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Topic: What kind of decoupling capacitors? (Read 7653 times) previous topic - next topic

TeslaIaint

I'm using an LM 7805 voltage regulator for my project. I copied the schematic below from the data sheet. http://www.fairchildsemi.com/ds/LM/LM7805.pdf

What type of capacitors do I use? After some research, it seems people have different opinions. Since there is no polarity shown on the data sheet, do I assume that they are ceramic?

CrossRoads

Usually for the larger values for the regulators, aluminum electryolytic is used.
Input caps can be larger if you supply is noisy.
I normally go with 10uF and 1uF, or something along those lines. 0.33uF and 0.1uF seem kind of small.
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.

CrossRoads

http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf
Uno for example uses 47uF aluminum electrolytic on both, with additional ceramic 0.1uF.
All caps should be rated for at least 16V; 25V and 50V might be cheaper and work just as well.
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.

cmiyc


Uno for example uses 47uF aluminum electrolytic on both, with additional ceramic 0.1uF.

The Uno isn't using a 7805, so not the best example.

Personally, I prefer to start with what a data sheet recommends and experiment from there.
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spcomputing

I usually work from 100uF and 100nF on the input of the 7805 on a breadboard and a 100uF on the output with 100nF on all uC/IC inputs.

Grumping Mike of course has good guidance:
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Maybe he can explain the datasheet example.

TeslaIaint

#5
Aug 02, 2012, 03:55 pm Last Edit: Aug 02, 2012, 04:39 pm by TeslaIaint Reason: 1
I think I'll combine what I've read and with your replies. I'll use a .33uF ceramic and a 10uF electrolytic in parallel on the input. Then I'll use a .1uF ceramic and a 1uF electrolytic in parallel on the output. I am using two Li-Ion cell phone batteries in series for a total of roughly 7.4V to go into the LM 7805. That will be driving two small 3-6V rated motors at 5V (35mA free running and 800mA stall current). Any input is appreciated.

Chagrin

Most linear regulator datasheets suggest tantalum or electrolytic capacitors as they have higher ESR.

MarkT

Different chips have different requirements, newer low-drop-out regulators can be more fussy - read the datasheet for these.  Often they need the output capacitor to have an ESR that's not too low nor too high - otherwise there is a risk of unwanted oscillation.  Decoupling for the regulator should be close to the regulator, decoupling for the actual load should be close to the load.

Older 78XX's are pretty stable and you can be fairly lax with decoupling (the output will already have decoupling at the load, the input may already be fairly stable (especially out of a SMPS which has its own decoupling).
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TeslaIaint

The only thing running on this part of the circuit is the motors I mentioned above. Do I even need to worry about it? The other part of my circuit should be isolated by the motor controller, right?

Docedison

If you use a battery for the 7805 you might well consider a minimum of 100uF on the input side with a .1 to  .33uF cap as a battery's internal resistance increases as it becomes depleted or needs charging. I start at 470uF and go as high as 1000 to 2200 uF depending on load and battery size. There is nothing wrong and a whole lot right in using Large value bypass capacitors they WILL prevent a lot of 'unpredictable' or 'unreliable' operation of electronics, don't cost a great deal more and don't draw any more current if they are bigger.
They will also extend a batteries useful life by reducing or eliminating the apparent higher internal resistance of a discharged battery (the main reason for larger values).
There are however better devices available called LDO regulators or Low Drop Out regulators. A 7805 requires a 2.5 to 3V higher source voltage than its output voltage and there are regulators that will go as low as 200 - 400 mV I/O differential and at currents equal to the 7805 (1A max).
If noise isn't a real issue like for anything non analog a switcher type regulator is a much better deal...
A linear regulator can be thought of as a voltage controlled resistor, It's load current and its quiescent current X the drop across the regulator is dissipated as heat.
For example consider a "Worst Case" scenario, a 24V supply driving a 7805 and a 3/4 A load.  24 source - 5V load = 19V drop across the regulator X .75A = 14.25 W as wasted heat and 3.75 W for the 5V 3/4 A load is almost 4X the power total power wasted as heat. An 80% efficient switcher will waste (.8 X 3.75) - 3.75 or .75 W as heat so the current drain from the 24V source is now 3.75W load + .75w loss/24V or 18 mA. .018A X 24V = 4.5W total power drawn from the source. In short a linear regulator regulates voltage and a switcher regulates power.... Efficiently.

Doc
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TeslaIaint

As always, thanks Doc. I researched the LDO's but they have a tiny current output. Or is it just the ones I saw? It will be about 1 amp between the two motors at a decent speed so I'm inclined toward the 7805. I guess I shouldn't worry about it too much. I just want my sensors to work, and they are "isolated" Just trying to learn the right way and understand.  I don't care about the robot or the motors working forever. Just until I get the results I need, then I'm ready to spend the money on the big motors and hardware. Eventually I'll be using a couple 200 W motors. That'll be an entirely different story. I'm trying to get this robot to follow a person, but have had only moderate luck It worked before....mostly....but I only used one sensor on a servo. Now I have two sensors and they are both mounted 30 degrees or so apart without the servo. I might even try three ultrasonic sensors. (maxbotix XL ez1) They seem pretty decent for people detection, but expensive. I just got two of them a couple days ago so haven't been able to test them completely yet. That's the big picture. The small picture is that I have to just do things in tiny steps. I'm not the greatest at writing arduino code so it's one step at a time.

JoeN

#11
Aug 05, 2012, 03:15 am Last Edit: Aug 05, 2012, 03:21 am by JoeN Reason: 1

Usually for the larger values for the regulators, aluminum electryolytic is used.
Input caps can be larger if you supply is noisy.
I normally go with 10uF and 1uF, or something along those lines. 0.33uF and 0.1uF seem kind of small.


There are tantalum caps for all the values mentioned here (.1, .33, 1, and 10uF - http://www.taydaelectronics.com/capacitors/tantalum-capacitors.html).  Are those acceptable assuming you keep it to 5V?  They are a lot smaller.
I will never ask you to do anything that I wouldn't do myself.

TeslaIaint

Thanks all for the replies

@ Doc  I misread your post earlier about the LDO regulators. I didn't see the part about the 1A output. The ones I saw were much less so I thought it was typical. I did a search on Mouser's website and found this. http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000443.pdf

The attachement is from the data sheet. does anybody know what specific types of capacitors those symbols are referring to? I assume the cap on the input is ceramic because there is no polarity. But what about the output cap that has the polarity?

I'll stick a heat sink on it as well. Even if it doesn't need it, it still looks bad a**


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