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....
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.
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.
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!
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.
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..
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.
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.
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:-