hello guys,
as i know capacitors are used with AC current so why when we get a toy or rc car (DC current) or something and check the circuits it has lots of capacitors in it ?
i need please some explanation about this so maybe if it is useful, then i use it in my project
i know what it does and these stuff but i never worked with it in a real application !!
and thank you for your replies 
First you need to make a distinction between steady-state DC (nothing ever changes), with
practical DC circuits (which change with time so are not purely DC - at the very least
they are switched on and off).
No practical circuit is pure DC anyway...
So capacitors are not just "something to do with AC" - they store charge, specifically
equal amounts of +ve and -ve charge on the two plates, and larger capacitors store
more charge for a given voltage difference.
Charge can be stored for several reasons - one is decoupling - you want to provide a
constant supply voltage to a varying load over timescales of milliseconds down to
nanoseconds - most capacitors on digital logic circuits are there precisely to do this
(power supplies are too slow and too far from the loads (the logic chips) to perform
this task, since logic loads change in a few nanoseconds, and at that timescale all
the wiring on your circuit board has very significant impedance due to stray inductance.
Logic circuits are, in effect, very high frequency AC environments.
You mention an RC car - the remote signal is a radio transmission - that's AC. The demodulated
signal from the receiver is a pulse train using PWM - that's AC. The brushless motor powering
the wheels is inherently an AC device that requires rapid high current switching between the
windings. (if its a brushed motor then it's power is controlled using high-power PWM, again
this is not DC).
No interesting circuit is ever purely DC!
so where do i have to use it ? the capacitor i mean ?
i put it in parallel with the motor and the rf links and pot ?
Which circuit? It all depends... Often one is placed across the motor terminals to reduce the switching
noise for a brushed DC motor (commutator switching noise). Value depends on size of motor etc...
aha so just the motor ? or rf link 2 and potentiometer ?
i attached a picture of the transmitter which i am working on, but let's consider the source power is just the 12V and arduino is connected to it and there is no 9V.
is it necessary to use a capacitor (50V, 1uF) here when finishing the circuit ?
The demodulated signal from the receiver is a pulse train using PWM - that's AC
Pulsed DC != AC
AWOL:
The demodulated signal from the receiver is a pulse train using PWM - that's AC
Pulsed DC != AC
Informally ac means any frequency above zero, dc means the zero frequency component
(often we speak of "ac component", "dc component" of a signal.
So perhaps we should say that pulsed dc = dc + ac ? I should more accurately have said
"PWM contains AC"...
i just said DC to point at a small circuit that is powered by a battery
but can you please tell me where to put capacitor in my circuit ? on motor which doesnt figure in the transmitter but there is the transmitter module do i have to put a capacitor on it ? if yes and a 50V 1uF is okey ?
Informally ac means any frequency above zero
Informally, AC means "alternating current".
Come to think of it, that's what it means formally too.
can you please tell me where to put capacitor in my circuit
Across the power supply:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
if yes and a 50V 1uF is okey ?
The voltage on a capacitor is the voltage that it can withstand, so normally you only but up to 80% of the rated voltage on a capacitor. Putting less is fine although capacitors with a higher voltage rating are physically bigger.
A value of 1uF is fine for the high frequency noise but the low frequenct stuff needs something larger like 47uF to 100uF. Have two capacitors one large and one small.
mmm well i am sorry i didn't understand this one : only but up to 80% of the rated voltage on a capacitor ?
so you saying a much bigger capacity is recommanded in a dc current about 12V battery ?!! why is that ?
and do i have to put a capacitor for the transmitter and receiver modules ? and for the pot ? other than the motor and servo ?
You only should put 80% of the rated voltage on a capacitor to extend its life. So using a 16V rated cap on a 12V motor is fine. Any higher rating is even better as regards extending its life. Do not use a 12V rated cap on 12V. Do not use a 6V rated cap on 12V or it will explode.
Dd you read that link?large values are for low frequency noise and regulation of the voltage. A capacitor across the supply of anything can do no harm and often does good.
Do not put large capacitors. Across signals as it reduced the speed they will change at.
Grumpy_Mike:
You only should put 80% of the rated voltage on a capacitor to extend its life. So using a 16V rated cap on a 12V motor is fine. Any higher rating is even better as regards extending its life. Do not use a 12V rated cap on 12V. Do not use a 6V rated cap on 12V or it will explode.Dd you read that link?large values are for low frequency noise and regulation of the voltage. A capacitor across the supply of anything can do no harm and often does good.
Do not put large capacitors. Across signals as it reduced the speed they will change at.
ok i read the article you gave me, i got some stuff + i put a circuit from a toy rc car and i say some of what the article talks about and it made me understand better, they used 2 big capacitors 47uF and a lots of ceramic orange capacitors, and i understood that to reduce the resistance of a capacitor we must use a bigger value of capacitance ... but in the next section that talks about inductance unfortunately i didn't get it 
can you please give me a simple explanation of it in just 1 line or something ? and the inductor part i didn't get it 2 (excuse my dummy knowledge but i am still a beginner with these stuff and i really need to understand it, i tried some research (maybe i didn't know what to search for that's why i decided to ask on the forum) because what i get is just explanation about the capacitor and how it works and not its real application so far)
and about the motor section, no matter how much voltage is the dc motor the typical values are as shown 470uF, 47uF and 0.1uF ?
and for the arduino do i have to use capacitors 2 like the normal ICs ? or it has been taking care of ?
and thanks for the help again i appreciate it
An inductor looks like a high resistance for high frequencies and a low resistances for low frequencies. Therefor if you use one then it stops high frequencies getting through to the rest of the circuit and disrupting it.
You only need to use an inductors when capacitors are not enough by themselves to sort out any problem.
The arduino already has capacitors built into it.
Yes those are typical values used with motors. It will not change with voltage.
Every capacitor has an effective inductance associated with it which limits the maximum high frequency it can short out. The bigger the capacitor the bigger this inductance is so that is why you need two.
aha ok great my project is an RC car well it need very high and efficient signals
so i must use everything you gave me in the link and by the way what about the RF modules ? the receiver and trasmitter fo i have to use capacitor or it's already build in ?
here is the link for the RF link i am using here :
http://www.ekt2.com/products/productdetails?ProductId=3f806822-c96b-43c6-8b93-153c980a4e2d
In general you only resort to inductors when simple capacitors are not enough. So try the simple stuff first. The transmitter and receiver should have them in already.
When I try that link I get an error page saying
This constraint cannot be enabled as not all values have corresponding parent values.
i donno what's wrong with that link !! me 2 the server has an error, i will upload a picture when the website will open !!
and about the capacitor then i can make a conclusion which is : a module should have already everything taking cared of, so we use these tricks just for "naked" components, i donno if this is a write way to express it but i guess u got the idea
so we use these tricks just for "naked" components,
Yes that is good enough to work with but if you do have problems extra capacitors are not going to do any harm.
aha great if i used a capacitor in a wrong place because i might do it lol
thanks a lot Grumpy_Mike i learned a lot from this post, info that i didn't knew about like the standard circuit for dc motor i appreciate your help
firashelou:
aha so just the motor ? or rf link 2 and potentiometer ?i attached a picture of the transmitter which i am working on, but let's consider the source power is just the 12V and arduino is connected to it and there is no 9V.
is it necessary to use a capacitor (50V, 1uF) here when finishing the circuit ?
Not sure what's going on with left side your breadboard here....
You have two resistors only connecting +12v & GND. And, you have two buttons with just one side connected to +12v and nothing else. .??
