Led In paralell?

congo_90

this is simple electronics i know but still i'd welcome your comments
Ok so i'm quite aware that LED's should never be wired in parallel but...

I was just wondering is it possible to have two series circuits share the same power source and switch? (see diagram)

If not my plan B is to have a sort of distribution box where the 12V can be distributed to all 4 LED's safely and once again, from one switch.

Lastly and most importantly is the type of cable i require. The source (battery) is 12V and i *think* 200-300 Amps (car battery). The cable will be coming from the positive straight and fused at some point along the line (probably close to battery for access.

Anyways thanks in advance!

DublinDilbert

congo_90 wrote: »

this is simple electronics i know but still i'd welcome your comments
Ok so i'm quite aware that LED's should never be wired in parallel but...

I was just wondering is it possible to have two series circuits share the same power source and switch? (see diagram)

If not my plan B is to have a sort of distribution box where the 12V can be distributed to all 4 LED's safely and once again, from one switch.

Lastly and most importantly is the type of cable i require. The source (battery) is 12V and i *think* 200-300 Amps (car battery). The cable will be coming from the positive straight and fused at some point along the line (probably close to battery for access.

Anyways thanks in advance!

Yes your circuit doesn't look too bad, there are a few things to look out for:-
1) Max forward voltage drop across the LEDs, depending on the LED colour the voltage drop will be different, say for a red LED its about 1.5V. This is why you can't connect 20 of them in series and run off a 12V battery.

2) kinda related to #1, your battery voltage should be great enough to overcome the forward voltage drop of the LED's + drive enough current through them.

3) The drive current through the LEDs is set by the series resistor. In one string of LEDs:-
Ileds = (Vbat-(Vled1+Vled2+....+Vledn))/R


The size of your cable will be set by 2 things:-
1) The total load of your circuit + a bit for good luck
2) The size of the fuse protecting the cable.

congo_90

@Dublindilbert, Once again you have helped me out (in the past you helped with similar things).
You're answer is pretty much what i was looking for,
I bought 15 Amp cable today along with a 15amp fuse. Do you think this will be sufficient? this fuse will support both circuits. The switch i'm using has a built in light so i'll also be running this constantly off the main wire with a resistor on it.

Resistors in use will be 1K ohm. hope it works and doesn't go up in smoke

kelly1

congo_90 wrote: »

Ok so i'm quite aware that LED's should never be wired in parallel but...

Hello, this is news to me. Why can't LED's be wired in parallel? Same voltage drop but double the current. What's the problem?

DublinDilbert

congo_90 wrote: »

@Dublindilbert, Once again you have helped me out (in the past you helped with similar things).
You're answer is pretty much what i was looking for,
I bought 15 Amp cable today along with a 15amp fuse. Do you think this will be sufficient? this fuse will support both circuits. The switch i'm using has a built in light so i'll also be running this constantly off the main wire with a resistor on it.

Resistors in use will be 1K ohm. hope it works and doesn't go up in smoke

15A Cable, how many LEDs are you hooking up?? :eek:

Typically LEDs require fairly small amount of current to light them up, used to be of the order of 5 - 10mA, although modern LEDs can now take up 250mA ( with much increased light output)....

If your worried about things going bang, best to set them up on a "bench" with a small power supply and go from there.... Also if you don't have a cheap multimeter pick one up, so you can measure the current etc...

DublinDilbert

kelly1 wrote: »

Hello, this is news to me. Why can't LED's be wired in parallel? Same voltage drop but double the current. What's the problem?

I see where your coming from on this one, and in theory if the LED's had "exactly" the same voltage drop you could do this.

The problem is no 2 LEDs will have exactly the same voltage drop, one might be 1.48 the other 1.49, for example. The one with the lower voltage drop will get the lions share of the current, hence will be much brighter and possibly damaged.....

kelly1

DublinDilbert wrote: »

I see where your coming from on this one, and in theory if the LED's had "exactly" the same voltage drop you could do this.

The problem is no 2 LEDs will have exactly the same voltage drop, one might be 1.48 the other 1.49, for example. The one with the lower voltage drop will get the lions share of the current, hence will be much brighter and possibly damaged.....

Don't mean to be pedantic but if they're wired in parallel, the voltage drop across each of them has to be the same. But I can see how the current would be different.

mathias

Your circuit is fine , in parallel each led must have its own resistor , choosing the right value of current limiting resistor for each parallel path is the key , heres a site that does it for you ,

http://www.quickar.com/ledbasics.htm

DublinDilbert

kelly1 wrote: »

Don't mean to be pedantic but if they're wired in parallel, the voltage drop across each of them has to be the same. But I can see how the current would be different.

Yes you are correct, the voltage across all the LEDs connected in parallel will be the same. The voltage across all of the LEDs will be that of the LED with the smallest voltage drop.

The LED with the smallest internal voltage drop will get most of the current, hence will be the brightest! The LED with the largest internal volt drop will be the dimmest or might not even light up at all.

When connecting LEDs in parallel there is fairly complex interactions with the VI curves for each LED, which will vary from part to part.

Why don't you try connect up 10 LEDs in parallel and let us know how you get on? it should be fairly easy to set up.

congo_90

DublinDilbert wrote: »

Yes you are correct, the voltage across all the LEDs connected in parallel will be the same. The voltage across all of the LEDs will be that of the LED with the smallest voltage drop.

The LED with the smallest internal voltage drop will get most of the current, hence will be the brightest! The LED with the largest internal volt drop will be the dimmest or might not even light up at all.

When connecting LEDs in parallel there is fairly complex interactions with the VI curves for each LED, which will vary from part to part.

Why don't you try connect up 10 LEDs in parallel and let us know how you get on? it should be fairly easy to set up.

10 LEDs? lol I dont need that many... Anyway just a little update; I blew a few LED's and established that 15A fuse along with a lotta wires connected to a car battery probably wasn't my *excuse the pun* brightest idea.
Tomorrow i'll stick a 5A fuse on the circuit instead and hopefully I won't fry anymore LED's!!

how high (in current) can I go to get Maximum brightness? say from a standard red led?

DublinDilbert

congo_90 wrote: »

10 LEDs? lol I dont need that many... Anyway just a little update; I blew a few LED's and established that 15A fuse along with a lotta wires connected to a car battery probably wasn't my *excuse the pun* brightest idea.
Tomorrow i'll stick a 5A fuse on the circuit instead and hopefully I won't fry anymore LED's!!

how high (in current) can I go to get Maximum brightness? say from a standard red led?

This is a common mistake when people start to use LEDs.

You need a current limiting resistor in series with the LEDs.

The fuse is a protection device. If you put in a 5A fuse the LEDs will still blow. If you put in a 1A fuse the LEDs will still below.

You'll need to get some sort of resistor to limit the current.

What's the recommended forward current for the LEDs from the Datasheet??
What's the typical forward voltage drop for the LEDs from the Datasheet??
How many LEDs are you putting in each string?
If you give us the answers to the above 3 questions, we'll show you how to calculate the resistor value.

Don't worry we've all done the same thing when starting to mess with LEDs

congo_90

DublinDilbert wrote: »

What's the recommended forward current for the LEDs from the Datasheet??
What's the typical forward voltage drop for the LEDs from the Datasheet??
How many LEDs are you putting in each string?
If you give us the answers to the above 3 questions, we'll show you how to calculate the resistor value.

Don't worry we've all done the same thing when starting to mess with LEDs

I'll have to get back to you tomorrow on those firsttwo2 questions but i will assume 2v and around *1.4 drop.. * and maximum two LED's per string.

I have 1K(ohm) resistors for each string. I'll need to fuse them inevitably or else risk damage to other components? I thought the fuse in unison with the resistor would prevent the LED's blowing. especially because the battery is high current. anyways will update tomorrow with the answers thanks again for all your help

DublinDilbert

congo_90 wrote: »

I'll have to get back to you tomorrow on those firsttwo2 questions but i will assume 2v and around *1.4 drop.. * and maximum two LED's per string.

I have 1K(ohm) resistors for each string. I'll need to fuse them inevitably or else risk damage to other components? I thought the fuse in unison with the resistor would prevent the LED's blowing. especially because the battery is high current. anyways will update tomorrow with the answers thanks again for all your help

The fuse is only there really to protect the cable in the event of a short circuit.

The resistor is what protects the LEDs from over current.

Yes the battery would be capable of delivering a large current, IF a small enough resistor is placed across it:-

Ohms law states V=I x R => I = V/R

If V = 12V and R=10 Ohm => I = 12/10 = 1.2A
If V = 12V and R=1 Ohm => I = 12/1 = 12A
If V = 12V and R=0.1 Ohm => I = 12/0.1 = 120A

Do you see how it works?

kelly1

Here's a good site:-

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

BTW, I take back what I said about it being OK to connect LEDs in parallel! :pac:

mathias

BTW, I take back what I said about it being OK to connect LEDs in parallel!

Theres no need to , LED's are fine in parallel as long as each parallel path has the correct current limiting resistor , ( The OP's circuit has a limiting resistor in each path which is fine ! ) see the quote from the site below ,

Avoid connecting LEDs in parallel!
Do not connect LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea.

theres now two sites posted here that will do the calculation for you. If you want to calculate yourself its quite simple and is below ,

The information you need to do the calculation is ,

The forward voltage drop of the LED
The Current for the required brightness , this is usually available off the data sheet , and will be in the order of Milliamps
The Voltage of the supply.

Then the calculation is Voltage of the Battery or source , minus the forward Voltage drop , then divided by the required current. This will give you a value in Ohms. Then buy the value of resistor closest to this. Choose the next highest value to your result. For instance if you get 420ohm then the nearest perferred value should be 470 ohm.

R = (VS - VL) / I

Where
R is the resistor value needed for each parallel path
VS is the supply voltage
VL is the forward voltage drop of the LED ( in the original drawing there are 2 series LED's in each parallel path so you need to add the Voltages for the 2 LED's in each path.)
I is the current needed for a certain brightness.

congo_90

for all to know,
these are the LED's i'm using. I never knew they had a lower viewing angle until tonight when i accidentally tested a 3mm and a 5mm on the same breadboard

Well, They're fitted now. Two 5mm led's yet I now need to run another 2 onto it.
I'm using a 2.2ohm resistor with 12v 15amps

The only thing is i'm concerned about running a total of 4 2.5v LED's off a 12v battery along with any other things ie; radio starter motor etc. Would i be safe to do this? it'll basically look like the first diagram with all 4 LED's

DublinDilbert

congo_90 wrote: »

for all to know,
these are the LED's i'm using. I never knew they had a lower viewing angle until tonight when i accidentally tested a 3mm and a 5mm on the same breadboard

Well, They're fitted now. Two 5mm led's yet I now need to run another 2 onto it.
I'm using a 2.2ohm resistor with 12v 15amps

The only thing is i'm concerned about running a total of 4 2.5v LED's off a 12v battery along with any other things ie; radio starter motor etc. Would i be safe to do this? it'll basically look like the first diagram with all 4 LED's

Sorry where does the 15A keep coming from?????
Where does the 2.2Ohm series resistor come from?? LEDs won't last any amount of time!

From Maplin:-

Forward voltage is 2.5V
Forward current is 30mA

Battery voltage with engine running is approx 13.5V

If you put 4 in series, the series resistor is calculated from:-
R = (13.5 - (2.5 * 4) ) / 0.03 = 3.5/0.03 = 116Ohms (use 120Ohms)

If you put 3 in series, the resistor is calculated from:-
R = (13.5 - (2.5 *3 ) ) / 0.03 = 3.5/0.03 = 200Ohms

congo_90

DublinDilbert wrote: »

Sorry where does the 15A keep coming from?????
Where does the 2.2Ohm series resistor come from?? LEDs won't last any amount of time!

From Maplin:-

Forward voltage is 2.5V
Forward current is 30mA

Battery voltage with engine running is approx 13.5V

If you put 4 in series, the series resistor is calculated from:-
R = (13.5 - (2.5 * 4) ) / 0.03 = 3.5/0.03 = 116Ohms (use 120Ohms)

If you put 3 in series, the resistor is calculated from:-
R = (13.5 - (2.5 *3 ) ) / 0.03 = 3.5/0.03 = 200Ohms

15 A was a mistake from my first attempts and now its pretty much staying there because i would have to replace a long length of wire etc otherwise...
so i went to maplins at lunch today before checking back here and was given 44ohm resistors.. now my circuit has two 2.2kohm and 2 440 ohm lol yes tis very confusing so to save myself a lotta hassle i just gave each led a resistor. now i've a dim one on either side.