Simple question about LEDs and resistors

First, I want to state that I'm not an electrician and that I don't know much about electricity.

I recently bought these battery operated led lights:
http://www.adafruit.com/products/1398

I want to cut the strand and keep only 16 leds. I want to power up the 16 leds with the two cr1220 batteries without using the controller. So the 16 leds would only be attached to the batteries.

My question: Would I need any sort of resistor in order not to harm the leds? I couldn't find any resistor inside the controller, so I'm not sure if it's needed.

I don't know any technical information besides what's posted on the website.

Hope it makes sense!

Welcome to CPF, Ghadames! :wave:

You can see there's only 2 wires running along the length of the string. Given the low voltage, the LEDs won't be in series. Which means there's basically just one voltage (due to wire resistance, slightly lower at the end vs. at the beginning of the string).

Which means that each LED will have an internal current-limiting component, or there's a resistor placed close to each LED. Probably visible to the naked eye, but internal or external resistor on each LED, isn't very interesting here.

First thing would be to measure in what range that "string voltage" is. Perhaps easiest would be to disassemble that 'controller' just far enough that it can be operated, with internal wiring accessible to do measurements. Then insert batteries, switch on, and measure between the outgoing wires what voltage goes into the string.

Probably you'll find that 'controller' is little more than a glorified switch + battery holder. At best it might have some electronics to reduce 2x battery voltage to something suitable for the string (probably somewhere in 3-4V range). Or does it actually allow changing the LEDs' brightness?

To change to X number of LEDs is easy: just count desired # of LEDs from the beginning of the string, and cut off the rest (on the end). Then isolate the new end such that it can't short-circuit accidentally. Probably you could do that right now, and the 'controller' wouldn't care. But I suggest you wait until you've opened up that 'controller' and checked what's inside.

I'd probably go for another power source. Common CR2032 cells are cheap / easy to find and have much, much more capacity than those tiny CR1220's. Better still would be a few AAA or even AA batteries in series.

Which brings up a few questions:

1) Do you have (access to) a voltmeter?
2) Can you disassemble 'controller' far enough to see what's inside, and perhaps post picture(s) of the inside?
3) Do you have (access to) a soldering iron?
4) If all you really wanted was a # of those LEDs on a string, why buy a much longer string + 'controller' ? LED strings can easily be found in many types / lengths etc these days. Chances are you'll replace that 'controller' anyway (and probably ditch it in the trash sooner than later ).

Ghadames said:

Hope it makes sense!

Click to expand...

Yeah it does, even without website info! LEDs are easy to work with. Much like ripping open a sandwich toaster & see how it heats your sandwich - no manual needed. :laughing:

Thank you so much, RetroTechie, for the detailed post!


The controller has different functions (fast blink, slow blink, etc), but I'm not interested in that.


Alright, I will go with the CR2032 cells. I don't want to use the AAA or AA batteries.


1)
Yes, I've measured the "string voltage":
http://www.cheapvoipequipment.com/IMG_6997.JPG
And I've included a picture of how I did it:
http://www.cheapvoipequipment.com/IMG_6998.JPG
2)
It's very small, and this is pretty much what it is. I don't think I need to do more dissembling. Here's a picture without the cell batteries:
http://www.cheapvoipequipment.com/IMG_6995.JPG
3) No I don't have soldering iron, but no worries, I think I can do this without one!
4) Because I specifically wanted those very tiny leds. This led string comes with 10 leds or 30 leds, so I didn't have any other option.

Given all this information, do I need to add a resistor or anything?
I personally would prefer to use only 1 CR2032 cell to light up the 16 leds. Would that be possible? Or do I absolutely need two cell batteries?

First, kudos for the measurement and pics. Properly done, and well described.

The description says it all in one sentence: "It is possible to cut the ribbon in half and then snap another battery pack onto the second piece. "

This means that running just 16 LEDs on one battery pack with no additional mods is perfectly fine.

HOWEVER, RetroTechie has suggested a mod that would provide much more life and cut your cost in batteries significantly (if you're going to go through more than one or two sets). It's quite possible that this mod would require a resistor to protect the string.

Why do I say this? The normal voltage of those cells is 3V or so. Two of them should be 6V. But you measure less than 4V on the string. There are three possible explanations for this:
1. There's some sort of current limiting or voltage reduction in the controller (possibly just a resistor you don't see).
2. The controller is using PWM to control the average voltage on the string, and the meter is measuring this, not the max.
3. The internal resistance of the 1220 cells is high enough to provide current limiting to protect the LEDs.

Personally, I think #3 is the most likely. I know that the internal resistance of a 1220 is pretty high. The internal resistance of a 2032 is much lower. That would (if #3 is the correct explanation) provide a substantially higher current to the string. That might still be okay, but it might not.

There is definitely some sort of chip in the controller. If it can do blinking and fading, almost certainly there's a microprocessor there using PWM to provide the effects. This makes explanation #2 more likely, but I'm sticking with #3.

You can answer the question pretty definitively with just a simple instrument that everyone carries around - your eyes. Just turn the string on full, and wave it rapidly through your field of vision. If the streaks you see are smooth and uniform, there's no PWM going on. If they look like dotted or interrupted lines, PWM is being used. When the string is not on full, you will certainly see this (well, I'm 99.44% sure). But when it's on full, I'm 95% sure you won't see this.

Here's another thing to try - when you cut the string, notice how much the brightness of the remaining LEDs changes. If I'm right, the brightness should increase noticeably. Of course this is a test you can only do once on a string. You should get the 'before' and 'after' as close as possible in time, because your eyes will not remember the brightness very long and the change won't be huge. I'd say it's safe to cut the string with it turned on. The 1220 batteries can't provide damaging current levels.

Oh, and one 2032 cell won't be enough. The voltage just isn't high enough.

Here are the results. Tests were done using CR1220 cell batteries.


When I turn the string on full and wave it rapidly, I do see dotted lines. When I first cut the first 16 leds (I did this after reading DIWdiver's post), I did not notice any significant changes in brightness. That was my first impression. But when I tried to connect the wires again, I did notice that the first 16 leds were slightly less brighter. I did this many times, and I can state that I do see a difference.


When I measure the two batteries together on their own (without them being inside the controller), I get 9v on the voltmeter.


BTW, when I did these tests, the knob of the voltmeter was set at 1.5v.

That sounds about right. If you make a battery swap and it gets LOADS brighter, you will want to add 1-10 ohms in series with the larger battery. That would indicate higher current consumption, harder LED running, and potentially reduced life.

If I understand you correctly, you want me to try the CR2032 cell batteries and if the leds get brighter, I would have to add "a 1-10 ohms in series"? My goal is to run the 16 leds with two CR2032 cells without using the controller.

My questions:

1. If I use the CR2032 cell batteries and the leds get brighter, can it harm them or burn them out?
2. When you say "1-10 ohms in series," does that mean adding a single resistor between the batteries and the first led?
3. Would this be a good 1-10 ohms resistor?
http://www.ebay.ca/itm/10-ohm-1-4w-...674?pt=LH_DefaultDomain_0&hash=item51ba1fceaa

1. Yes
2. Yes
3. Yes

But I'm concerned that you read 9V on the voltmeter. You should see only around 6V. Surely something's off there.

The fact that you see dotted lines when you wave the string AND see reduced brightness with more LEDs is very interesting. This suggests that PWM is being used to reduce the power dissipation and brightness in the LEDs, AND the internal resistance of the batteries is also having an effect. Given this, I think that using the larger batteries and removing the controller will definitely require a series resistor.

AnAppleSnail's recommendation of 1-10 ohms seems right. The resistor you linked to looks fine. There are many others that would work too. To get resistances less than 10 ohms, simply connect several resistors in parallel. The total resistance is 10/n, where n is the number of resistors connected in parallel.