# L.E.D. mA



## CaptFrank (Jan 29, 2005)

I have another technical question for the Pros on this forum.

I bought some red 5000 mcd L.E.D.s. 

They use 1.8 -2.2 V at 30 mA.

A Radio Shack clerk told me "AA" batteries have too high an 
amperage for the L.E.D.s.

*If I use two "AA" batteries to power them, how do I regulate
the amperage?*


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## Scott Hasty (Jul 10, 2003)

Going to Radio Shack for electronics advice is like going to a plumber for a coronary bypass.  

Current ratings (30 mA or milliAmps) is the amount of current a device will draw from a power supply (or battery). You want to have a power supply that is rated higher than the load (your LED). AA's have a maximum capacity of 2A, so you are PLENTY fine!

As far as your current LED's and power supply, I would drop your current to 20mA. At three volts, you will need to put a 40 ohm resistor in there. I recommend you go to six volts. In this case, you would need a 190 ohm resistor in there.

Scottie


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## CaptFrank (Jan 29, 2005)

Wow!

Resistor's deal with voltage _and_ amperage?

Great!

Thank you! :thumbsup: 

If I go to six volts, should I stick with "AA"s?


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## Scott Hasty (Jul 10, 2003)

Yep, depending on the circuit, they can be a voltage drop (series) or a curent drop (parallel). Heck, if you want something smaller, go with AAA's. Personally, I'm a BIG fan of wall warts. The advantage is you don't have to cut a panel out to put your battery holder and the plug for the wart can double as the mounting point for the model.

Scottie


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## omnimodel (Oct 9, 2004)

If you're using LEDs with different current and voltage ratings (for example Red LEDs in the nacelles and White in the hull), I'd strongly recommend wiring them in parallel (one resistor per LED). This will ensure that each bulb is getting the proper voltage and current.

To calculate resistor values for various bulbs and power supplied, I'd recommend this site:
http://linear1.org/ckts/led.php

As far as what power supply to use... that's up to you. It should have more mA than the LEDs will draw; so if you've only got a couple of LEDs, batteries will be fine. If you have a bunch of LEDs, you'll need to move up to a wall wart. Just total up the total mA your bulbs will be drawing (in you example above, 60mA for the pair) and then figure the appropriate supply.

Finally, for optimal LED life, never trust your power supply to regulate the voltage and current. Always use the appropriate resistor, and your LEDs should last indefinitely.


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## CaptFrank (Jan 29, 2005)

I went back to Radio Shaft and looked for the resistors 
you recommended. The closest I could find: a 5-pack of 
47-Ohm Resistors 1/2 watt 5% tolerance.

I bought one to test. I connected a red L.E.D. to a 
resistor, and then to 2 "AA" batteries. It worked! 
It lit, and stayed lit! :thumbsup: 

I think I can use these for my Millenium Falcon.

Do I have to put Resistors on all lights? I want to use Blue, too, 
and I have no problem with them.


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## RossW (Jan 12, 2000)

That's because blue LEDs have a max. forward voltage of around 3.2V, so you're slightly underpowering them with the 3V from your batteries.


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## CaptFrank (Jan 29, 2005)

How bad would it be if I used three "AA" batteries?
I like how bright it makes the L.E.D.s.






Oh yeah, ...the plumber is coming by to do my 
coronary bypass...


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## Otto69 (Jan 2, 2004)

That depends on what batteries you use, and how you wire them.

If you wire batteries of the same type in series, the voltage doubles, but the amps/current stays the same.

If you wire batteries of th same type in parallel, the voltage stays the same, but the amount of current reserve doubles (i.e. it runs longer at the same voltage).

A random web search showing this as well as how you wire the terminals is here: 

http://zbatteries.com/zbattery/seriesparallel.html

Congrats on pursuing this, it's not rocket science, but it puts you a step ahead of a huge percentage of the general population . Resistors have relatively simple rules for series and parallel as well which you could pursue if you wish. WIth resisters and batteries you can do nearly any static lighting display you want (i.e. no fade in/out).


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## CaptFrank (Jan 29, 2005)

> _If you wire batteries of the same type in parallel, the voltage stays the same, but the amount of current reserve doubles (i.e. it runs longer at the same voltage). _


So, if I wire two "AA" batteries in parallel to another two, they'll light my L.E.D.s twice as long as one set?

And with the resistors, the Reds will still work?


That's a great article, by the way. Thanks!


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## Otto69 (Jan 2, 2004)

To your question about batteries in parallel, yes.

I don't quite get the reds question, sorry. But note that the way this all works is, if you have too much resistance in a circuit you simply have dimmer LEDs. If you have to little, you may burn out the LEDs immediately, or sooner, since the more current drawn the more heat is generated. But aside from that it's not a big deal. Experiement a little. You can also put resisters in series to increase resistance if you can't find exactly the numbers you want.

Something also to consider nowadays, with the easy availability of battiers of all types for various cameras etc, is to go to the store and check out the form factor and voltage of various batteries. You may find something that fits a model kit more easily than AA's or AAA's. It may not be cheaper, and it may or may not last as long, but my point is that no one says you have to use AA's and AAA's only. The rules for series and parallel apply to any battery of any voltage. For example there are 1" diameter, very thin, flat round batteries which are 3 volts (you may see some of these on CPU mother boards).


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## CaptFrank (Jan 29, 2005)

*Prequel Post*

Allow me to fully explain about the "Reds".

I bought 100 5000mcd Red L.E.D.s from a factory in 
Hong Kong. (Only $9.99!)
The seller included a 3 volt, watch battery (about the 
size of a nickel) for testing. When I used the watch 
battery, the Reds look OK. So I connected one to two 
"AA"s. I planned on using them as the power source for 
the Millenium Falcon I'm building.
The Red L.E.D. lit, then dimmed considerably, almost going 
out. I thought the batteries might be dead, so I tested them 
on a White one. No problem. The same with a Blue one. No 
problem. 
I deduced that it might be an Amperage problem. I went to my 
local Radio Shaft and inquired about amps.

Then, I started this thread.

That should bring everyone up to date.


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## jwrjr (Oct 31, 2003)

There, oddly enough, is your problem. A battery looks outwardly like a thing with voltage that delivers amps. Reality is a little more complicated than that. Think of a battery as having a resistor in series with it inside the battery casing. That is why when you draw more current from a battery the voltage drops, only to go back up when you remove the load. The resistor inside the lithium cell is good for powering a led. My guess would be that the led got hot when you powered it from the AA cells. That could cause the light output to go down. In an ac adapter the resistor is smaller still. A led can explode if you directly (without resistor) power it from an adapter. I've seen it happen.


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## Otto69 (Jan 2, 2004)

The LEDs will only pull as much current as they're rated for I think?

Your watch battery is probably 3 Volts or more, but AA's are only about 1.5 V. I suspect you may be under voltage, you might try rigging up some AAs in series to test the theory. Or use a voltmeter to measure the watch battery.


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## jwrjr (Oct 31, 2003)

Sorry, Otto, but a led will pull as much current as it can, until it burns out. The purpose of the resistor in series with the led is to limit the current flowing through it..


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## Otto69 (Jan 2, 2004)

Interesting...so an LED acts like a dead short basically? Regardless, is this a feature unique to something like an LED, which converts electrical engergy to light, such that it can always try to make more light by pulling more current, until it pops? Or is this a feature of all semi-conductors?



I just found this website which covers resisters in serial with LEDs:

http://www.eidusa.com/Electronics_LEDs.htm


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## jwrjr (Oct 31, 2003)

A led is, after all, a diode. The differences being the forward voltage and, of course, that it emits light when forward biased.


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## CaptFrank (Jan 29, 2005)

Sorry to dredge this thread up, but I didn't see any point to 
starting a new one.

Now a year later, I am trying to light another model.
I.m struggling to remember, and understand, what I learned.
I want to do it right, so it will last. And I don't want it 
to be a fire hazard.

I am planning to use 6 5000 mcd blue L.E.D.s.

The calculator:

http://linear1.org/ckts/led.php

and

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

suggests an array that would draw 120 mA from the source.

If I use a Radio Shaft adapter 

http://www.radioshack.com/product/index.jsp?productId=2049706&cp=2032056.2032136.2032153&pg=2&numProdsPerPage=20&parentPage=family

that provides 300 mA, what happens
to the excess mA? Is it absorbed by the resistors? 
Will it be too much for my 6 little L.E.D.s?


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## Scott Hasty (Jul 10, 2003)

If your circuit draws 120 mA and your supply provides 300 mA, you are in current heaven! Basically, you want your supply to equal or exceed your circuit's current requirements. You are in good shape, Capt!

Scottie


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## Steve244 (Jul 22, 2001)

yeah, what he said. The "current limiting resistor" does just that (limit current). Think of it as restricting the flow of electricity. Not a big enough resistor (or none) and too much electricity flows which can burn out the device (LED in this case). The 300mA capacity of the power supply is it's limit. Its perfectly happy providing less juice.


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## Scott Hasty (Jul 10, 2003)

I have to caveat my response. When buying wal warts such as:

http://www.radioshack.com/product/i...rentPage=family

Be careful! They may be rated at 6 volts, but can have a MUCH higher output! I have a VAST collection of warts that were used for everything from phone chargers to answering machines and their outputs varied horrifically. In one example, it was rated at 4.5 volts and measured almost 15 volts out. I don't want to scare you, but invest in a DMM (digital multimeter) or a [for us oldtimers] VOM (VoltOhmMilliammeter) so you can measure what is coming out of that plug!


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## CaptFrank (Jan 29, 2005)

Cool!


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## Steve244 (Jul 22, 2001)

what he (Scott) said. The problem being that cheap AC/DC converters don't usually have a voltage regulator. So there could be a problem with all the excess amperage. It's like a motor without a throttle. Spend a little more and get a regulated power supply. Batteries don't have this problem.


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## enterprise_fan (May 23, 2004)

I bought a set of Philips 60 L.E.D. LIGHTS last year at Christmas with the intention of putting is my PL 1/350 Enterprise. After reading this thread I don't know. I was going to dissamble the lights from their holders to conserve space when lighting her up. I had planed on wiring them in parallel throught out the ship. There aren't going to any blinking nav light just plain internal lighting. The set came with a plug, with a small fuse inside, that pluges right into the wall socket without use of any wal wart. Do I still need to have resitors on the L.E.D. bulbs? Can I still use the existing wiring that came with the set or do I need different wire?

Here is the info that is on the back of the box:

Philips 60 White L.E.D. lights
Electrical Rating:
120V, 0.04 Amps, 4.8 watts

Is this a foolish endeavor an disaster in the makings?

Thanks to all the wonderful people here at this forum who have helped the newcomers. :thumbsup: :wave:


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## Steve244 (Jul 22, 2001)

You haven't said what you plan to use as a power source.

Yeah you'd need resistors. The string of lights as is, is wired in series. The number of LEDs carefully calculated to provide the resistance necessary to keep the thing from burning out. Fewer LED's gives less resistance and lower voltage requirements, you'll have a 4th of July light display if you plan on using 120V.

White LEDs typically are about 4V. In series, 30 would require 120V. I bet the string of xmas lights is actually two series circuits, each having 30 LEDs in series.

If you light one by itself (or in parallel with others), use the calculator to determine the required size resistor, depending on the voltage rating of your power supply. You can assume the current requirement of the white LEDs is around 20mA.

eg: a 9V battery would require a 270 ohm (or higher) resistor per LED to protect it from too much current.


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## Scott Hasty (Jul 10, 2003)

For what you paid for those lights, you probably could have bought 2 100 packs of LEDs (in different colors) with resistors off of that auction site. Unless you got them after Christmas. 

Steve is probably correct; two strings of thirty, each in series. Each string owuld draw 20 mA for a total 40 mA draw.


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## terryr (Feb 11, 2001)

enterprise_fan said:


> I bought a set of Philips 60 L.E.D. LIGHTS last year at Christmas with the intention of putting is my PL 1/350 Enterprise. ...The set came with a plug, with a small fuse inside, that pluges right into the wall socket without use of any wal wart.


It must have some sort of converter in the plug or wiring. I never heard of A.C. LEDs.


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## Steve244 (Jul 22, 2001)

Nope, they plug 'em in directly to 60Hz AC. Which implies that they turn on and off 30 times per second and are actually off half the time. I've never seen these working, but at that frequency there must be some visible flicker.

Edit: Electrical Engineer, First Class ******, just wrote me to advise ***** ******** ** *** ****** **** *********. *******!


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## enterprise_fan (May 23, 2004)

The lights plug directly into the household current. It would seem that there are two set of 30 lights in series because there are three wires inter-weaving through the set. If there is a flicker it must be very fast because it is not very noticeable.

Oh and-by-the-way the extent of my electrical knowledge is the top of the battery is + and the bottom is -.  :tongue:


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## Steve244 (Jul 22, 2001)

You're in trouble if you run 9V!


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## enterprise_fan (May 23, 2004)

I took the spare "bulb" and touched the ends to a 9V battery to see if it would light up, to my suprise it did. I held it on there for a few seconds to see if it would heat up and it did. Like I said before my electrical know how is very limited. Math is not my strong point so using the LED calculator is like trying a blind man how to draw mickey mouse. I have a very slim idea what I'm doing. All I know is that I have 60 lights, I need resistors and I need a power source (battery or wal wart). What kind of power source do I need to power 60 lights. What size of resistors and how many do I need to keep from recreating the distruction of the Enterprise in STIII_TSFS.  

Thanks for all your help


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## Steve244 (Jul 22, 2001)

60 is a lotta lights. You can do it with a 9V battery, but it won't last too long (15 minutes?). How many LED's do you plan to use?

If you wire each one in parallel, that is have a separate plus/minus wire for each bulb, you need a 270ohm (or higher) resistor for each LED using a 9V power supply. The higher resistor you use, the less light output. Too high and it won't light. Too low and the life of the LED will be shortened. Experiment some with a breadboard, a battery and a selection of resistors. It's fun and will entertain the cat.

The constraints of wiring the LED's inside the model will determine if you can wire all parallel or some in series. The number you can wire in series depends on the voltage of your power supply.

If your power supply is 9V, you can wire no more than 2 LEDs in series (they're each 4 volts). You'll still need a resistor (you have 1V too much!). You would need a 51ohm resistor in this case (or the next higher available size).

I don't think it's a good idea to go over 9V.


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## terryr (Feb 11, 2001)

This is a good page for LED overview.

http://www.kpsec.freeuk.com/components/led.htm

and a color chart

http://www.theledlight.com/color_chart.html


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## enterprise_fan (May 23, 2004)

Now let me see if I got this straight.

The LEDs need to be connected in parallel so if one should go bad the rest will still lit. Each LED in that circuit needs to have a resistor connected to the + side to increase the life of the bulb. The power supply depends on the number of LEDs.

If the above information is correct I have the beginnings of a game plan. I’m still working on number of LEDs for each group. I just don’t know what size resistor to use, what size wire to use and what power supply would work best.

I would like to connect groups of LEDs in parallel for each part of the ship as follows: 

A group 8 for each nacelle
A group 25 for the saucer 
A group 20 for the engineering 
A group 7 for the dorsal & torpedo launcher 
Total of 60 LEDs

Each group will then be connected to the main power supply, what ever it may be.

Somewhere in this group someone suggested that the interior be painted. A undercoat layer of black to minimize light leaks then a layer of silver/chrome to reflect light from what ever is used to illuminate the portholes of the model. I plan on doing to my big "E" so the LEDs I do use should be enough. I’m still working out some of the details but that is my plan so far. 

All the ideas that have been given here are good ideas. That is why a lot of us are here, to get ideas on how to do things that make people say “Now did he do that?”

Thanks to all for your help. :wave:


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## Steve244 (Jul 22, 2001)

That'll work. 

There is a third option which will conserve on resistors and wiring. Check out the parallel LED diagram and calculator (3rd down) on this page. This will only work when the LEDs are identical. Make note of the resistor power rating if you do this. You can burn out a resistor this way and also create excessive waste heat.

Another idea is to use armature wire. The insulation is clear resin so the wires are ultra thin (suitable for electric motor windings). Regular insulated wire will quickly fill cavities. Also run just two wires (+/- ) direct from the battery to each area and then branch out to the LEDs in parallel.

Cautionary note:
Even though LED's create almost no heat (they're an efficient light source), I'd not want to see what happens if you rig it and leave the lights on for a long period. According to the original specs, they use almost 5 watts. Not a lot, but consider how hot a 5 watt night light gets to the touch, and these will be enclosed. I'm not talking burning hot, but plastic softening hot is a possibility...

Are you sure you need that many LEDs? Half or 1/4 that number in an enclosed reflective area should be plenty bright.

I haven't lit a model that big before, but you might want to get advice from some who have before committing to that many LEDs inside. I hope you like soldering!


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## enterprise_fan (May 23, 2004)

According to the box the LEDs are 4 watts so the resistors should be around 51 ohm right. You might be right on reducing the number of LEDs I use. I'll see where I can reduce the number of LEDs. I still don't know size (gauge) wire to use.

My local HobbyTown USA has red, green and yellow LEDs the size of rice so I might put in some the nav lights, on a separate line, after all. Would I need smaller wire and smaller resistors for them if I decide to use them?

The power supply will be 9v wal wart.


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