Need some LED/power help

J

jedibugs

Well-Known Member
Hey gang. So, I just made a lighted display for my Sonic Screwdriver (here http://www.therpf.com/showthread.php?t=153734 ) and I was super proud of it. However, it seems that I'm not powering my LEDs properly, as it's dead after maybe an hour of total usage.

I'm using 4 LEDs, each rated at 3.3v (3.6v max) at 25Ma. Now, all that means very little to me, so I'm hoping someone who knows more can maybe give me some pointers here. I'm powering each LED with two AAA batteries. At first, it all seemed to work just fine, the lights were quite bright and there was no flicker or anything. Now the batteries appear to be totally dead.

In case it matters, I've got it wired as such:
-AAA+-AAA++LED--AAA+-AAA++LED...etc, with the negative lead from the last LED going back to the negative terminal of the first battery to make a complete circuit. Very straightforward.

As each AAA has an output of 1.5v, each LED is only getting 3v, so I'm thinking that may have drained the AAAs, but after only an hour?? I also have no idea what the 25Ma means (milliamperes? Even if that's right, I don't know what it means).

So, in short, can anyone recommend what kind of batteries and/or alternate wiring I should use to make this work properly?

Thanks in advance for any help!

-Shawn


Sent from my iPad using Tapatalk HD
 
If you burned out the LED's, save your money and hit the dollar stores for a cheap LED flashlight. You can take it apart, remove the LED's and run new lengths of wire... You will have a small chip board in the flashlight with a switch and all. Ive had to do this numerous times for instant lighting in a pinch.
 
robstyle said:
If you burned out the LED's, save your money and hit the dollar stores for a cheap LED flashlight. You can take it apart, remove the LED's and run new lengths of wire... You will have a small chip board in the flashlight with a switch and all. Ive had to do this numerous times for instant lighting in a pinch.
Click to expand...

The LEDs definitely aren't burnt out, I've verified that. But that's still a quite nice idea.
However, as my current LEDs are hot-glued in, I'd rather see if I can just swap the power and/or wire it differently.

Thanks, though... I may just end up doing that!

-Shawn


Sent from my iPad using Tapatalk HD
 
Problem is that the LEDs only need 25ma to work. But with the LEDs connected in your system there are no resistors. This means the stored energy in the batteries just drains through the LEDs.

You need to add a resistor to each of the LEDs. Somewhere around 60 ohms, then you can run all four off a three AAA batteries.

I can send you a drawing if you like.

Ed


jedibugs said:
Hey gang. So, I just made a lighted display for my Sonic Screwdriver (here My Sonic Screwdriver Display Case ) and I was super proud of it. However, it seems that I'm not powering my LEDs properly, as it's dead after maybe an hour of total usage.

I'm using 4 LEDs, each rated at 3.3v (3.6v max) at 25Ma. Now, all that means very little to me, so I'm hoping someone who knows more can maybe give me some pointers here. I'm powering each LED with two AAA batteries. At first, it all seemed to work just fine, the lights were quite bright and there was no flicker or anything. Now the batteries appear to be totally dead.

In case it matters, I've got it wired as such:
-AAA+-AAA++LED--AAA+-AAA++LED...etc, with the negative lead from the last LED going back to the negative terminal of the first battery to make a complete circuit. Very straightforward.

As each AAA has an output of 1.5v, each LED is only getting 3v, so I'm thinking that may have drained the AAAs, but after only an hour?? I also have no idea what the 25Ma means (milliamperes? Even if that's right, I don't know what it means).

So, in short, can anyone recommend what kind of batteries and/or alternate wiring I should use to make this work properly?

Thanks in advance for any help!

-Shawn


Sent from my iPad using Tapatalk HD
Click to expand...
 
NTBBCPD said:
Problem is that the LEDs only need 25ma to work. But with the LEDs connected in your system there are no resistors. This means the stored energy in the batteries just drains through the LEDs.

You need to add a resistor to each of the LEDs. Somewhere around 60 ohms, then you can run all four off a three AAA batteries.

I can send you a drawing if you like.

Ed
Click to expand...

Ed, that would be super-awesome of you!


Thanks,

-Shawn


Sent from my iPad using Tapatalk HD
 
Last edited:
If i'm reading it right and you're wiring all your batteries and LEDs as one single series circuit that is completely bonkers. If you want to just want to run on regular batteries the best way to go is to use a single pair of aa or aaa is series + of first to - of second then run the LEDs in parallel. + of each LED to battery + and - of each LED to battery -. There are better ways, either using 3 aaa/aa for 4.5v source or a single 3.7 liion cell but you'll need to resistor is appropriately. The above way will be be slightly underdriving the LEDs but it's simple and you won't need resistors.
 
So I used the LED wizard and I was right, It suggests 56 ohm resistors..
14919-leds-case.jpg


You should buy 4 resistors, 56 ohm , 1/4 watt. Radio Shack sells them

If they don’t have 56 ohm, any from 50 to 60 will do.

The 1/4 watt is the amount of heat the resisters produce. 1/4 watt is nothing, no chance of fire.

Stack three batteries together, this will give you 4.5 volts.

The Positive side goes to the positive lead LEDs and the negative side goes to the resistors.

Resistors of this type have no positive or negative so you can put them in with out worry.

The batteries have 1150mA per hour of energy and this circuit will draw 100 mA so the power should last between 7-10 hours...




Ed
 
Subvirtua said:
If i'm reading it right and you're wiring all your batteries and LEDs as one single series circuit that is completely bonkers. If you want to just want to run on regular batteries the best way to go is to use a single pair of aa or aaa is series + of first to - of second then run the LEDs in parallel. + of each LED to battery + and - of each LED to battery -. There are better ways, either using 3 aaa/aa for 4.5v source or a single 3.7 liion cell but you'll need to resistor is appropriately. The above way will be be slightly underdriving the LEDs but it's simple and you won't need resistors.
Click to expand...

Yeah, I never pretended to know what I was doing. I do have a bunch of resistors sitting around, so I think I should be able to fix this when I get home today, thanks to all the help I've gotten here.


Sent from my iPad using Tapatalk HD
 
Thanks, Ed. That should be a huge help. Guess I'll just have to use the lights sparingly.

-Shawn

NTBBCPD said:
So I used the LED wizard and I was right, It suggests 56 ohm resistors..
14919-leds-case.jpg


You should buy 4 resistors, 56 ohm , 1/4 watt. Radio Shack sells them

If they don’t have 56 ohm, any from 50 to 60 will do.

The 1/4 watt is the amount of heat the resisters produce. 1/4 watt is nothing, no chance of fire.

Stack three batteries together, this will give you 4.5 volts.

The Positive side goes to the positive lead LEDs and the negative side goes to the resistors.

Resistors of this type have no positive or negative so you can put them in with out worry.

The batteries have 1150mA per hour of energy and this circuit will draw 100 mA so the power should last between 7-10 hours...




Ed
Click to expand...




Sent from my iPad using Tapatalk HD
 
Thanks, Ed. That should sort me out quite well. I guess my dream of leaving the lights on for large chunks of time has been dashed, though. Stupid physics.



NTBBCPD said:
So I used the LED wizard and I was right, It suggests 56 ohm resistors..
14919-leds-case.jpg


You should buy 4 resistors, 56 ohm , 1/4 watt. Radio Shack sells them

If they don’t have 56 ohm, any from 50 to 60 will do.

The 1/4 watt is the amount of heat the resisters produce. 1/4 watt is nothing, no chance of fire.

Stack three batteries together, this will give you 4.5 volts.

The Positive side goes to the positive lead LEDs and the negative side goes to the resistors.

Resistors of this type have no positive or negative so you can put them in with out worry.

The batteries have 1150mA per hour of energy and this circuit will draw 100 mA so the power should last between 7-10 hours...




Ed
Click to expand...




Sent from my iPad using Tapatalk HD
 
LED's wont light much below their rated voltage (3.3V in your case) so once the batteries drop below that level, they quit lighting. The batteries will likely still read a volt or so across their terminals, but just not enough to light your led's.

If I read your post correctly, you're using a total of 8 AAA batteries. All connected in series should give you 12v with fresh batteries. I would wire it up like this:

attachment.php


I used 20ma instead of 25, since that reduces the current draw by 20% which will make them last longer and will probably not be noticeably less bright.

Doing it this way requires your battery voltage to drop to around 50% before the LED's won't light anymore. Get rechargable NiMH batteries, they have good capacity and obviously you can recharge them...
 
Wow, so I was REALLY over-thinking this. In the end, the solution turned out to be dead simple:
7552568102_a2f7dc0d4d_c.jpg


Sent from my iPad using Tapatalk HD
 
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