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Old Oct 04, 2012, 02:40 PM
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Desoldering resistors on LED strips to run at a lower voltage and amperage

I'm in the process of setting up lights for a 4 cell plane. In order to do so I wanted to bring down the voltage (a fully charged 4 cell runs the strip lights a bit too hot for my taste). I have a couple of these step up/step down regulators that I didn't use for another project. When I measured the current of 3 strips, it was pulling about .9 amps. Sadly that means I'd have to run one of those regulators per each strip since they max out .5 amps, or get a larger regulator that can handle about 1-2 amps. However, this seems like such a waste since a good portion of that is just going towards heat energy.

So I was wondering... How bad of an idea would it be to desolder all the resistors, replacing them with simple wire, and just tune the regulator down to ~6v (exact voltage TBD) running no resistors at all. When I took an electronics course back in the day they said you always have to run a resistor, but practically speaking is this necessary? In my initial tests on a small strip they ran fine and very, very cool to touch, as was the regulator.
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Old Oct 05, 2012, 08:44 AM
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Bonnie Scotland
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The resistors are there to limit the current, not reduce the voltage, so if you remove them the LED strips will draw more current to the point where they burn out. Besides the above electrical reason, what you propose is going to be really fiddly to do.
LED strips work best with 3 cell lipos so all you really need to do is power them separately from a small 3 cell pack. At the sort of figures you are getting a 1000mAH pack should last more than 1 hour and will only weigh about 80g. Im' guessing if it's a 4 cell plane it could handle this small extra weight.
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Old Oct 05, 2012, 03:52 PM
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This is for a Crash Test Hobby Assassin running a 4 cell 1000 mah battery, so running another battery just for lights is not a practical option. Desoldering these resistors is surprisingly quick with a 40 watt iron, particularly when your objective does not involve ever reusing said resistors again. Given the size of the airframe and that there is only one resistor per three lights, I would only have to desolder about 20-35 for the setup I'm thinking of so fiddly is not really a concern. The particular regulator I mentioned puts out a max of .5 amps, so amperage is greatly reduced from the battery.

That all said, I'm still not sure if the LEDs will be able to run off the regulator I mentioned alone in the long run or if I'd have to set it up in another way. Alternatively I could just get a step down regulator that can run the lights off a 4 cell, which of course would probably be the easiest route but I already have the other regulators.
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Old Oct 05, 2012, 05:18 PM
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Anyway, sorry, so it sounds like the no resistor idea is a bad one. I'll look into alternatives.

Thanks.
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Old Oct 07, 2012, 01:13 AM
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You can tap off of 3 cells from your balance lead. Some people say the LED strips don't draw enough power to affect the balance of the pack some disagree. With a 1000mAh pack not sure? I have not personally tried it myself. A thread in this section from not long ago shows a drawing of which wires in the balance lead to use for 3 cells.
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Old Oct 08, 2012, 03:14 PM
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At 12V 1 meter of LED strip draws 200ma. I use a dimming circuit to drop the current to 70ma and they are still plenty bright.

Your best option is to use a smaller 3 cell battery, 11.1V, to run the lights. Regulating the 14V down will work but it always throws power away in the form of heat. Say we are going for 11V regulated and sticking with the 200ma, which actually drops as the V drops. 3V drop and 200ma is .6W. Whether using resistors or diodes to drop the voltage you will still have to use something with a 1W body. A single 3.5V, 1W Zeiner would work.
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Old Oct 09, 2012, 01:53 AM
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I ended up using less lights than I originally planned - around 1 meter total, which pull under .4 amps. So I decided to go with a GWS ICS-100Li (to both act as a switch and dimmer) wired to the .5 amp step up/step down regulator set to ~10v to keep the light strips cooler with no noticeable dimness. The two of them combined came in at 10 grams, which I greatly prefer over having another battery on board that I'd have to make room for and remember to charge.

I think this is what I will do if I am going to run lights off of anything greater than a 3 cells from now on, and even on a 3 cell I'll probably use the esc. If I need more than .5 amps, I'll go with a 3 amp step down, which only adds about 5 grams more than the .5 amp.
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Old Oct 21, 2012, 12:29 PM
ltc
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Quote:
Originally Posted by Abditus View Post
Anyway, sorry, so it sounds like the no resistor idea is a bad one. I'll look into alternatives.

Thanks.
The 'no resistor' is not a bad idea; it is just not compatible with a fixed voltage source.

If you substitute a voltage source for a current source, it will work perfectly. The current source will automatically adjust its voltage output to maintain a fixed (programmable) current thru any string of LED's.

Most commercial LED applications use current mode drivers.
You can buy a dedicated IC or simply convert a simple 3 terminal linear voltage regulator into a current regulator.
It is very straightforward and there are demo boards and application notes available on the web.
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