My "noob" questions :P

I’ve decided to start a general thread for the many “noob” questions I am sure to be spouting soon. Let’s start with this one:

Capacitors are rated at a certain voltage (which I know not to exceed, thanks to this video :o), but what happens if you feed it current significantly lower than what it’s rated for? I saw a great deal on about 1000 50v capacitors of varying…er…capacity…, but I don’t know if/how they would function using the Arduino’s 5 Volt DC. Any help would be…helpful…:stuck_out_tongue:

Capacitors are rated at a certain voltage

Actualy they are rated at a certain “maximum working voltage”. Operating them at lower then that rating is fine.

Lefty

The rating is a safety limit, you don’t need to go anywhere near it if you don’t want to. It’s absolutely fine to use a capacitor rated for 500v in a low voltage circuit if that’s what you happen to have to hand, although it will be a bit bulkier than it “needs” to be.

I’ve dismantled a number of commercial products recently to salvage parts, and I’ve noticed that in low-voltage circuits (anything under 50v) they commonly use capacitors rated to cope with twice the voltage they’ll actually use. So 5v circuits often seem to have capacitors rated for 10v. If this is a sensible rule of thumb, those “50v” capacitors will be fine for anything up to 25v. Using them with your 5v Arduino projects just has an even more massive safety margin.

(on the other hand: Where higher voltage capacitors are used (eg 50v rated on a 5v line) in carefully-designed commercial products, I suspect it’s probably only because there was no significant cost or space saving in using a lower voltage rated one. At least for electrolytics, if you have two caps the same size, the one with a higher voltage rating will be much larger and maybe a bit more costly. All this matters a lot if you want to make thousands of an ultra-compact consumer product, but very little to the average Arduino project.)

I suspect it’s probably only because there was no significant cost or space saving in using a lower voltage rated one.

No it is to improve the reliability of the design. Capacitor have a ware out mechanistic and using them at lower than the maximum increases their life.

Also some regulators like UL require capacitors to be run at a rating 60 to 80% lower than their maximum.

That’s interesting, is there really a life expectancy increase when you use a voltage rating beyond 2x what it will actually encounter? Where can I can read up on this?

Where can I can read up on this?

For starters :-

http://www.ami.ac.uk/courses/topics/0190_drat/index.html

http://bookpower.blog.dianyuan.com/u/41/1147846507.pdf

http://www.digikey.com/Web%20Export/Supplier%20Content/CDE_338/PDF/CDE_PredictingLifeTemp.pdf

http://www.edaboard.com/ftopic333677.html

Or google “capacitor lifetime”

Thank you very much!

Time for a (hopefully) less nooby question :P.

Let’s say that you can only draw a total of 20 mA from a power source (the Arduino in this example) safely (I know that the arduino can supply more than 20 mA total, but this is just an example). If you were to attach 2 leds (which both draw 20 mA) to two different pins, and then alternate power between them very rapidly (I’m guessing 35+ times a second), would it appear as if both are lit at the same time? The human eye seems to be a tricky thing, but I think you could in theory get a lot of LEDs to look like they’re lit at the same time without them actually being lit. Any advice as to how rapidly they’d have to switch, or if it would even work at all? :slight_smile:

The human eye seems to be a tricky thing, but I think you could in theory get a lot of LEDs to look like they’re lit at the same time without them actually being lit. Any advice as to how rapidly they’d have to switch, or if it would even work at all?

That’s how multiplexed LED displays work, but more for saving on pin count then current. I think the eye won’t discern flicker at greater then 16 per second or higher, kind of like the motion picture shutter speed system.

Lefty

Oh I never knew that’s how multiplexers worked. So they just alternate (very quickly) which led is getting the power? That’s really smart :).

Thanks though :D!

New question:

If I were to want to buy some hookup wire for breadboarding with the Arduino (specifically this breadboard), what AWG wire should I get?

I’m looking at these on Radioshack:

Thanks in advance :)!

http://www.sparkfun.com/commerce/product_info.php?products_id=8023

So 22 AWG then? I was hoping to be able to buy it locally, so should this one at RadioShack be comparable?

No. You need blue and yellow. And pink. Don’t forget the pink.

Yeah, that will work fine.

Don’t forget the pink.

LOL. Alright, well thanks! :smiley:

I’m back! This time, with questions about torque!

I’m looking at servos (for use with future Arduino Projects), and I found some (awesomely priced) servos on ebay:

http://cgi.ebay.com/2-x-9g-Torque-Servo-for-RC-Plane-Helicopter-Boat-Car_W0QQitemZ170399440974QQcmdZViewItemQQptZRadio_Control_Parts_Accessories?hash=item27ac99204e

I don’t really understand how torque is measured. This servo seems to list its torque in two way; 9g (in the title), and 1.5KG/CM. I always thought torque was measured in foot-pounds (or meter-KG??), so the 9g doesn’t make much sense to me.

Can anyone decode this into what it really means?

EDIT: Ooopsies, I think 9g is the weight XD! :stuck_out_tongue: My bad; Disregard that portion of the question…

In general torque T (with respect to an axis of rotation) is the vector product of the position vector R (going from the axis to the point r where the force is applied) and the force F. T is therefore is an axial vector. You can use the “right hand method” to determine it’s direction.

thumb: R
index finger: F
middle finger: T

If you’re just interested in numbers (absolute values):

T = R*F * sin(phi)

phi is the angle between R and F.

In a physical sense the unit of kg*cm (or foot-pound) is wrong, as kg is a unit of mass and not force. The correct SI unit is Nm. Nevertheless most of the time mass is used instead of force, as people seem to relate to it much easier.

Practical example:

A servo with a max. torque of say 2kg*cm could still rotate if you attached a mass of 2kg (about 20N) on a string to the disc at 1cm from the axle. Or 1kg at 2cm, 0.5kg at 4cm … The product is constant.

Very informative. Thanks! :smiley: