How to Get Started with LED and Resistor Mods

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
The most frequent question that people ask me when it comes to LED mods, is "how do you know what resistor(s) to use?"

001Resistors.jpg


With this write up, (on the assumption you are already familiar with soldering) I hope to answer some of those questions and shed some light on the options that are available to people wanting to learn and perform LED mods on their vehicle but aren't sure where to start.

First off, you'll want to figure out the size, color, and shape of the LEDs you want to use. For most of the mods I've seen performed, 5mm is the size of choice. You can also use 3mm, or some of the larger sized bulbs depending on the mod you have in mind. The 2 dominant bulb shapes are round tops, and flat tops. Flat tops tend to have a wider beam pattern, so I recommend those. The best deals are usually Ebay vendors in China. The downside is the wait time for your package. Pictured here, the longer leg is the positive lead, the shorter one is the negative.

001RoundTopLED.jpg
001FlatTopLED.jpg


If you run a quick Google search, you will come up with a pretty good list of LED resistor calculators, that will all essentially ask you for the same pieces of information. Supplied voltage (output from your alternator), LED current (the current your LEDs can take) and voltage drop (the voltage the LEDs use)

When you find an Ebay ad (or from any other seller) that suits your needs, scroll down to the Details section and you should see information listed that's similar to this ad from a batch that I purchased. The 2 requirements for an LED resistor calculator have been boxed.

01LEDDetailsP.jpg


Pulling up the resistor calculator, you input the numbers. First example is calculating the resistor needed for this particular LED in a single setup. I have seen my alternator put out 14.5V max, so I always use this number as my supply voltage. From the Ebay ad, it lists the Forward Voltage in a range from 3.2V to 3.8V so I averaged that to 3.5V and enter that. The continuous current is 30 milliamps. Running the calculator spits out the exact resistance (ohms and wattage) and then a rounded value.

02SingleResistorCalculator.jpg


Depending on where you look for resistors, you may find a value available that's close to the calculated one. Anything within the calculated to rounded range would be fine. In this case a 366 ohm 1/3 watt to a 390 ohm 1/2 watt resistor would do the trick.

Taking this one step further, if you have multiple LEDs, you can string them in series (back to back). Doing this allows you to use fewer resistors and conserve space. Generally you can max out at 3 LEDs together before you run out of voltage to supply the circuit. So if we apply the same numbers for a 3 LED set we get the following. Between a 133 ohm 1/8 watt and 150 ohm 1/4 watt resistor.

03SeriesResistorCalculator.jpg


The same logic applies for wiring up the LEDs in parallel with each other, receiving power from the resistor without the voltage drop from another LED in front of it.

04ParallelResistor.jpg


So if you have a 3 LED mod, you can wire them up with any of the above methods. 1 resistor per LED, or 1 lower rated resistor for all 3 LEDs if you wire them like the diagrams. Once you've got a handle on that, you can step your game up and get into LED arrays. Doing this combines the last 2 steps, taking sets of LEDs in series, and wiring those up in parallel with each other. This comes in handy when you're dealing with a large number of LEDs, for instance creating dome light circuit boards.


Here's the cargo light that I made containing 60 blue 5mm flat top LEDs. I took sets of 3 LEDs in series, and wired them in groups of 4 in parallel with each other, and put a resistor in front of that group. Duplicated that 5 times on the circuit board, and connected the pre-resistance lead to the positive source for the light's socket. The location of the resistors is marked by a green star, the red line/arrow indicates the flow of power from the resistor to the beginning of the LED array. The yellow line/arrow indicates the flow of voltage down through each series of 3 set of LEDs, and they join to a common ground which ties to the negative lead for the light.


04LEDArrayP.jpg


The formula for the resistance needed for the array is as follows:

_________1___________
(1/R1)+(1/R2)+(1/R3)+...

where R1, R2, etc is the resistance needed for the set of LEDs in series. So to go back to the previous examples, a set of 3 LEDs in series requires a 150 ohm 1/8 watt resistor. Plugging that into the formula gives you about a 37ohm resistor. To make this a lil easier, I preloaded an Excel sheet into a zip file with this formula, with combinations of up to 10 parallel sets of LEDs in series for anyone to use. It can be tweaked to input different resistance levels, so if you had multiple sets of 3 LEDs in series, some sets of 2 LEDs in series, single LED/resistor combos, or combinations of all of the above, the same logic applies. Whatever resistance needed for that 'in series set', can be entered and the ohm rating will be calculated for you.

If there's anything I missed or overlooked, any of the other experienced LED folks can feel free to chime in and add anything. If anyone has any questions, post em up and we'll try to get them answered and your mod bug properly fed.
 

ItsOnVoy

Member
Nov 21, 2011
2,401
Thanks for the great write up :biggrin:
 

kardain

Member
Dec 16, 2011
557
There are only two resources I use depending on what computer/PED I have access to at the time... either LED series parallel array wizard if I'm at my desktop/laptop, or Electrodroid if I only have my tablet or phone handy.... The first link takes a bit of the guesswork out when doing a 60 bulb array.

Good suggestion with the 14.5 source voltage. Most other sites recommend to resist based off of 12v. However, by resisting off 14.5 it adds a bit of a threshold in case of one LED in your series fusing. The remaining functional LEDs should be able to soak the voltage without popping (personally, I use 14.4, but what's a .1 difference LOL)

Another downside to the China LEDs that should be mentioned is a lax quality control. If using them, it is not uncommon to have at least 10% of the pack be DOA. Before wiring in the LED to your circuit, test it using a 3v button cell battery (there's one in the OEM keyfob if you don't want to buy one). Red/orange are the lowest voltage requirement (2.2v), but a brief test on one of these batteries will not kill it. Also, the button cell is enough voltage to light the remaining spectrum (white requires the highest voltage, 3.6).

Trust me, I've put enough strings together with cheap ebay LEDs just to find later out that a couple were DOA.

I can't think of anything really further to add to an otherwise superb writeup. Then again, it is past my bedtime so thinking isn't on my priority list anyway.

But, I'm available as well to assist with the LED questions :smile:
 

MAY03LT

Member
Nov 18, 2011
3,412
Very nice sir. Thanks!:thumbsup:
 

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
Thanks fellas. I figured there were more ways to get the job done, I found a number of resources when I was trying to figure things out in the spring. But for those who's search-fu isn't quite strong enough to match their desire to get started, maybe laying out some examples will be the push people need to get the ball rolling.

Besides, you can't yell at people to search for something that's not here right? Now we can!! :raspberry:
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
Really appreciate the explanation. May try this myself now that I grasp it a bit better.
 

CdnGMan

Member
Nov 22, 2011
1,393
Toronto
Thanks for that write - up Carlton... very much appreciated...
I'm not sure that I have a complete grip on the calculations, but it does make more sense than it did before I read it. :wink:

Is using a 9V battery acceptable when you're looking to test your LED soldering / wiring skills?
If not, what would you suggest instead?
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
a 9V battery should suffice for testing though it will result in a dimmer glow. You can always get a cheap power supply and use it to test.
 

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
Glad it helped some guys, that was the intent.

Greg, I'm sure Stef is glad as well, since this could possibly mean something coming off his "Stef do" list instead of putting something on it :dielaugh: I agree with Larry, I use an old computer power supply with some alligator clips, as my test bench, since I happened to have one laying around in an old parts bin. If you want to go that route, let me know and I'll tell you how to tweak it so that works for you. But a 9V battery would work just to make sure your connections are good, and voltage is flowing through your circuit. :thumbsup:
 

kardain

Member
Dec 16, 2011
557

Freddy G.

Member
Dec 4, 2011
55
I'd much rather pay for the shipping to and from and pay you for the work. Gonna need your address. lol
 

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
Freddy G. said:
I'd much rather pay for the shipping to and from and pay you for the work. Gonna need your address. lol

:raspberry: I know you would. But you'll be much happier once you do it yourself, and see how easy it really is.

:offtopic: That's also why any meets up here are never near my house, just in case you decide to finally come to one.
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
Maybe if you also add the link to the calculator you used so that they have it for direct reference it may help others. You know someone will ask or say the one they are using looks nothing like those pictured.

Blckshdw said:
Glad it helped some guys, that was the intent.

Greg, I'm sure Stef is glad as well, since this could possibly mean something coming off his "Stef do" list instead of putting something on it :dielaugh: I agree with Larry, I use an old computer power supply with some alligator clips, as my test bench, since I happened to have one laying around in an old parts bin. If you want to go that route, let me know and I'll tell you how to tweak it so that works for you. But a 9V battery would work just to make sure your connections are good, and voltage is flowing through your circuit. :thumbsup:


The plus side of using an old computer power supply is you also have 5 volts available to test individual LEDs. and if you need lower power yet add a resistor to test them.
 

kardain

Member
Dec 16, 2011
557
djthumper said:
The plus side of using an old computer power supply is you also have 5 volts available to test individual LEDs. and if you need lower power yet add a resistor to test them.

Pin 13 on the main connector is +3.3v, perfect for white/blue/green tests with no resistor.
 

fletch09

Member
Nov 20, 2011
1,982
nice write up :thumbsup:
LED noob question:
the circuit board, in your cargo light, is just a board w/ holes in it so you can arrange the bulbs
as you wish, and all the connections are made behind. correct?
or are there certain spots to do the soldering?
a pic of the back, if you have one, would prob. answer my noob question.
 

djthumper

Administrator
Nov 20, 2011
14,950
North Las Vegas
kardain said:
Pin 13 on the main connector is +3.3v, perfect for white/blue/green tests with no resistor.

Thanks, I forgot about that.
 

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
kardain said:
Pin 13 on the main connector is +3.3v, perfect for white/blue/green tests with no resistor.

Learned me somethin new today. :biggrin: That will come in handy.
 

Blckshdw

Original poster
Moderator
Nov 20, 2011
10,605
Tampa Bay Area, FL
fletch09 said:
nice write up :thumbsup:
LED noob question:
the circuit board, in your cargo light, is just a board w/ holes in it so you can arrange the bulbs
as you wish, and all the connections are made behind. correct?
or are there certain spots to do the soldering?
a pic of the back, if you have one, would prob. answer my noob question.

Yep, you thought right. I only took one back-of-board pic, since it's not that pretty. But yeah, I bent the legs of the LEDs to overlap each other to make the necessary connections, trimmed them to get rid of the excess, and soldered them up. This one as you can see, was in process, positive lead is going out of the top, the ground is coming down out of the bottom of the board. The first array has been soldered, the 2nd array has been laid out, and was about to be trimmed and soldered. Hope this helps some.

IMAG0094.jpg
 
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Mike w

Member
Jun 24, 2014
287
I can do this thanks for the valued information. I know how to solder so that's a plus. I can't wait to do all the lights in the instrument panel.
 
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