General question about powering an arduino

[SXRguyinMA]

or this maybe?

[x88x]

you're talking about the green wire on the 24-pin connector right?

No, no, no, I see the confusion now. There's a special 5V 'standby' line, I think usually blue. Basically it lets the MBB check that the PSU is actually functioning or something.

As for the capacitor, I think if you just put the cap in parallel with the arduino and put a diode between the PSU and the cap (so the cap doesn't discharge into the PSU), that should work. I would try it first on some other 5V device that doesn't matter if it gets burned out, just to make sure I'm not full of crap.

[SXRguyinMA]

lmao sounds good. also, The only caps I'm finding are 35v or 50v. If I only hook a 5v line to it will it only charge 5v up to its rated uf capacity? or shoudl I try to find a 5v cap?

[x88x]

Correct. No matter what the voltage rating (as long as it's >=5V), any cap just hooked up to a straight 5V line will only charge to 5V. To get it higher than the source voltage you would need a DC-DC boost circuit like in my coilgun thread.

[crenn]

http://www.sparkfun.com/commerce/pro...roducts_id=746

[x88x]

If you're looking for something a little cheaper, there's also this:
0.47F 5.5V : $1.49
http://www.goldmine-elec-products.co...?number=G16573

As opposed to the one crenn linked
10F 2.5V : $4.95
http://www.sparkfun.com/commerce/pro...roducts_id=746

You could combine two of the 10F ones in series to get a 5F 5V cap bank for $10, and that's definitely a great price for that, but idk if you need 5F of capacity. I'm also not sure how you would go about calculating that. With the smaller ones off Electronics Goldmine, you could run as many as you need in parallel, for any multiple of 0.47F. So if you only need, say, 2F of capacitance to run the arduino and servo for the time needed, you could hook up 5 of the 0.47F ones in parallel for $7.50. ...and, actually, now that I type that all out, I would just go with two of the one that crenn linked, but I'm not gonna delete all that because..yeah.

[SXRguyinMA]

lol I ended up getting the ones crenn linked only because sparkfun had a few other misc goodies (switces, RS232 to USB converter) that I wanted, so it was easier. Should hopefully see them in a week or so! then I can get cracking on this!

Even if the 5F ends up being too much that just means the Arduino will run longer than I need until the caps are drained right?

[x88x]

Right. Think of a cap kinda like a bucket of water with a hole in the bottom. The capacitance rating determines the size of the bucket, the voltage determines the size of the hole. So, you fill up the 'bucket' with 5V, and when that input voltage goes away it puts out 5V until it's empty.

[SXRguyinMA]

cool thanks!

I'm pretty knowledgeable about some electrical stuff, but when it gets into trasistors and capacitors and the like I'm a little lost

[SXRguyinMA]

crenn, I had a guy over on the arduino furms say those caps you suggested probably won't work because they're too high resistance (40 ohms each). He suggested these ones, but then I'd need 10 of them (~$40) in parallel for 4.7F, and they are .5ohm each

http://search.digikey.com/scripts/Dk...me=283-3014-ND

any suggestions? I've already got the ones from sparkfun coming, so I'll give it a shot. he thinks it might be too much resistance (80 ohms total) to run the servo properly

here's the thread from arduino forums

http://www.arduino.cc/cgi-bin/yabb2/...m=1279813240/0