Author Topic: SMD LEDS Connected In-Series With One Resistor  (Read 734 times)

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tehachapifan

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SMD LEDS Connected In-Series With One Resistor
« on: February 03, 2016, 09:57:49 PM »
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So, on my current SW1500 bashes, I'm planning on lighted numberboards this time and I hope to use 2 0603 SMD LEDs with forward current of 15mA/20mA max each in-series with one 620 ohm resistor, which is the supplied resistor with these LEDs. These will be connected to one of two extra function outputs on a Zimo MX621 decoder. I'm actually hoping for rather dim lights as compared to the rest, which I believe the hookup in-series will help accomplish, but I want to make sure I'm not about to commit a decoder or shell-damaging error. Will this hookup work like I think it will?
« Last Edit: February 03, 2016, 10:03:41 PM by tehachapifan »
Russ

peteski

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Re: SMS LEDS Connected In-Series With One Resistor
« Reply #1 on: February 03, 2016, 10:09:51 PM »
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Yes, multiple series-connected LEDs with a series-connected resistor is a perfectly valid way to hook up LEDs.  They will need double the voltage (of a single LED) to start glowing, (about 5.5V) but since you run DCC (around 12V) that is not a problem.

As far as brightness goes, white LEDs are very efficient. I think that even with a 620 ohm resistor they will be way too bright for number boards.  I suspect that you will need a much higher value resistor to get them dim enough to look realistic.  Probably in the range of 2200 ohm or even much higher.  If you are planning on working with LEDs, I recommend that you buy yourself on of those SMD resistor kits available on eBay. That way you will have lots of different values to play around with.  I have pretty much every standard value resistor in 0603, 0805, 1206 sizes. Makes my hobby much easier.  :)  The 1206 size is pretty much save (power-dissipation-wise) for most installations.  Those are 1/4W resistors.  The 0805s are 1/8W resistors.
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tehachapifan

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #2 on: February 03, 2016, 10:14:54 PM »
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Thanks, Peteski! I thought it would work but wanted to bounce it off the board first to make sure. I'll get ahold of some different resistors as you suggest to deal with the brightness.
Russ

craigolio1

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #3 on: February 04, 2016, 12:37:00 PM »
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Keep in mind that LEDs in series are like mini Christmas lights.  If one goes you lose all of them and have to sort out which one it is.  This can be a pain in our locos as we have all of these bits tucked into such tight spaces.  Trust me, I know this pain.  You can wire them in parallel and still use just one resistor.  If one LED goes then the current is sent to the remaining LEDs.  If you are using the minimum specified value then the result would be over current burning out the rest of the LEDs.  Since you're lighting number boards you will use, as Peteski suggested, a much higher value resistor (I do for head lights as well other wise they are as bright as the sub) so this won't be a problem.  Also, using a higher value resistor reduces the power consumed and therefor reduces the power rating (physical size) of the resistor needed.

Peteski please correct my errors.

Craig

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #4 on: February 04, 2016, 06:51:04 PM »
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While the single resistor with parallel LEDs is doable, it is not a best practice for powering LEDs.  I avoid it in most cases.   As Craig mentions, if one of the LEDs malfunctions (it becomes an open circuit) then the current which was passing though it will be then distributed among the functioning LEDs.  In our example where we are powering our LEDs with fraction of their nominal operating current, that is not a problem.

But the remaining LED will still glow brighter while the broken LED will be dark. So, you will have to still disassemble the model to repair the problem. So I don't really see this type of hookup as advantageous.

There is also another mode of LED failure where the LED might short out internal. It will have very low resistance and no longer glow. In that case all the current going through the circuit will be shunted through the shorted LED, preventing the remaining good LED from glowing.

If you think about this, if you were using the series-connected model then those 2 modes of failure will have the opposite effect from the parallel-connected model.

Since LEDs have non-linear current vs. voltage curve it is advantageous for each LED to have its own current limiting resistor. The series-connected  LED chain wit a single resistor is pretty much the same as a single LD with a current limiting resistor (since in series-connected circuit the same current always flows through all the components in the circuit).

Keep in mind that LEDs are solid-state semiconductor devices.  The are *VERY* reliable and unless improperly powered should not fail within your lifetime.  They are so reliable that if you look at most of the commercially available LED lightsbulbs or fixtures, they all use long chains of series-connected LEDs.  If commercial companies use series-connected LEDs, that should reassure you about the reliability of the LEDs.  :)

So, my recommendation is to use either a resistor per each LED or chains of series-connected LEDs with a single resistor per chain.
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tehachapifan

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #5 on: February 04, 2016, 10:17:51 PM »
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Thanks for all the input! :D I think I will probably go ahead with the single resistor and the in-series hookup.
Russ

carlso

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #6 on: February 05, 2016, 07:09:27 PM »
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Peteski,

I have a question for you and I think it goes along with Russ's thread. I do not understand the lighting except to know that a resistor is required for an LED.

You mentioned sizes 603, 805, & 1206 and said the 1206 would be safe. What do these 3 numbers represent, is it the physical size ? ? I am doing the ESU Loksound Select Micro and speaker install in an Atlas/KATO diesel and want to have the Mars light and regular headlight and plan on using the nano LED's that Evan Designs sells. I am going to set up one & maybe two smd caps. that I have. They are 220mfu 16v units. I planned on using the 805 2.2Kohm 1/8 watt. OK or go with the 1206 and 1/4 watt?

Russ, please pardon me for hi-jack.

Carl
Carl Sowell
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Southern New Mexico N Scalers, Las Cruces, New Mexico

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #7 on: February 05, 2016, 09:20:41 PM »
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That is correct Carl. Surface Mount Devices (SMD) are identified by their physical footprint.size.  In US we use Imperial sizes.  Looking at the designation is is quite easy to instantly know their size. Separate the 4-digit designation into two 2-digit numbers. Those are their footprint dimensions in hundreds of an inch.  So 0603 component's footprint is 0.06" long and 0.03" wide.  Or 1206 is 0.12" long and 0.06" wide.  Once you know that, it is really easy to visualize their size.  The thickness is not part of that nomenclature and it can vary.  Here is a good page which shows you a chart of sizes and the power rating (for resistors): https://en.wikipedia.org/wiki/Surface-mount_technology#Packages

As far as power dissipation goes, quoting http://www.electronics-tutorials.ws/resistor/res_7.html :
The Resistor Power Rating is sometimes called the Resistors Wattage Rating and is defined as the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance.

But keep in mind that a resistor which is utilized at its 100% power dissipation rating will be very hot.

I recommend reading through the the above web page - there is lots of useful info (like several ways of calculating the power dissipation) and recommendations on how to choose the proper power resistor.

Basically I recommend to use a resistor which is rated at at least triple of the actual power it will dissipate. In enclosed space (like inside a locomotive) and when the resistor is not soldered to a circuit board which acts a a small heat sink, I would even recommend to triple the chosen power rating. Not because I worry that the resistor will burn up, but I worry that if the resistor is close to some plastic part, it might get hot enough to deform the plastic.

As far as the other question you have, I will assume few things:

1. The voltage between the common positive (blue wire) and a function output is 12V
2. You will be using a single white LED and a single resistor (in series) for each function.

One unknown is the current which will flow through the LED circuit.  That will be chosen by you to get the desired brightness.

There are several ways to calculate resistor value and power dissipation.  I will use the one I prefer.

White LEDs have forward voltage "Vf" (voltage across their terminal when they are illuminated) of around 3V.  That fluctuates slightly depending on the specific chemistry of the LED and the current flowing through it, but for what we are doing 3V is a good value.

To calculate the resistor value we subtract the Vf from the input voltage of the LED/resistor circuit (which is 12V as assumed above).  So that is 12-3=9(V)  Voltage will be represented by the letter V later on.

Next, we need to pick the current we want to flow through the LED (and by definition through the series-connected resistor).   With a small white LED in the headlight lens you might find that 3mA (or 0.003A) gives you a good brightness.  Current will be represented by the letter "I" later on. Now we have enough information to calculate the current passing through the resistor.
To calculate the resistor value (R) we use the Ohms Law Triangle.  Specifically: V/I=R.  So for this circuit it will be 9/0.003=3000(ohms). Or 3k ohm.

Since in a series-connected circuit (by definition) the same current passes through all the components in that circuit, if you feed 12V to a series-connected 3k ohm resistor and a white LED, 3mA will pass through both of them.

Power dissipated by that resistor (or "P") can be calculated using a Resistor Power Triangle (shown on the web page I initially mentioned.  Here I'll use I*V=P or 9*0.003=0.027(Watt).  If you triple that it is 0.081W. In this instance using an SMD 0603 resistor (rated at 0.1W) would be acceptable.

Here is the same circuit but you want 10mA passing though the LED (so it glows extra bright).
Resistor value will be 9/0.01=900(ohms).
Power dissipated by the resistor will be 9*0.01=0.09W Triple that is 0.27W.  Here I would use a 1206 size SMD resistor rated at 0.25W (that is close enough to be triple the rating).

You don't have to worry about the power dissipated by a small LEDs. Some of the power is converted to light and the remaining power is dissipated as heat (but that is minuscule in our application).

Last thing: some resistor values obtained in the above calculations are not available as standard-value resistors which are available (as the least expensive 5% tolerance resistors).  In that case select the next higher value.  That will slightly reduce the current you wanted to pass through the LED but will be close enough.  Google "standard resistor values" then find a table or a calculator showing 5% tolerance resistors.

I hope this helps.

You mentioned installing some capacitors. That is totally unrelated to calculating resistor values for LEDs.  I suspect those caps are the ones used in sound decoders to prevent sound cutouts due to intermittent electric track pickup.  I prefer using tantalum caps rated for 20 or 25V but if your DCC booster does not output more than 12V then 16V caps should be safe to use (few people here use 16V caps in their decoders).
--- Peteski de Snarkski

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carlso

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #8 on: February 06, 2016, 02:12:55 PM »
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Thank you Peteski for the great reply. I have printed your info as well as the link info.

Carl
Carl Sowell
El Paso, Texas
Southern New Mexico N Scalers, Las Cruces, New Mexico

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #9 on: February 07, 2016, 10:54:06 PM »
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Thank you Peteski for the great reply. I have printed your info as well as the link info.

You're welcome Carl.
--- Peteski de Snarkski

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PeyRil

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #10 on: February 11, 2016, 05:26:17 AM »
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That's a great simplified explanation Peteski and very easy to follow! Do you use products from Ngineering? They have quite a selection of SMD LED's and resistors along with other lighting components. They also have a page with a few calculators too for those who don't get along with numbers. http://www.ngineering.com/led_calculators.htm

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Re: SMD LEDS Connected In-Series With One Resistor
« Reply #11 on: February 11, 2016, 05:41:46 AM »
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That's a great simplified explanation Peteski and very easy to follow! Do you use products from Ngineering? They have quite a selection of SMD LED's and resistors along with other lighting components. They also have a page with a few calculators too for those who don't get along with numbers. http://www.ngineering.com/led_calculators.htm

Thanks! I was hoping to make it easy to follow without getting too technical.  :)
I've been messing around with electronics for close to 40 years (both, as a profession and as a hobby).  I do use some of the Ngineering LEDs and other items, but I don't use any online calculators - it is all in my head.  This is all grade-school math (if you know what is what). 8)  I usually shop for my electronic components through Digikey, Mouser, eBay or electronic surplus dealers.
--- Peteski de Snarkski

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