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Electronics tech Anything to do with the electronics in a model. Lights, Radio, ESC, Servo, Basic electrical.


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  #21  
Old 09-13-2010, 03:16 AM
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Default Re: All About LED's

Quote:
Originally Posted by IDontKnow View Post
I am sorry, but you are incorrect there. If wiring in parallel you can do it with only one resistor as long as the wattage is high enough as in 1/2 to 1watt resistors.
10 LED's on my flatbeds both wired in parallel with 1 resistor and been working perfectly for the last 6 months! No overheating of anything all LED's are bright and uniform in light output. Current draw? Minimal Charge the 7.2 volt 4000mAH battery about once every two to three months. The resistor size is 330ohm 1/2 watt
Will post details on the Bezels and LED's when I find it again
The big reason I suggest running a resistor on each LED in parallel is simple. No two LEDs are identical in their construction, which basically means the resistance of each LED, even from the same production batch, varies. Why is this a problem? Because in a parallel circuit the current will always take the path of least resistance. Or in other words, the LED that happens to have the least resistance. What happens is that LED will be subject to higher current flow then it is rated for. The more LEDs you have in your parallel circuit, the worse this problem becomes. Remember in parallel circuits that voltage stays the same to all loads. Current, however divides across them. This is also why I suggest a series circuit. In series circuits, current remains the same across all loads, regardless of each component's resistance measurement. The voltage divides.

To safely run multiple LEDs in parallel, each really should have it's own resistor, or your weakest LED will be have the most current flowing through it. Yes, you can get away with one resistor for multiple LEDs in parallel, but is not optimum for the life or performance of the LEDs.

In regards to current draw, parallel circuits are additive. It may not sound like a huge deal when you are running ten LEDs and each only draws .020 amps, but if you have 100 LEDs on a truck and trailer combo, that is suddenly a 2 amp draw! That will start to be a noticable draw on your battery. A 4000 mili amp pack will be dead in two hours with 100 LEDs, wired in parallel, alone. That does not account for any current draw from the motor, while driving. If you are one of the guys who likes to run more then 100 LEDs, it is definitely something to consider.
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Last edited by Espeefan; 09-13-2010 at 03:22 AM.
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  #22  
Old 09-13-2010, 09:02 AM
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Default Re: All About LED's

Okay, never had a problem up to now so we will just have to agree to disagree
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  #23  
Old 09-13-2010, 09:49 AM
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Default Re: All About LED's

How about we all agree there's different ways to do things?

Either way, it would be nice if this were cleaned up in favor of a definitive guide, with both schools of thought. Kinda sucks though when you have to search through the thread for the tech.
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Last edited by Thorsteenster; 09-13-2010 at 09:54 AM.
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  #24  
Old 09-13-2010, 11:22 AM
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Default Re: All About LED's

Thor, I agree that LEDs can be wired in parallel or in series. There is no arguement there. I just want guys to know that there can be a potential problem if they only use one resistor to drop the current for multiple LEDs in parallel. The last thing you want to do is tear apart a model to replace and LED that has burned out, after it is completed. When you have a lot of multiple LEDs to power, wiring them in series is by far the most efficient way to do it.
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Old 09-13-2010, 11:58 AM
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Default Re: All About LED's

Okay fellas, I just got done uploading some schematics that I spent a good deal of time drawing up this weekend. I know I hoped to get them uploaded last weekend, but I just ran out of time. My apologies.

First of all, a link to my favorite online calculator, for figuring out the proper resistance value required for your LEDs.

http://led.linear1.org/led.wiz

When you punch in your LED's specs, look to the packaging or information card to get the voltages and current draws correct. If none are available, check online, from the dealer you purchased the LEDs from. Often times they have data sheets that give you everything you need.

Let's start by doing a simple LED circuit for a model.



Now I often use terminal blocks when I need to take a voltage source and split it up into multiple circuits. They are quite handy and help keep wiring clean. You do not have to use them at all, if you don't want. It's just a nice option to have. Never know when you might want to add a circuit, might need to make some repairs, or expand the electrical system of your truck. It's also handy for maintenace, should you need to remove the body. You can also use micro plugs so the LEDs stay with the body or cab of the truck, when you remove it. It helps to keep things tidy.

This basic circuit above is one that you can build completely from scratch, as a stand alone system. There are options out there for turning the LEDs on and off. Toggle switches are cheap and effective. Hands free operation can be done with battle switches, which plug into your radio receiver. My choice are the battle switches from these guys. Other places offer their own switches too.

http://www.dimensionengineering.com/BattleSwitch.htm

One thing to know about LEDs is that they are polarity sensitive. If you hook them up backwards, they will not light up. You will not damage them in doing so, unless the voltage is to high, but if you manage to mix up the polarity of one, while wiring it, any other LEDs wired in series with it will not light either. So if in doubt, check that your LED polarity is correct. There are testers you can buy to do this, or you can make your own tester using a couple batteries. It's also good to test the LEDs you are going to use, to be sure they work. I've never run across a dud myself, but no one likes to be forced to do rework. I'd hate to tear apart a circuit to replace a bad one.



The easiest way to identify the polarity of an LED is by looking at the leads. The longest one is always going to be your positive lead. If the leads are trimmed the same length, there are other things to look for. Inside the lense, the portion of the LED that has a little 'flag' is going to be your negitive - the cathode.
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Last edited by Espeefan; 01-23-2011 at 01:06 AM. Reason: updated with LED calculator URL
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  #26  
Old 09-13-2010, 12:07 PM
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Default Re: All About LED's

One thing you are bound to come across when designing and wiring up multiple LEDs is this. Your supply voltage will only be able to power so many LEDs in series. If you have a 7.2 volt battery pack, and your LEDs require 3 volts to light up, the max you can run in series is two LEDs. If you have 12 volts to work with, then you will be able to 3 LEDs in series.

What do we do when we want to run more then that?



Enter the series-parallel circuit. Basically we are unlimited to the number of parallel circuits we can run off of our battery, even if we can only drive 2 or 3 LEDs wired in series. So what you do is make multiple series circuits and parallel them off the power supply (battery). Each set of LEDs wired in parallel all receive the full battery voltage. Within each additional parallel LED circuit we have, the voltage divides across the LEDs wired in series. By wiring in series-parallel, there is no limit to the number of LEDs we can run.

If you want an odd number of LEDs, you can just as easily run a group of 2 or 3 in series, and in a parallel circuit off your supply voltage, run 1, or 2 in series. The resistance of your resistor will need to change because of the fewer LEDs in that circuit, but it's all accounted for, in the LED array calculator I provided a link to above.
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Old 09-13-2010, 12:09 PM
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Default Re: All About LED's

So how about a real world example?



I drew up this schematic to show how you could wire up a truck with LEDs. Just to give you some ideas. This would be a complete stand alone circuit. No MFU or other lighting kit necessary.
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  #28  
Old 09-13-2010, 12:11 PM
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Default Re: All About LED's

And, how about a trailer? We need lights on them too.

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Old 09-13-2010, 12:19 PM
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Default Re: All About LED's

A couple more things I wanted to point out - Your negitive wires become your 'common'. What I mean when I say that is this. All the negitive wires are of the same polarity, potential, and tied together. If you like, you can tap any negitive wire, from any circuit, into another, and your circuit will be complete. Multiple, seperate circuits can share the negitive wires without a problem. As long as one negitive wire makes a connection back to the battery pack or terminal block, you can solder any other negitive wire to it. The wires share the common link to the negitive terminal. One thing to consider if you decide to tie multiple negitive wires to just one negitive wire, is to be keep in mind the size of the wire. Don't tie all negitive wires from multiple circuits to one common wire alone. You could be asking to much of the single wire, to handle all the current flow of the other circuits. Best to use a heavier wire for the one common, or to simply tie the negitives together where it is convenient, for the sake of simplicity or neatness of the wiring, but for other circuits, run the negitive back to the main terminal.

Also, your switching control should be done with the 'hot' wires. The positive wires. This way the power is off, when the switch is off. If you control the switching with the negitive wires, the circuit works the same way, however the potential voltage is there and waiting in the circuit, where your loads are. In industery or residential wiring this is a bad thing. A circuit has the potential to be 'live', even if the switch is off. The voltages are not going to hurt you here, but it's good habbit to let your switch kill the power to anything down stream of it.

The last thing I would like to add is this. It's a good idea to have some short circuit or over current protection in your wiring, in the form of a fuse, or re-settable circuit breaker. Should anything short out, your wiring will be safe and nothing will melt. Size your fuses just slightly over the max current draw of your circuit. To do that you will need to calculate the max current draw. It is not absolutely necessary to have some circuit protection, but it is something to consider.

I hope this information will be helpful to those of you who are trying to figure things out. I may draw up some other circuits a bit later, but the principals of these applies to any you may design on your own.
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Last edited by Espeefan; 09-13-2010 at 12:28 PM.
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  #30  
Old 09-13-2010, 12:24 PM
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Default Re: All About LED's

Holy smokes, nice work! Worthy of it's own thread even. Nice to have some actual diagrams in situations for RC use.
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  #31  
Old 09-13-2010, 12:31 PM
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Default Re: All About LED's

Thanks Thor. As time allows I can explain things a little more in depth, but that should get you guys going.
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Old 09-14-2010, 09:09 PM
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Default Re: All About LED's

Wow, That is more then I expected. But that really makes sense though. Thanks for all the time you put into it. I do have some lights, but just the ones I hooked inot my TLU-01. I ordered a bunch of LED's and some resistor and I'm going to try to hook a bunch up. I need some running light's on my truck and want some onm my Trailer. Not as many as in you diagram but enough to make it noticable.
Thanks Again!!
John
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Old 09-16-2010, 08:09 PM
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Default Re: All About LED's

I got all my LED's and resistors now. And a little wiring. I've been working sun-up to sun-down all week and will prolly continue through til next week sometime. But I should get a chance to get some wired up next week and as soon as I do, I'll post and let you know how it went. I can't wait to see how it goes. I got some of the black LED holders but not sure if I wanna use them or not.
So that lead's to another question. What do you guys suggest I use as holders to hold them in place? How does everyone else mount their LED's?
Thanks a ton,
John
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  #34  
Old 09-17-2010, 08:26 AM
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Default Re: All About LED's

Hey John, glad this helped you out a little. So far I haven't gotten to the point of adding LEDs to my semi truck. Lots of other things to do first! But I have added LEDs to some of my other RC models, or have some in process. Non-semi truck models, so I just stick the LEDs in the light buckets, as they are intended to be.

With the big rigs, it seems you'd be just fine mounting the LEDs through a properly sized hole. 3mm or 5mm. Most LEDs are tappered, so they get tight as you insert them. A little dab of shoe goo, rubber cement, or something you can remove later is a good way to hold them sercure. Otherwise, you can try some small O-rings, which can sometimes be passed off as a rubber gasket, but I prefer the cleaner look of no O-rings. Been looking for some bezels to use, but it's hard to find something that looks the part. If you insert the LEDs into some thicker metal (cab skirts, for example) then they really don't need much support. For something thin, like styrene, or sheet metal, you can always make a backer plate with styrene, glue it to the location you need the support, and drill a hole through both pieces. My favorite method is to just rely on the tight fit of the LED, sized to the hole. It's the easiest and cleanest way to go. Sometimes, if you need the LED to seat deeper into the hole, you might need to sand the bottom of the LED's base, because some are tappered and get wider there.
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Old 09-17-2010, 10:11 AM
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Default Re: All About LED's

I've found PVC caps to work well for buckets/enclosures for some applications, 1/2" I think, the smallest size.
I generally use hot glue or Shoe Goo. A great way to apply Shoe Goo is, not sure what it's called but the pharmacist gives them out, baby medicine shooter or something. Kinda like a syringe but no needle. Pull the plunger out and squeeze the Shoe Goo in. Works great, I'm actually only getting it where I want it now!
Also, here's a set of tail lights I made with some red plexi and styrene. The reverse lens I drilled out and filed square and used a piece of lexan.

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Old 09-17-2010, 01:17 PM
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Boy, that's a sweet looking tail light, Thor!
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Old 09-17-2010, 01:24 PM
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Default Re: All About LED's

I'll be making V2 here at some point. I sold that body (HPI Bronco) in favor of a ProLine Bronco with full hard top.
I have an MFC02 for it, so it'll get the works.
I think I'll try X-Acto-scribing some lines in it too.
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Old 09-17-2010, 02:02 PM
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Actually, I think your finger print on the lense makes it look perfect!
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Old 09-17-2010, 02:50 PM
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Default Re: All About LED's

Doh! A little too close a close up lol. It was inside too......
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Old 09-17-2010, 02:57 PM
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Default Re: All About LED's

wow thats some impressive charts!
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