Hi,
I am quite new to electronics however I have completed a couple of simple circuits (with the help of the software Crocodile Clips).
My new project is to light up 270 LED's with a 24volt battery. I am struggling to do so as I keep blowing the resistor (in the simulation), due to the reason I am merely guessing values.

They are 3mm LED's i am not sure of the voltage etc but for this is what the software tells me the LED's properties are:

Part parameters:
Maximum brightness current: 10 mA
Forward voltage at 10 mA: 2.0 V
Maximum reverse voltage: 5 V
Maximum current: 30 mA

I am running the circuit each led in paralel. So I have the battery connecting to a resistor (?) then connecting to the positive of each LED and back round again to the negative of the battery.

Please excuse my very limited knowledge.

Pulling my hair out. Any ideas would be much appreciated. Thanks

 

Hello,

You can make 27 strings of 10 leds with a resistor in series.
The resistor value will be (battery voltage - (number of leds * voltage of leds)) / current through the leds.
I will calculate the resistor for 15 mA through the leds.
So (24 - (10 * 2)) Volts / 15 mA = 4 Volts / 15 mA = 0.266 kOhm.
Take the next higher value for the resistor => 270 Ohms.

Bertus

 

LEDs are not controlled by voltage. They are controlled by current. You must use a current controlling circuit. You can not put LEDs in parallel. They don't share the current properly.

http://forum.allaboutcircuits.com/showthread.php?t=19075

 

Here's a handy LED array calculator. Put in your parameters, and it will spit out several possibly solutions. Keep in mind that the battery will probably not be exactly 24V to start with, depending on chemistry, and the voltage will drop as the battery discharges, so you should tweak the design parameters to reflect actual battery voltage to avoid overdriving your LEDs.

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

Also keep in mind that the resistors will respond linearly to changes in voltage, so the current through the LEDs--and therefore brightness--will decrease as the battery discharges. If that's problematic for your application, consider using some sort of current regulator.

 

Thank you for your assistance Bertus. I am going to follow your method and will let you know how I get on. Thavinator: great tool! Just what I need. Cheers

 

Hello,

As you might have noticed I have taken a certain voltage drop across the resistor.
When the battery is 24 Volts the current will be 4 / 270 = 14.8 mA.
When the battery drops to 23 Volts, the current will be 3 / 270 = 11.1 mA, still enough for good brightness.
When the battery has a higher voltage , say 25 Volts, the current will be 5 / 270 = 18.5 mA, wich is still safe for the leds.

The ledwizard that thavinator has pointed you to, will probably give 11 leds with a lower value resistor.
The current regulation will not be as good as with my suggestion.

Bertus

 

Be careful with online calculators. They can sometimes give stupid answers, like 1 ohm. No way you're going to add up LEDs to within .02 volts of the supply and protect them with 1 ohm. Check back here if you suspect you got a stupid answer.

ps, my "rule of thumb" is that you have to waste about 15% of the supply voltage in the resistor to get reliable results.

 

Bertus, I attempted your suggestion (along with one of the solutions from the tool thavinator posted) and cannot get the circuit working as the battery seems to blow. I have a screenshot of the circuit here : http://www.whatsonyourwall.net/270led.jpg

What am I doing wrong?

Thanks
Stephen

 

The picture is obscured but I'd say you connected both terminals of the battery to the same wire.

 

Stupid answer. The actual current is about 4x 10^3 ma, not 10^24 ma.
Good on you for checking back with this.

 

Bretm, I have uploaded a new picture without the caution window: http://www.whatsonyourwall.net/270led2.jpg

If you are correct how should the battery be connected. Again, very sorry as I am new to electronics and am only starting to learn the basics.

Thanks

 

The negative part of the supply has to run all the way around to the resistor connecting wire.

 

Ah of course. I have now done that however the LEDS are not lighting up on the simulation, but there is current flowing through them. I guess its a matter of atapting the values of the resistors.

Thanks for your help mate

The negative part of the supply has to run all the way around to the resistor connecting wire.

 

No joy at all at getting them to even put out a dim light.

The negative part of the supply has to run all the way around to the resistor connecting wire.

 

Are you trying to get yourwall.net to glow or are you connecting LEDs and resistors?

 

I do not understand? I am definetly not trying to spam. Its the server I have access to. I will put further screenshots in dropbox if its a problem.

Are you trying to get yourwall.net to glow or are you connecting LEDs and resistors?

 

Complete misunderstanding.

Are you connecting electronic parts with wires on them to see if they glow?

 

I have uploaded the latest image of the simulation (in Crocodile Clips) here: https://dl.dropbox.com/u/40399020/270led3.jpg

Yes. I am trying to complete the circuit in the simulation so I can build it. Its actually for a 24v lorry battery to light up a series of LEDS on a board. I just am trying to get it working in the simulation properly so I know what values of resistors etc to order.

I am only learning the basics of electronics and apologies if I am being dumb with this one. Any help is appreciated. Thanks

Complete misunderstanding.

Are you connecting electronic parts with wires on them to see if they glow?

 

Reverse the battery. I wasn't paying attention.

 

Thanks mate. It seems to be working perfectly now. Thanks to everyone for their help. Fantastic

Reverse the battery. I wasn't paying attention.