# Why should you use two resistors in parallel on an LED?

So I was looking over the Arduino R3 schematics and noticed this little design choice:

What is the reason for doing something like this? I mean it's hard to know what the designers were thinking, but maybe it was done this way to save space. Do you get any other benefits?

• If you already have lots of those 4-pack 1k smd resistors on your board then paralleling a spare pair is more cost-effective than having a single 500R and avoids an extra line on your BOM. – brhans Dec 16 '15 at 17:29
• In fact, even if you didn't have a spare pair it might still be more cost-effective to drop a whole new 4-pack and only use half of it - just to avoid the extra BOM line ... – brhans Dec 16 '15 at 17:34
• /me too @Passerby. I was tempted to ask for a link to the complete circuit diagram. Using an existing component from the BOM also saves a reel on the pick and place machine, which may just be what you need to use a cheaper assembly line. – jippie Dec 16 '15 at 17:44
• We finally got a decent picture of a small part! +1 – Olin Lathrop Dec 16 '15 at 18:36
• @mkeith, an additional BOM line can be relatively expensive if you have to deal with minimum order quantities (buying full reels of components) or if your pick & place machine is full so that adding a BOM line means having to run an extra machine setup and pick & place process. – The Photon Dec 16 '15 at 19:07

Don't look to the arduino designs as examples of stellar electrical engineering.

However, there can be a legitimate case for doing this. This part contains 4 resistors. If it was already there for another reason, especially if several more of them are used on the same board, then using two of the resistors that would otherwise go unused in parallel to make a 500 Ω resistor is a reasonable thing to do.

It can often save more money overall to use fewer different parts, than a smaller number of total parts but more different ones. For cheap parts like resistors, the dominant cost is not the price of the part, but the cost of purchasing, stocking, setting up the pick and place machine, etc.

• +1 just for the first line: Don't look to the arduino designs as examples of stellar electrical engineering. – Connor Wolf Dec 16 '15 at 19:02
• If it was already there for another reason.. .. reads like a guess, not an answer. Perhaps an edit for clarity? It's a fact: 8 x 1kΩ with next nearest values at 22Ω and 10kΩ per arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf The only conjecture is whether the parallel-1kΩ choice was originally motivated by BOM optimisation, but that seems a pretty safe bet in this case. Yes I chuckled at the first line too, but I wouldn't +1 it! Off topic and contradictory .. unless the intention is to imply that BOM optimisation is not "stellar electrical engineering"? – tardate Dec 17 '15 at 22:39
• @Olin Are there any specific reasons why the Arduino pcbs aren't stellar designs? I mean, I know they're really basic from an EE perspective, but in general they are pretty well implemented, no? – 0xDBFB7 Dec 19 '15 at 13:41
• @dc1: The few times someone has posted arduino schematics here, I was not impressed. For example electronics.stackexchange.com/q/24486/4512. However, this is a different question. If you want a critique of a arduino schematic, post one in a question and ask about that. – Olin Lathrop Dec 19 '15 at 19:04
• @Olin Man, the schematic posted is pretty dang messy. I guess I had a false image of how well Arduinos were designed. – 0xDBFB7 Dec 19 '15 at 21:33

For the same reasons as Olin mentioned, using two distinctly seperate resistors in parallel can be a saving if those resistors are used elsewhere on the PCB. Line items in the build need to be stocked and counted and there is a real annual cost for this.

Additional to all the valid point given already, another reason could be a better heat dissipation.

Since these resitors are limiting current on a LED they could probably get relatively hot, given their small size. In this case the power they absorb is around 20mW.

Having two items instead of one allows for better heat dissipation.

• This is wrong. The power dissipated by a 500 Ohm resistor in series with a LED (about 2V drop) cannot exceed 30mW. – Dmitry Grigoryev Dec 17 '15 at 9:39
• Thanks @DmitryGrigoryev to do the math correctly. I fixed my answer. – Marcel Dec 17 '15 at 9:52
• They don't absorb 20mW. They emit 20mW. And that is unlikely to make them hot. – abligh Dec 17 '15 at 18:51

Historically, using two resistors in parallel can give you a resistance which is not available in a standard package. In this case, two 1K resistors gives 500 ohms. A the nearest 'standard' single resistor is 470 ohms.

This may not be as relevant today as it once was, but was a way to obtain very specific resistances.

• Mmmh, I doubt the value of the limiting resistor for that LED was so critical as to determine that design decision. – Lorenzo Donati -- Codidact.com Dec 16 '15 at 21:14

I asked this myself an engineer doing layout reviews. He gave me three answers, from which two were already mentioned here:

1. Distribution of heat dissipation
2. Reuse of known, proven, qualified, used, ... components
3. New: Increase of reliability for the case one component fails. However, I do not know if this applies in this case.
• I don't think the increase in realiability applies to this case. In fact, reliability is a much more broad and complicated subject (unfortunately), than merrily duplicating components... – jose.angel.jimenez Dec 17 '15 at 10:47
• Reliability could actually be worse, if you consider short-circuit kinds of failures. – Dmitry Grigoryev Dec 17 '15 at 11:00
• More parts to fail means you're more likely to have a failure... However, it could limit damage... – Brian Knoblauch Dec 17 '15 at 12:15