BJT as switch...

[Ian Rogers]

Worst case senario...... 56 ohm shorting 9V rail.... no LED's and no transistor..... 9V / 56Ω = 161mA... Therefore... 9V * 161mA = 1.45Watts.. Even on a heat sink it will be okay..

LED's will give about 1.8v drop so 7.2V / 56Ω = 128.5mA... 7.2V * 128.5mA = 925mW....

I think you have something wired wrong.....

 

[alec_t]

Actually there are 1500LED

Really? In that case, allowing for the forward voltage of each (red) LED to be ~2V and some overhead for control you could run 500 chains of LEDs from a 9V supply, each chain having 3 LEDs in series and drawing 4mA. The total current would be 500 x 4mA = 2A, so your proposed supply with a 2A rating would allow no safety margin. You would also need 500 resistors and a transistor rated at >2A . But before you even think about wiring up such huge numbers of things, get this working with just one string of 3 LEDs.

 

[misterT]

OK, should i connect 15 led per 110ohm resistor of 2watt..

Are you saying that you are going to use one resistor for a group of LEDs connected in parallel?

If you connect LEDs in parallel you will need a resistor for each LED. That is why series connection is much more simpler and more efficient. Series connection needs only one resistor.
You can also combine series and parallel connections like this:




EDIT: Ok, maybe you can use a single resistor for many LEDs in parallel, but all the LEDs need to be close to "identical". If you have bunch of different color, size and brand LEDs then you are in trouble.

 

[Ian Rogers]

Remember that Ritesh wants to "adjust the brightness" of the LED's.... That's mainly why he wants the parallel connection...

 

[misterT]

Remember that Ritesh wants to "adjust the brightness" of the LED's.... That's mainly why he wants the parallel connection...

Individually or in groups? Or all together?

 

[Ian Rogers]

No Idea!!! I've find it hard to get the answers of my postings.... Very difficult!!

 

[blueroomelectronics]

Why do you guys bother. Just makes me want to pull my hair out.

 

[rumpfy]

I'm with blueroom;

Cant put Leds in series cos its too hard!
so where does the series resistor go if its too hard!
cant use them in parallel cos they wont share current equally
yes you can, I've done it
Then alec_t says; 'no you cant use leds in single parallel'
Really!!!
I'm just exhausted!

 

[koolguy]

Individually or in groups? Or all together?

Actually I am making LED sign board for which equal brightness is nice and so, i deiced to make // connection for so many LEDs.

The ETO older version is better...!!

 

[Ian Rogers]

The ETO older version is better...!!

I don't agree.... Once this is up and running, it will be far superior...

 

[alec_t]

Then alec_t says; 'no you cant use leds in single parallel'

I said no such thing! Someone else may have.

 

[Ian Rogers]

I said no such thing! Someone else may have.

It was MrT

Anywho!!! Whats your take on parallel LED's.... I think it should be okay as long as they are equivalent LED's

 

[alec_t]

My take on parallel LEDs:
* It's done for cheapness.
*IF they are perfectly matched (unlikely) there's no problem.
*If they're not, then the current distribution among the LEDs will be uneven, so those with higher than average currents will tend to have shorter lives than the others. Whether or not that's acceptable is a commercial consideration.
*Parallel connection of strings of non-matched series-connected LEDs is preferable for longevity.

 

[koolguy]

OK, so please tell me what will be better step for // connection of Led with 2 watt 110ohm and bc337 at any suggested voltage?

 

[alec_t]

See post #22

 

[koolguy]

See post #22

can't we use single // led no series, understand me making Led sign board with series connection at very close gap is very very difficult without PCB..

 

[alec_t]

Do you want to burn out all your LEDs? Why is a series connection more difficult than a parallel one? Both involve soldering to 2 terminals of each LED.

 

[koolguy]

Do you want to burn out all your LEDs? Why is a series connection more difficult than a parallel one? Both involve soldering to 2 terminals of each LED.

In // connection i just solder in one time directly in series every time i have to cut and solder take lot of energy and nesting of wires.....

 

[alec_t]

in series every time i have to cut and solder take lot of energy

Oh, you poor thing! I suspect the energy expended trying to solder to a common bus-bar acting as a heatsink might be even greater . There are actually fewer joints involved with the series chain method:-

 

[koolguy]

OK, for series tell me what to do for White LED ?
how many series led how many R to connect with it?