Electronics Moron Q pt 2 [wiring multiple LEDs in series and parallel]

ok back to the the moron q about wiring leds i asked a couple days ago - now i understand how resistors work for a single led, but if i wanted to hook up a string of leds off of one molex connector, how do i go about doing this?

[edit by linear: topic clarification]

 

Depends how many you're wanting.

Quick lesson about series and parallel:

all elements in a series circuit carry the same current
all elemnts in a parallel circuit have the same voltage across them

So really the best approach is to string as many LEDs as possible in series, but you will quickly find yourself constrained by supply voltage. So we'll have to compromise a bit...

Let's assume you want blue LEDs that are rated 3.7V @ 20 mA. Let's further assume +12V supply.

Three LEDs in series will drop 3 * 3.7 = 11.1 V. Adding a fourth one to your series circuit will causeno current to flow, because there's not enough voltage across all four diodes to make them all conduct. So three would be the practical series limit for a 12V supply.

Finding the resistor for this series combination:

R = (12 - 11.1) / 0.02 = 45 ohms (use a 47 ohm)
P = (12 - 11.1) * 0.02 = 18 mW (so a cheap 1/4W resistor will be fine)

Now if you consider this series circuit of three LEDs + one 47 ohm resistor your basic 'building block,' you can then wire these in parallel up to the current limit of your power supply.

Let's say you scavenged a wall wart from an old modem or something, rated +12V, 200 mA.

Each of your series strings is drawing a little under 20 mA, so you could place ten of these in parallel across this supply. In practice, I'd be a little uncomfortable going so closde to the rated current, but if it's a junk transformer anyhow, you probably don't have much to lose. Just make sure it's 12VDC not AC.

If you are using a computer PSU, the current available is huge from the +12V rail, so knock yourself out.

 

ummmm.....i understand about series now...but......uhhh......whats parallel....and no, im not 10, i just dont know anything about electronics. i get the entire stringing a series of 3 blue leds , makes sense now that you explained it, but what did you mean by running as many as i want in parallel??

 

how to wire it up

ok, if you wire something in series, it means connecting the +ve end to the -Ve end in a chain like so:

this means way the current draw is the same as for on, but the voltage is more.
in parallel all the +ve sides are joined to the +ve power supply, and all the -ve sides are joined to the -ve power supply, like so:

This uses the voltage of 1 led, but the currents add up to be big.
so, to get the best of both world as the other person said you use that chain of say three leds and a resistor and wire each chain in parallel like so:

this way you can just keep adding chains untill your power supply is close to its maximum current rating. but don't go to close or you may have a sticky and very smelly mess on your hands are your psu melts!
Good luck and if you have any more Q's just ask

 

i think it makes sense and with a little tinkering i could probably do it, but does anyone have any pics of leds in parallel, that would probably make it crystal clear....thanks

 

i made a pic of what i think it loks like but i have no friggin clue how to post pics, so if first i could get help with this (how to post pics) then we can maybe address the real question

 

posting pics

posting a pic is a little complex, because you need some web hosting of your own. Basically you just need some web space somewhere (eg your isp proboly gave you 5meg free or sum thing) once you have some web space, you need to 'upload' your picture to it. most of the time this is through ftp, but some providers (like geocities have a web page you can do this through.) so. one you've uploaded a picture to your own webspace, it will wave its own url.
for example say you had ntl as an isp and you had signed up with the user name 'mine'. you would then have the webspace
www.mine.ntlworld.com
or sumthing simmilar. so if you but a picture called test.jpg onto that site the url would become
http://www.mine.ntlworld.com/test.jpg
so, when you come to post you see all of the buttons just above the 'Your Reply' Window, you need to click the 'IMG' button, and enter the pictures url into the box which comes up.
a bit of code will then appear in you post pointing to the picture. when you have finished and submitted you post, all being well it should appear in the right place.

I hope i've been sort of clear. i haven't gone into too much detail on uploading and stuff, because i don't want to sound too patronising. its your still stuck, i'll c what i can do.

hope this help a bit

 

dunno if this helps

here, i've done another pic of some leds in parallel. this time useing the graphical aspect of my bit of software. as you can see from this all the flat sides of the LED (the anode) are joined together and taken to the negative on the battery. and ll the normal sides of the LED (the Cathode) are joined and taken to the positive on the battery.

in real life you would proboly need some resisters though, but we explained that earlier

if this doesn't help, i may have to dig out my very ancient and dody digital camera and photo some real ones.
oh, and if you give me the specs of the leds you want to use, and the powersupply, and how many you want, i can draw you a diagram to follow

 

ok, now i feel like a moron - i didnt know that i had to have my own hosting i just figured that i some how uploaded my pics to this server - but i see now....as for teh diagram thats more or less what i had drawn - i think im good for now but im sure ill be back next week when i actually wire everything up. thanks.

 

ummm....quick Q teh reason i asked this Q in the first palce was because i was going to light some leds in my HD window - small problem, i had 3 leds running @ 3.5V each and 20mA - did some math (75 ohm resistor), wired em in series, and tada! no light, however, this guy: http://www.moddin.net/article.asp?ArticleID=33&PageNo=2 used a 440 ohm resistor, and it worked...did i do something wrong...

3 leds (3.5V and 20 mA each)
12V powersupply
75 ohm resistor


any help?

 

ummm, i don't know why your not getting any light. I think the resistor value is a but low, i work it out to be 525Ù
R=3.5x3/0.02 = 525 so go for a 510Ù resistor.

still, having too low a resistor shouldn't stop you, the led's should light, very brightly, and if left connected for a bit will all burn out.
the only thing i can guess is that one of the led's in the chain is broken, or you've wired them up the wrong way around, or just one the wrong way.
i'm sure you've tested all thoses options, but its all i can sujest without seeing a detailed pic

 

i got my resistor value from the led calc found on this site. as for the wiring, i tested all the possibilities you mentioned and then some. (wired properly, (+) to (-) and (-) to (+) and so on, and the resistor was on the last led's long leg) any other idears?

 

ooppps, yeah i did do that resistor ting a bit wrong. i forgot to subtract the total led voltage from 12V.
Appart from what i've already said, i really have no idea what could b the problem.

Have you got any other LED's that you can 'play' with to try again. i would sujest some cheap red or green ones. then when you've got them working move onto the blue ones. obviously you'll need to work out the resistor values again.

appart from that. i have no more ideas. Does anyone else??

 

So lets see if I've got this right.

If I wanted a single string of 1.8v LEDs powered from my 12v molex lead and connected them in parallel (say using two tracks on a piece of stripboard) - all I'd need is a single 340 ohm resistor and I could connect as many LEDs as I want (within the bounds of common sense)?

Time to get rid of my delicate CCFLs and use LEDs instead!

 

The resistor will change as you add LEDs.

If you add LEDs in parallel, the resistor dissipates more power but stays the same resistance value (ohms)

If you add LEDs in series, your resistor dissipates less power, and the resistance gets smaller.

Look at 6 of your 1.8V LEDs in series:

R = (12 - 10.8) / 0.02 = 60 ohms
P = (12 - 10.8) * 0.02 = 24 mW

But for 4 in series:

R = (12 - 7.2) / 0.02 = 240 ohms
P = (12 - 7.2) * 0.02 = 96 mW

6 in parallel:

R = (5 - 1.8) / 0.12 = 27 ohms
P = (5 - 1.8) * 0.12 = 384 mW (1/4 W resistor won't cut it--it just gets worse at 12V supply)

4 in parallel:
R = (5 - 1.8) / 0.08 = 40 ohms
P = (5 - 1.8) * 0.08 = 256 mW (I wouldn't push my luck with a 1/4 W resistor here either)

Those milliwatts dissipated in the resistor are converted to heat.

 

okay feeling like a bit of a n00b here, but my physics teacher sucked last year, we spent like a week on electronics. Can you put the resister _anywhere_ in the circuit so long as the current has to pass through it in order to get to the led?

 

here is an image to illustrate my question, would the resisters in these two circuits have the same effect?

 

I was thinking of using up to 20 or so LEDs (1.0cd, 1.8v, 300mA, 100mW) in a single row, using stripboard for ease of use.

As for the resistor - I could always use a 2W rather than a 1/4W. They only cost pennies anyway.

Or maybe I should use the 5v lead from my 400W Macron PSU instead of the 12v.

You mentioned something about heat - exactly how hot could the resistor get?

 

Sobriquet: the two circuits you have drawn are equivalent. Series connected elements all carry the same current by definition, the order does not affect this.

Slartibartfast: 300mA has to be a typo. How about 30 mA? Or is 300 mA supposed to me 15 mA * 20 LEDs?

Anyway, using the 12V supply will let you do 6 in series (10.8V dropped across the six) witha 60 ohm resistor.

You could put three strings of six in parallel, I'd use a resistor per string (because I have a drawer full of 1/4W resistors mainly) but you could limit current to the parallel circuit with a single resistor:

R = (12 - 10.8) / 0.06 = 20 ohms
P = (12 - 10.8) * 0.06 = 72 mW <- a 1/4 W resistor would handle this just fine.

All 20 in series is not an option, because the voltage drop required to light them exceeds your available supply voltage.

 

Yup - the mA is a typo. Their actual rating is as you surmised - 30mA.

Problem is I really want a single row (or double row maybe) of LEDs to obtain the effect of a long strip of light (like a CCFL does).

Could I connect the first six in series (by cutting tracks on the stripboard) and the rest in parallel?