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Topic: Most basic capacitor circuit. (Read 6575 times) previous topic - next topic

Jaykey


Okay, I recently did a post were I "designed" a simple LED circuit. By actually working off a datasheet and "engineering" this extra simple project I gained a much more solid grasp of what I did then if I had simply read.

I would like to do the same for capacitors.

What is the simplest circuits that uses caps?

I know they store a charge, but WHY would you want this? The resistor converts the voltage to the necessary amps(current) and appropriate voltage to operate a devicem such as an LED. What does a CAP really do.

PS. Can you give me just enough info to where I can engineer this circuit without your doing it for me. WHEN & WHY caps are used, and the simplest device I could design to use this.

retrolefty

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What does a CAP really do.


Resistors are a little simpler then caps, as resistors only perform one function, they limit current according to ohms law. Caps can hold a charge and are used like, for example in sample and hold circuits, where a analog voltage is switched to a cap which charges up to the value of the applied signal, then the signal can be disconnected and the cap switched to a circuit designed to read the now stabilized voltage charge on the cap. A cap can also be used to pass AC voltage through it, but block any DC voltage. That is used to filter AC noise voltage to ground in DC power supplies and other filtering applications. Also caps can be used to pass audio (AC voltage) signals in series between one stage of an amplifier to the next stage (say pre-amp to main power amp) without having the DC voltages used by the amp stages from effecting each other.

So I guess I'm saying that there is no single circuit using a cap that will explain all the properties and uses of capacitors. The math is more complex also in that the equivalent resistance of a cap, called capacitance reactance varies with the frequency of the applied signal. That is why resistors are taught in electronics DC fundamentals before one learns about AC fundamentals where capacitors and inductors are introduced. It's best to study caps as part of  a more typical written tutorial covering AC fundementals I think.


Yankee

I found this video very helpful (and entertaining) in understanding capacitors: http://www.youtube.com/watch?v=ORW7Qr4Nhtk&feature=relmfu. Once Jeri has prepared you check out this in-depth tutorial: http://www.electronics-tutorials.ws/capacitor/cap_1.html

westfw

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What is the simplest circuits that uses caps?
I know they store a charge, but WHY would you want this?

Well, the simplest circuit is probably the bypass caps used in digital circuits, whose purpose is to store "some" charge very close to the pieces of circuit that switch charge around (since long wires are not "ideal" and would interfere some with rapid movements of charge.)

But most uses of capacitors do not use them so much for storing charge, but in ways that depend on the nice mathematical behavior exhibited by the voltage and current parameters around the capacitor.  While a resistor is simply V = I*R,  a capacitor does V = (1/C)(Integral(I(t), dt)).   That's calculus, and may not make much sense depending on how much math you've had, but:

1) having a simple component that does "hard" math can be useful.
2) The important part is that the voltage depends on TIME as well as current and capacitance.  Thus a capacitor can be used in assorted applications that involve timing things.

A particularly simple "blinking light" using a capacitor is this neon light circuit:

The resistor limits the charging current, so the voltage on the capacitor gradually builds up to a point where the neon bulb turns on (neon bulbs have certain "magic" properties that allow this to work; notably having infinite resistance until you hit a particular voltage.  You can't build this with an LED.)

You might also look here: http://www.adafruit.com/forums/viewtopic.php?f=8&t=17207

CrossRoads

#4
Feb 20, 2011, 08:31 am Last Edit: Feb 20, 2011, 04:16 pm by CrossRoads Reason: 1
You could also go get a 555 timer chip and experiment with Rs  & Cs there to see capacitors in action more clearly.
Get yourself an oscilloscope, such as www.dpscope.com, I have one, it works very well (and price has dropped! $70 for kit, $80 assembled, $9.50 US shipping),
or Virtual Analyzer (free) from www.sillanumsoft.org/ and make a probe for your sound card line-in input. I also have this, have used with a good Sony microphone to look at frequencies out of an amp I made, trying to get 60/120/180 Hz filtered out, microphone placed in front of the speaker.
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.

westfw

Here's a pretty simple circuit.  Easy to put together if you have some large caps hanging around.  The schematic calls for a 3000uF cap, but anything from about 1000 to 10000 will probably be interesting.  visible events will last a couple of seconds for each 1000 uF.

SW1 is supposed to be a SPDT toggle switch or similar.  It's currently in the off position.
When you flip SW1 to the ON position, current will flow from the battery, through LED2 (which will light up), though the 1k resistor, and into the capacitor.  As this happens the voltage across the capacitor will increase, which in turn will cause the current to decrease (because the voltage drop across the resistor becomes smaller.)  After one time constant (R*C = about 3 seconds for the values shown) the current will be about 36% of the start current.  After two time constants, it'll be about 10%.  The LED gets gradually dimmer.  Charge has been stored on the capacitor, and we have measured time.

Now flip the switch back.  The capacitor begins to discharge through R1 and LED1, running the cycle in reverse using the stored energy.

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