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Old Oct 05, 2013, 02:20 PM
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Troy's Avatar
United States, CA, Lake Forest
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Step down voltage from 4S to power LEDs...options

Fairly easy topic but just weighing my options to step down voltage from a 4S pack (16.8V peak) to around 10-11V to safely power some LEDs on my quad. I just want to solder a pigtail off of my power distribution board to whatever voltage regulator I end up using. The LEDs are on strips and on a round LED board requiring 10-11V.

In the past I have used simple diodes to step down voltage to safe levels for servos that require 5.5V or less using a 2S lipo. I used (4) 1 amp silicon diodes to drop 8.4V to 5.6V successfully here: http://www.rcgroups.com/forums/showp...6&postcount=31

So, I could simply just use 8 silicon diodes in series (1 or 3 amp easily found at Radio Shack) to get down to ~11V.

Or, I have a Castle programmable BEC that is a bit more bulky and heavy.

Or, I could find a simple voltage regulator at Radio Shack but not really sure what to look for. Maybe this would suffice although slightly higher voltage:
http://www.radioshack.com/product/in...ductId=2062600
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Old Oct 05, 2013, 03:12 PM
"MAYONNAISE"
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Le Treport, France
Joined Jun 2004
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the castle CC BEC would be the fine solution ... aboard a plane.

aboard a quad ( what a horrible thing ... ) and for 10-11v output an adjustable 317 regulator with a little heat sink will be fine enough.

Alain
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Old Oct 05, 2013, 03:45 PM
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A DC-DC converter with a LM2596 can do that with good efficiency

http://www.ebay.com/itm/DC-DC-LM2596...-/380663899701
http://www.ebay.com/itm/DC-DC-Buck-C...-/221264399466

How much current do you need?
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Old Oct 05, 2013, 04:33 PM
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Not sure of the current yet. Probably a good thing to check but I don't have the LED strips yet, just the LED disk you see here. I'm going to guess less than 1 amp even with the other LEDs added on later.
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Old Oct 05, 2013, 05:55 PM
Dave the Rave
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For what it's worth, most (if not all) LED strip lights are actually designed to operate on a car's electrical system, which can be as much as 13.8 volts when it's being charged by the alternator. So your target voltage of 10-11 volts might be a little low. You really aren't that far off with a 4S battery, even a fully charged one. You would probably be fine with just a couple of 1N5400 diodes, they will drop it about 1.2 -1.5 volts and put you within the voltage range those LEDs are designed to work on anyway, and can handle at least 2-3 amps of current.
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Old Oct 06, 2013, 11:34 AM
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I'll throw this little tidbit into the discussion....
When dealing with LEDs, you aren't designing around voltages, you design around current draw. The typical LEDs draw 20 mA per. voltage drops can range from 1.2Vdc upwards of 4.0 Vdc. I would put a resistor in series with each leg of LEDs for the least weight and cost.
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Old Oct 14, 2013, 05:58 AM
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Agreed... ANY voltage will work as long as one places the proper current limiting resistor in series with the led(s). I did a job that required detecting 5,000V (very low current) pulses. The simple solution was a LED (in an optoisolator) and high value resistor. It worked much to my amazement.

jeffparisse
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Old Oct 15, 2013, 03:37 AM
Dave the Rave
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Troy:

Now that some of the other posters have added their advice, allow me to point out that the LED strips to which you refer, and that includes the discs being made specifically for multi-rotors, already have the necessary current-limiting resistors for the LEDs mounted on the strips. That's how and why these products can be sold as being suitable for 12 volt DC applications, the resistors are sized properly for that voltage range. The LEDs are wired in series in sets of three, with a resistor for each trio of LEDs, then the sets of three are all wired in parallel along the length of the strip. They will work at the proper current draw when connected to any voltage source from about 9.5 volts to 13 or more. So don't "overthink" your problem, all you need to do is drop the voltage from your pack by a volt or two, and that can safely be done with a couple of diodes. The disadvantage of using some voltage regulators is that if the source voltage is too close to the desired output voltage, the regulator will drop out and you'll get no output at all. Most regulators need at least 2 full volts more in input voltage than you desire in output voltage. So if your battery begins to get too low, you may lose your lights completely. The diodes will not do that.
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Old Oct 15, 2013, 11:12 PM
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This is great info, thanks for taking the time to post.
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Old Oct 16, 2013, 03:15 AM
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Quote:
The disadvantage of using some voltage regulators is that if the source voltage is too close to the desired output voltage, the regulator will drop out and you'll get no output at all. Most regulators need at least 2 full volts more in input voltage than you desire in output voltage. So if your battery begins to get too low, you may lose your lights completely. The diodes will not do that.
Most (if not all) linear voltage regulators behave like a diode when the dropout voltage is reached. The regulator output will simply be input voltage - dropout voltage. Dropout doesn't refer to the regulator shutting down, it means that the regulator has dropped out of regulating mode.
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Old Oct 19, 2013, 06:01 PM
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Different regs drop out at different voltages over input by design. Look for "LDO" in the specs (datasheet) for one with a low dropout voltage (usually <2V).

Diodes are good for a volt, maybe two but a linear reg should be used after that.

jeffparisse
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