Electronics > Electronics

Using a capacitor to smooth out a power supply

(1/3) > >>

ErikY:
I am playing around with some ultrasonic sensors, and I am planning on using the analog output to read the values.

I am reading a book that mentions when using these, it is recommended to place a 100uF capacitor between teh +5v and the GND of the sensor in order to smooth out the power supply and get more accurate readings.

I am kind of shocked by this, as I would have expected a .1uF capacitor to do this job, not a 100uF capacitor.

Can anyone tell me why you would need to use a cap with such high capacitance in this application, when to smooth out an ATMega micro-controller, most only use a .1uF capacitor?

Thanks!

jwatte:
Maybe the sensor has really spiky power draw, when generating the ultrasonic pulse.
Maybe they are assuming that you use long leads to connect the sensor.
Maybe they are just being conservative.

Admin:
When was this book published? What sonar, specifically, was it referring to?

The older sonar you could buy ~5 years ago, as a hobbyist, was a bit noisy. The sonar sold today are (should?) already have proper cleaning caps installed.

You can easily determine if you need additional caps by using an oscope between power and ground while running the sonar, and looking for spikes, etc.

ErikY:
jwatte and Admin,

Thanks for your replies.

The ultrasonic sensor is this one:

https://www.sparkfun.com/products/639

An ultrasonic rangefinder from Maxbotix.

It draws 2mA.

I read the data sheet and it does not mention the need for any caps at all.

The book was published in 2011, so it's pretty new.

The book recommends, and I would do anyway, using regulated 5V as the + source.


This issue raised a questions to me, which I probably should have been more specific about.

Reading the basic electronics tutorial on the main SoR site, mentions if your cap is too large, it will take a long time to charge the capacitor when starting up. I suppose at 100uF that is probably not too big of an issue, I just read that very literally, that you want to make sure the cap is not too big.

The question is, other than the time to load, are there any other downsides to using too big of a cap?

Regarding the oscope, I will definitely do that. My wife actually bought me a pocket oscilliscope for Christmas, and I have not yet used it, this will be a good reason to use it.

Another question that comes up is, do I need to put this on a board with all other sensors, motors, etc. that will be on my actual bot, so that I can test it in a real life situation, or can this be isolated for testing for the need for a cap here?

I am kind of embarrassed, but I really don't know how to use an oscope, or the one she bought me in particular, came with no instructions. I am going to have to do some learning and digging.

jwatte:

--- Quote ---are there any other downsides to using too big of a cap?

--- End quote ---

Cost and bulk are pretty big ones :-) Also, larger capacitors may be less reliable because of optimizing for cost or bulk. And a larger capacitor has more electrolyte to spill over your project when it ruptures ;-)
Also, inrush current may become bigger, which may be a problem (that's the turn-on spike you're talking about.)

You can easily solve the capacitor inrush current and time-to-voltage function by finding the ESR of the capacitor, and solving the 1/(2 pi RC) filtering differential equation.

Navigation

[0] Message Index

[#] Next page

Go to full version