24 mins ago










  • $begingroup$
    As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
    $endgroup$
    – Toor
    17 mins ago


















$begingroup$
even though they have different +ve signal source?
$endgroup$
– DonP
24 mins ago




$begingroup$
even though they have different +ve signal source?
$endgroup$
– DonP
24 mins ago












$begingroup$
As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
$endgroup$
– Toor
17 mins ago






$begingroup$
As the others have pointed out above, it's okay if you only ever turn on one at a time. You'll run into trouble if you try to turn on more than one at a time.
$endgroup$
– Toor
17 mins ago




















draft saved

draft discarded




















































Thanks for contributing an answer to Electrical Engineering Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid …



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f424033%2fcan-i-use-a-single-resistor-for-multiple-led-with-different-ve-sources%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







-

Popular posts from this blog

Why is a white electrical wire connected to 2 black wires?

Image
1 I just opened my ceiling light to replace it with another mount. I see a white wire connected to 2 black wires, is that right? The middle switch controls this ceiling light. electrical ceiling light share | improve this question asked 4 hours ago konyak konyak 108 4 New contributor konyak is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct. ...
Read more

Waikiki

Image
Denne artikkelen kan ha godt av ein språkvask , som reinskar opp målføringa og/eller innfører same språkstilen overalt. Denne artikkelen kan ha godt av ei opprydding for å nå ein høgare standard og/eller for å verta i tråd med standardoppsettet. Sjå korleis du redigerer ei side og stilmanualen for hjelp. Waikiki Land   USA Stat Hawaii Stad Honolulu Wikimedia Commons: Waikiki Utsyn over Waikiki Waikīkī er ein bydel i Honolulu, sør på øya Oahu, som er ei av Hawaii-øyane. Waikīkīstranda er ein av dei meste berømte strendene i verda. Waikīkī strekkjer seg langs stillehavskysten og har ei rekkje velrenommerte hotell. Bydelen er Hawaii-turismens høyborg. På Waikīkī Beach står ei statue av den lokale sportsstjerna Duka Kahanamoku. Kjelder | Denne artikkelen bygger på «Waikiki» frå Wikipedia på bokmål, den 4. november 2018. Denne artikkelen manglar kjelder eller referansar . Hjelp Wikipedia med å finna truverdige kjelder...
Read more

What are all the squawk codes?

Image
2 $begingroup$ I love aviation and I love learning new things about it. I was wondering if anyone knew all of the squawk codes and in particular what squawk 7500 means. squawk-codes share | improve this question edited 15 mins ago Jamiec ♦ 14.8k 4 60 91 asked 5 hours ago Zac Callaghan Zac Callaghan 11 2 New contributor Zac Callaghan is a new co...
Read more