One resistor instead of multiple in parallel (For parallel LEDs) ?

I'm making a circuit that lights up some LEDs

My power source is a 9V battery and I've calculated that I need 1ohm resistor on all series of LEDs (See Circuit "A" in the photo below)

Can I use a single resistor at the beginning of the circuit to light up all the LEDs ? If yes, what would be the value. (See Circuit "B" in the photo below)

Now, If I want to array like that about 100 LEDs.. will a 9V battery be enough to keep the light on for a couple of hours? What is the formula to calculate that?

Thanks

Can I use a single resistor at the beginning of the circuit to light up all the LEDs ?

No.

The current will not share equally down the two serial strings of LEDs

will a 9V battery be enough to keep the light on for a couple of hours?

What sort of 9V battery, a small square PP3 type battery definitely will not.

If you want control of individual LEDs, rows, or columns that this matters. It matters because then the amount of current being drawn changes. When current through a resistor changes so does the voltage it drops.

1 ohm? I'd like to see your math. That doesn't sound likely.

If I want to array like that about 100 LEDs.. will a 9V battery be enough to keep the light on for a couple of hours?

9V batteries have about 500mAh of capacity. So if your leds draw 20mA each, that means you are drawing 2A. So your battery (assuming it can actually supply 2Amps continuously) will last less than 15 minutes.

99 leds, in strings of 3 equals 33 strings. At a nominal 20ma per string the load current is around 660ma. assuming 2 hours running and no more than 50% depletion of the battery you will need a capacity of .66x2x2 = 2.6AH. Assuming you are using a NiCad or similar type model RC battery rated at least 2500mAH you are going to be OK, providing you appreciate that these are generally 8.4 volts, not 9. Forget about zinc-carbon type 9 volt batteries, they are only for people with money to waste.

If one assumes each LED forward voltage is around 1.5 volts then your series resistors will have to "dump" around 4 volts. At 20ma this makes them 4000/20 = 200ohms !!! say 220ohms. Power rating is 16/220 so 1/4watt units will be suitable.

If the LEDs were 1.5 each, I'd suggest 5/string instead and make the most of the 9V. (9 - (5 x 1.5))/.02A = 75 ohm resistor, dissipating 30mW. Measure the LEDs, might even be able to get away with 6 in series. 20 strings, 400mA. 6 AA batteries @ 2500mAH will last several hours.

If all the LEDs are from the same manufacturing batch you might get away with it - but they need to be running at the same temperature. Basically the forward voltage is a function of current and temperature and varies between batches. Current is sharply (exponentially) sensitive to the forward voltage for a diode.

I was told by a friend to use 2 AA battery with a step-up circuit from 3V to 9 or 12V, what do you guys think?

Is there an alternative/better solution for me to create an array of leds?

Thanks

I was told by a friend to use 2 AA battery with a step-up circuit from 3V to 9 or 12V, what do you guys think?

Change your friends. That won't last very long and switch circuits that can handle that much current are expensive. Maybe if you actually said what you wanted to do we could be a bit more specific.

Grumpy_Mike:

I was told by a friend to use 2 AA battery with a step-up circuit from 3V to 9 or 12V, what do you guys think?

Change your friends. That won't last very long and switch circuits that can handle that much current are expensive. Maybe if you actually said what you wanted to do we could be a bit more specific.

I just said it.. I want to build an pannel with ~100 leds

I got a lot of High-Intensity White LEDs (3V) and I need a light source for doing videos

You may find that a large bank of white high intensity leds gives a rather "cold" colour for video work. You might be better working with the three primary colours and adjusting each colour to provide a suitable colour "temperature"

Also if your led forward voltage is 3 volts and they are high intensity you really need to provide model number before you get informed answers to your question, since you will not be able to run 3 in series off 9 volts and 20ma will not give "high intensity" brilliance. This may suggest where your 1ohm resistor came from but such a low value is unlikely to provide satisfactory current regulation.

Here are the specs of the LEDs

About the color, I will deal with the color balance in the camera… Eventually if I use that a lot I will go for different color LEDs but for the moment this is what I have in my hands

  • Emitted Colour : White
  • Size (mm) : 5mm
  • Lens Colour : Water Clear
  • Peak Wave Length (nm) : N/A
  • Forward Voltage (V) : 3.2 ~ 3.8
  • Reverse Current (uA) : <=30
  • Luminous Intensity Typ Iv (mcd) : Average in 20000
  • Life Rating : 100,000 Hours
  • Viewing Angle : 20 ~ 25 Degree
  • Absolute Maximum Ratings (Ta=25°C)
  • Max Power Dissipation : 80mw
  • Max Continuous Forward Current : 30mA
  • Max Peak Forward Current : 75mA
  • Reverse Voltage : 5~6V
  • Lead Soldering Temperature : 240°C (<5Sec)
  • Operating Temperature Range : -25°C ~ +85°C
  • Preservative Temperature Range : -30°C ~ +100°C

I need a light source for doing videos

OK that’s the vital piece of information we were missing.

So it looks like you want to run them at 20mA. So that does not explain where a value of 1 ohm came from.

As a quick calculation:-
Choose your battery and then arrange the LEDs in series allowing about 3V to drop across the resistor.
So for a 12V battery that would be 3 LEDs in series.
At a nominal forward volts drop of 3.5V per diode you have 10.5V across the diodes and 12 - 10.5 = 1.5V so at 20mA you need a resistor of
1.5 / 0.02 = 75 Ohms

Thanks Grumpy_Mike

Here is the website I used to calculate the 1 ohm.

http://led.linear1.org/led.wiz

About the 12V input.. I want to use a small battery.. that's why I started with a 9V battery but I'm starting to realize that a couple of AA batteries would be better.. I don't want to have 8 AA battery in serial, what are my options? That's why I came up with a step-up circuit as a friend suggested

Here is the website I used to calculate the 1 ohm.

Yes but what figures did you put in. When an LED resistor calculator produces a result of less than about 33R it is time to throw away the design and come up with something else.

what are my options

  1. Buy several 20 LED torches from a cheap shop, I have seen them for less than £3.00
  2. With 9V you could only have 2 LEDs in series.
  3. Two AA batteries would not even light up one LED
  4. Three AA batteries would be needed for one LED

That’s why I came up with a step-up circuit

You need lots of them because the current they can provide is limited.

Here is the website I used to calculate the 1 ohm.

You put in 3V for the forward bias didn't you? Instead of something >3.2 which is what your datasheet shows.

The calculator got tripped up because the forward biases of your ideal LEDs was exactly the same as your ideal voltage source. In the real world, nothing is so ideal.

Would a battery pack like that be enough to power ~100 LEDs ? http://www.ebay.ca/itm/8AA-RC-Ni-MH-12V-1800mAh-Battery-Packs-Green-NO-483-New-/150643064943?pt=LH_DefaultDomain_0&hash=item231306c06f Here are the specs: 12 V, 1800 mAh

@James C4S, you're right, I have put the Power source as 3V and the LEDs at 3V/20mA

Yes that battery would last about two and a half hours for 33 strings of 3 LEDs per string running 20mA per string.

I think I'd suggest a 7.4 Li-Ion or Li-Po battery pack. A 4-cell battery pack like one of these: http://www.tenergybattery.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=127&category_id=7&manufacturer_id=0&option=com_virtuemart&Itemid=27 7.4V so you have 2 LEDs in series, 50 strings x 20mA = 1 amp, @6600mAH can run for 6 hours (6 batteries)

http://www.tenergybattery.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=131&category_id=7&manufacturer_id=0&option=com_virtuemart&Itemid=27 or 4 cells in series, have 25 strings of 4 LEDs each. 25 * .02 = .5Amp, @ 2200mAH can run for 4 hours. (4 batteries)

Seeing a trend here? 1 cell = 1 LED string, 2 cell = 2 LED string, 4 cell = 4 LED string.

http://www.tenergybattery.com/index.php?option=com_virtuemart&page=shop.browse&category_id=7&Itemid=27

7.4V so you have 2 LEDs in series

But the spec says the forward volts drop could be up to 3.8V so two LEDs is 7.6V, you can’t supply this from a 7.4V battery.

LED forward voltages vary so much that they never match the datasheet "worst case" value in my experience! The datasheet give the voltage you'll need in the worst case (worst batch, coldest temperature, highest current). For instance I have a high power blue LED that measures 2.9V (nowhere near the 3.8V max voltage that you have to design for) and a high power red LED that measures 2.59V (red is typically 2.1V in datasheets!).

For this reason many driver chips use constant-current drive for LEDs.