Noob needs a little help on picking the resistors.

Hello all, I'm sort of a noob to LED's so please excuse my ignorance. I found a great website to calculate the resistors I need for using LED's standalone, in serial, or in parallel circuits: http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

great site check it out. Here is my confusion though. If I'm running an say 5, 3.6v 20ma LED's in a parallel and I'm using a 3.6v power supply, it says I don't need a resistor. Though I thought LED's were constant current diodes and NEEDED a resistor no matter what. So I'm a little confused. Also, would it be bad to run a 3.6v LED off of a 3.6v supply? Rather better to use say a 4.5v with a resistor? Ok final and last question, I don't know if it belongs here but it has to do with what I'm working on. If I get a 7.2v 3300mAh battery and connect it to some LED's with resistors that are pulling 3.6v and a total of 600mAh, would I get about 5.5 hours of battery (3300/600) or would it actually about double because I'm not pulling the full 7.2v's through the use of resistors? Any help would be TREMENDOUSLY appreciated.

Thank You

Re: Noob needs a little help on picking the resist

I suggest using a resistor on the leds, even if only a low value one. Heres why:

First off LEDs are only nominally 3.6V. Their actual forward voltage drop depends primarily on current flow through them, the higher the current, the higher the Vf (forward voltage). Vf also changes inversly with temperature, as temp goes up Vf goes down.

Keep in mind that primary source of temperature rise in an LED is the drive current, and this heat causes the Vf to go down, and as Vf decreases more current will flow causing more heating - thermal runaway.

A series resistor helps ballast or regulate the current, the larger the resistor the better the regulation but the worse the efficency. Yes, if you match the supply voltage to the led Vf then the resistor calculates to zero, but then the supply voltage has to adjust to the changing Vf of the diode. This is what a regulator does.

No two leds are identical, so when paralleling them, the one with the lowest Vf will pass more of thre current. However this tends to raise its Vf and balance the current out a bit. So matching only needs to be close, not exact.

About the 3300mAh battery, the battery still puts out 7.2V @ 600mA regardless of the LED voltage. You will probably see less than 5.5hrs because the 3300mAh is probably a 10 hour rating, so its capacity at 5.5 hour discarge rate is slightly less.

A CC1W regulator (down convertor) might be just the ticket here to regain the resistor losses.

Good luck,
Crux

Re: Noob needs a little help on picking the resist

First off do not go by what voltage batteries have marked on them as that is a nominal voltage and actual voltage usually is higher when new or freshly charged. If your actual LEDs have an exact Vf of 3.6v and your battery source is 3.6v exactly no resistor should be needed if the variance of the Vf of all your LEDs in parallel is minimal.

As for your 7.2v battery somehow I doubt it starts out at 7.2v but more like 8.4v hot off the charger and tapers down to 7.2 under a decent load with runtime. I would recommend if you are running at 7.2v to pair up your LEDs in series/parallel. Two LEDs/string and wire each string in parallel with a resistor to drop the excess voltage which may be as high as .6v.

Re: Noob needs a little help on picking the resist

Ok, thank you guys. I think that helped alot. One other question though.. If say I put a higher resistor, say one that is for 4.5v battery bank and it then drops its voltage as it is used, won't this change the current to the LED and thus change the wavelength? I need a pretty constant wavelength. Basically the LED's I have are rated 20ma 3.6v. So should I shoot for a higher resistor to compensate for the higher voltage at start, or shoot for the resistor that will give me 20ma at 3.6v when the battery reaches its nominal voltage. From what I've read, supplying a higher voltage will increase an LED's wavelength and vice versa, or was I misinformed? I need something for about 20 to 30 of the 20ma 3.6v LED's... so I don't think I'd be able to use a series because I don't want to use to high a voltage battery. I'm sorry, this is my first attempt to do anything electrical so it's pretty new to me.

Thanks again for the help

Re: Noob needs a little help on picking the resist

Treat LEDs as current devices. I recommend you get an ammeter and run them at various currents like 5,10,15,20,25,30,35, and 40 ma. at the 30+ma range you may decide to limit your runtimes if you are worried about reducing the life a little.
Sometimes it is best to try it before you put it together
as us telling you doesn't describe the tint shift as seeing it for yourself and LEDs vary in tint. I have had lights with 3 distinguishable different tint LEDs in them before.

Re: Noob needs a little help on picking the resist

[ QUOTE ]
Crux said:
A CC1W regulator (down convertor) might be just the ticket here to regain the resistor losses.

Good luck,
Crux

[/ QUOTE ]

Would I be able to use this to get a constant current of 20ma in a parellel to each LED? I guess what I need, if it is possible, is that no matter what the supply voltage is (since it will drop or fluctuate) I need something that will give each LED a constant current of 20ma. Again, I'm new to electronics so I don't know if this is easily done. As far as the ammeter, I forgot to mention I'm using true UV LED's.. so I can't really detect tint with my eye. I hope I'm not biting into more than I can chew, because I really would like to get this project to work. Again, I appreciate all the help and patience with my lack of knowledge on the subject.

Re: Noob needs a little help on picking the resist

If you are using UV LEDs I don't think tint would be an issue but from the little I know I *think* they are a little more sensitive to overdriving. You could use a buck convertor or a cheaper current regulator circuit with something like an LM317 in it to drop the excess from your ~7.2v battery. You would get better runtime I think using 6 LEDs in 3 strings of 2 series LEDs in parallel as the difference in Vin and Vf would be about 1.2v max vs 4.8v for an only parallel option. I would use resistors to drop the excess voltage. 1.2v@20 per string (3resistors for 2 strings) you would need 3 60ohm resistors. If the LEDs are matched well for Vf you could try a single 20 ohm resistor but that would afford you less protection should one string fail.

If you want to use a current regulation or buck circuit and run the LEDs in parallel at a certain level input the LEDs will drop off in current.

What type of battery are you planning to use?
Some rechargables do not take kindly to draining them below a certain voltage and a 3.6v load off of a 7.2v lithium ion with a regulator could drop it below that voltage. I do not recall what lower limit is safe but on nimh/cell I believe it is around .9v/cell so a 6 cell pack would be about 5.4v which leaves you about 3v headroom for a regulator. A resistored setup would probably act about the same if you used 2 leds in series for 7.2v/string.

Re: Noob needs a little help on picking the resist

[ QUOTE ]
Lynx_Arc said:
What type of battery are you planning to use?
Some rechargables do not take kindly to draining them below a certain voltage and a 3.6v load off of a 7.2v lithium ion with a regulator could drop it below that voltage. I do not recall what lower limit is safe but on nimh/cell I believe it is around .9v/cell so a 6 cell pack would be about 5.4v which leaves you about 3v headroom for a regulator. A resistored setup would probably act about the same if you used 2 leds in series for 7.2v/string.

[/ QUOTE ]

I was planning on using either Lithium Ion rechargable or NiMH. I've actually been having a time thinking about battery configuration. I was thinking about using a parallel 3.7v Li-Ion setup.. but now I don't know because of voltage issues. When you talk about a safe lower limit, like with the NiMH, you mean that basically the cells can be drained to .9v and not have problems, correct? So say 6 NiMH cells can each be drained to .9v, a total of 5.4v's, and be "safe"? What percentage of discharge are batteries at their nominal voltage? or is it a steady rate of decrease? Would there be a way to safeguard the LED's by opening the circuit when it gets to 5.4v's? Maybe I should just hire an electrician to make me schematics for this thing /ubbthreads/images/graemlins/icon23.gif.

Thanks Again

Re: Noob needs a little help on picking the resist

I've done a little researching.. what if I were to use a voltage regulator.. say with 6v and use the regulator with heat sink to get it down to 4v or so and then use resistors to achieve the desired amps? Does a voltage regulator keep the voltage pretty constant or will it fluctuate also? So say I took a 7805 and connected say 6v or so to it and then used resistors to regulate the amps to the LED's in parallel.. would that suffice? Or would a LM317 with r1= 100 and r2 = 200 (I think that math is correct) be better? Would I need anything else in the circuit? Circuits using LM317 seem to use capacitators to for some purpose.. Once I get the 4v on with the LM317T I should just be able to use resistors again?

Re: Noob needs a little help on picking the resist

Unless you need a certain amount of runtime that a 3.6v(4.2peak) cannot provide easily I would lean towards a resistored setup of this type. A 7.2v(8.4peak) setup would either require wiring LEDs in series/parallel strings, a regulator or buck (DC-DC converter) and possibly more heatsinking.

Even a 3AAA nimh setup should give you about 6-8 hours approx at spec while a AA setup may lead to over 20 hours.
A 3300mah lithium ion setup would probably run 30+ hours. Somehow I doubt you would be runnning a UV light that amount continuously without access to a charger setup.

Re: Noob needs a little help on picking the resist

Except I'm going to be running about 20 of the LED's.. totaling 400mAh.. so I'd love to be able to use AA NiMH but I need about 12hr runtime. So if I were to just run them on a 3.6 volt bank, they would be ok with just a resistor setup (say small resistor to compensate for the higher start voltage)? I'm sorry I was alittle confused by your last post. I also know that the LED's have a max forward voltage rating of like 4.5, would that be cutting it close?

Re: Noob needs a little help on picking the resist

Just a quick gremmer correction..."running about 20 of the LED's.. totaling 400mAh.." The correct termonology would be 400mA. mA (milli-Amps) is a term for current draw, mAh-(milli-Amp hours) is for current over time, ie. 10mA for 10hrs would be current * time, or 100mAh.
As far as wavelength, current differences wont really change the 'color', sometimes SEVERE overdriving and/or heat can affect this.
What is the final 'product' goung to be. This will also help us with what to suggest for driving options. If it is going to be in a display case for moderate use, then a current regulator would work better. If for a handheld unit for short uses, a resistored setup would work).

Jeff

Re: Noob needs a little help on picking the resist

Yeah, sorry I meant milliamps.. not amphours.. It is going to be something for moderate use, say 12hr periods. Basically I need something that will run the LED's at their specified wavelength, at least for the majority of the time (because I need certain wavelenght UV) for at least 9 hours. I've been having a heck of a time trying to figure out battery configurations, circuit configurations and such. The problem with the battery is that I need something that can supply 400ma's for about 10-12 hours, but the LED's are rated at 3.6v 20ma. So I know voltage drops from batteries with use which makes me think I need something moderately high voltage to ensure it runs. ANY suggestions are much appreciated.

Right now I am considering run 2 parallel 7.2v 3300mah batteries into a LM317 or 7805 and then using a resistor for EACH LED in parallel. This to me seems to be the most efficient. However, I'm worried about the heatsink heat.. how much heat will be generated from say a 2-3v dissapation?
I need to know how much voltage drop there is in battery use? I mean how long does it take to drop a 1.2v to .9v and at what rate? I could probably get away with a 6v 3300mah bank if I new it wouldn't get to 4.5 volts (5 * .09v)quicker than say a 7.2v to 5.4v's (6 * .9v).

Re: Noob needs a little help on picking the resist

That changes things, you had me thinking in terms of 5 LEDs. 20 is a huge difference and 12hours is considerably long runtime for a 20 LED portable device. As for max Vf of LEDs usually that is maximum they can take without frying, definately not recommended.

If you want to use AAs You could go with two sets of 4 in a parallel and resistors giving you about 5000mah @ 5.6v max. This should give you ample runtime with the advantage of being able to swap out 4 cell packs while another is running. A design consideration in swapping one pack out to compensate for uneven voltage would be putting a diode in for each pack to isolate them from trying to charge the weaker pack.

Re: Noob needs a little help on picking the resist

Does this make any sense or am I way out of my league?



maybe the resistors aren't exactly correct?

Re: Noob needs a little help on picking the resist

Unless your voltage input is higher using a regulator may not make good sense as they need ample headroom to operate I *think* about 3v. The additional 20ohm resistors wouldn't be needed as the regulator would be set to deliver proper current.

Re: Noob needs a little help on picking the resist

Yeah.. I looked and it needs about 3v headroom, which wouldn't work especially as the battery dropped since the LED's are 3.6v. Or would it since at 5.4v (0.9v x 6) it would still be about 2v over and the majority of the would be alittle less than 3v's? Which really brings me back to my original problem. I need something to compensate for the voltage dropping from the battery. Something that will keep the current and voltage constant to keep the wavelenght as constant as possible. Is there any info on the voltage drop rate for batteries? If I use a battery and it stays at around 3.6v for say 80% of the time.. that would be fine, but for some reason I doubt they do that. Don't batteries use basically most of their voltage in the first 30% of their charge or something? I know Like the last 50% goes quick and using a battery past that isn't good.

Re: Noob needs a little help on picking the resist

On another forum, someone said the LM137 isn't really good for battery applications. Rather I should use a LM2575T-ADJ to bring the voltage down to 3.8v and then use resistors for the LED's to regulate current. This sounds like the best idea to me but the schematics for the LM2575 are a little scary looking to me.

What is that Arrow point to the R2??? Any ideas about this design?

Re: Noob needs a little help on picking the resist

That last circuit appears to be some sort of DC-DC buck circuit (my guess). I am not the electronics guru of the forum I only read and glean knowledge but my calculations say you need 3.6v x 0.4A x 12hr or approx 17.28watt hours of power to drive all your LEDs. Assuming a 2500mah nimh has approx 3watt hours in them you would need at least 6 of them. This assumes no resistor or regulator circuit losses so adding in those with a guess of approx 75% efficiency my guess is you need 23.04watt/hours or about 8 batteries.
I am guessing either you can run 2 sets of 4 in parallel with resistors or 8 in series with a buck circuit as a regulator on 8 cells would be like an electronic resistor dropping in excess of 6v, while a buck circuit would be able to convert with more efficiency.

Re: Noob needs a little help on picking the resist

Ok well thank you for all the help Lynx_Arc. I really appreciate it. Here is a link to the discussion on the other forum if you care to see what comes about: http://www.edaboard.com/ftopic128083.html

also.. I made the LM317 circuit using 4.5v and dropping it to 1.85v to power a 1.7v 20ma Red led. It worked great. There was no heat build up on the LM317 that I could tell after a few mins of use. I wonder if this would still be ok for the 7.2v to 3.6v application since it's still pretty low voltage. I think it may have more to do with battery efficieny, but I don't know. Plus drawing 400-600ma instead of 20ma might cause a little more heat. Someone said the voltage loss is 50% lost through heat, making it less effecient I guess? It's a much simpler circuit though. Anyway, I appreciate all the help, my next problem is incorporating a photoresistor that turns off with light and on with dark.. see the thread if your interested. Thanks again!