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Topic: Question about capacitors (Read 3573 times) previous topic - next topic

alexander79p

Hi,
I have to connect a LED strip to the power source. The manufacturer of the LED strip suggests to add a capacitor across the + and - terminals. This prevents the initial onrush of current from damaging the LED strip.

My question is: I need a 1000 µF capacitor. I found it, but it is a 10 volt capacitor, while the power source is 5V. Is this a problem?

Thanks :-)
Alex

OldSteve

No problem at all. As long as the capacitor's voltage rating exceeds the voltage you'll charge it to, you're good-to-go.
Please do not PM me for help. I am not a personal consultant.
And others will benefit as well if you post your question publicly on the forums.

gpsmikey

I agree with OldSteve's comment, however, I question exactly what the function of the capacitor really is.  "This prevents the initial onrush of current from damaging the LED strip" is a very odd statement.  If the supply feeding the strip is not regulated and is current limited, then it would slow the rise time of the voltage across the LED strip, but with a regulated supply, the only thing I see it doing is causing the supply to drop momentarily as it current limits charging the capacitor (probably resetting any control electronics you have running off that supply).  Unlike light bulbs with a filament for example, LEDs don't change resistance as they turn on - they are on or off and the output depends on the current through them, but the drop across them does not change (much).  Typically, a RC (resistor / capacitor) network can be used to "ramp up" the voltage, but it is a function of the series resistor (that limits the inrush current) and the capacitor across the input.  In this case, they are giving you a capacitor with no idea of what the input resistance is.  Very odd.
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

alexander79p

@gpsmikey: This is the page in which you can find that sentence:

https://learn.adafruit.com/adafruit-neopixel-uberguide/basic-connections
Alex

gpsmikey

Yep - it does say that (" Before connecting NeoPixels to any large power source (DC "wall wart" or even a large battery), add a capacitor (1000 µF, 6.3V or higher) across the + and - terminals as shown above" ) on the second page, HOWEVER, my comment stands - if I put a 1000ufd cap across a 12v car battery, you will get a slight spark as it charges the cap, but it has no effect on the battery voltage.  If I put the cap across a smaller supply, it will cause the supply voltage to dip.  Neither of those is helpful.  Only if you have a series resistor feeding the capacitor (creating an RC filter) will you "limit the inrush".  Putting the cap across a low impedance supply really accomplishes nothing (unless they figure the supply wires provide the series resistance???)  You have read the directions - I'm just not sure the "directions writer" knows what they are talking about.  :smiley-confuse:
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

CrossRoads

The cap would have best effect when placed across the LED strip power terminals, not the power source.
That way it buffers any voltage drop down the wires from the power supply to the strip.
Same as putting a large cap across the input terminals on a PCB, it helps with any power fluctuations on the wiring.

1000uF seems like a lot unless you have a really long strip tho. 100uF is probably more than enough.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

gpsmikey

I agree with using the cap to buffer the supply to the LED's - what bothers me is their statement about it "limiting the inrush current" when it is across the supply.  Seems very odd.
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

BigBobby

My question is: I need a 1000 µF capacitor. I found it, but it is a 10 volt capacitor, while the power source is 5V. Is this a problem?
Some capacitors have reduced life and/or capacitance when used near their voltage rating.  It's usually a good idea to use them around half their rating (which you're doing so you're fine).

alexander79p

Which resistance value do you suggest me for the resistor? Should it be applied on the + side or on the - side of the capacitor?
Alex

gpsmikey

I assume that question is about the series resistor to limit inrush current, but I have no idea because their statement does not make sense to me.  It would be in the positive side (always good to keep grounds at ground), but I have no idea what their design is and why they say that.  I can see an RC network for example if driving a small motor to give it a "soft start" (unless you are using PWM then you do it differently), but it does not make sense to me for LEDs
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

alexander79p

Is it safe, for the LED strip, to connect it to the power source in the normal way, without neither capacitor nor RC filter?
Alex

MarkT

The reason this is specified for neopixels is that they are very sensitive to overvoltage from poor power
supplies, IIRC.  Specifically neopixels, not standard LED strips.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

alexander79p

Should I use an RC circuit in order to protect this LED strip?
Alex

TheMunkee

I too am curious about this. My power supply is going to be about 15 feet away and I'm going to be connecting four grbw rings. As I understand, each chip uses about 60ma at 41 a ring x 4 rings is about 10 amps. I was going to design a little pcb board in "fritzing". I wanted to protect the power supply from short circuits and vise versa  and protection the rings from each other. I thought a few fuses and a big cap for each would do it but if I can do anything else to keep everything running more reliably, then I would love to hear more.

jendalinda

These LEDs are pretty power hungry. I would put that capacitor to the strip terminals directly. It will work as a power reservoir.

Btw what is the purpose of the resistor in the signal path?

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