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Some Common Uses for Capacitors.

A circuit with both AC and DC Sources

A circuit with both AC and DC Sources

In the last electrical post, “Capacitors and Alternating Current“, I promised to talk about some very common uses for capacitors.  Separating AC from DC is a very common use for capacitors.

The circuit diagram to the left shows a 5 V DC source combined with a 2 V peak-to-peak AC source.  This is feeding a RC series circuit.

The voltage in and across the Capacitor and Resistor with 100 Hz AC.

The voltage in and across the Capacitor and Resistor with 100 Hz AC.

The calculated voltage across both elements, Vin the black line, as well as the voltage across the resistor, red line, and the voltage across the capacitor, blue line, is shown in the graph to the right if the frequency of the AC source is 100 Hz.  Notice none of the DC voltage appears across the resistor.  The AC is split across the resistor and capacitor.

The voltage in and across the Capacitor and Resistor with 1000 Hz AC.

The voltage in and across the Capacitor and Resistor with 1000 Hz AC.


If the frequency is increased from 100Hz to 1 Khz, much more of the AC appears across the resistor and very little appears across the capacitor.   It is a little harder to see in the graph, but notice also the ripple in voltage across the capacitor is about 90 degrees behind the voltage in and the voltage across the resistor is close to being in phase with AC part of the voltage in.

Blocking DC from one amplifier to the next amplifier.

Blocking DC from one amplifier to the next amplifier.

The ability of a capacitor to block DC and pass AC is often used in the coupling of amplifiers as showing this the diagram to the right.  In transistor and other electronic circuits it is often necessary to set the DC voltages for different connections at certain levels to set the operating point of the device.  As circuits are added in series to obtain more amplification the DC values add up.  Often the easiest solution is to block the DC from one amplification stage to the next stage.   This does have a trade-off.  The low frequencies may also be attenuated.

Related to the previous example, sometimes only high frequencies are desired to be passed on to a device and the low frequencies are attenuated.   This would be called a high-pass filter because it passes the high frequencies.  On many loud speaker systems a capacitor is put in series with the small tweeter speakers.  This is part of a system called “the crossover network” and prevents the tweeters from seeing the low frequencies which could overdrive those devices.   This and the DC blocking circuits cause endless discussions from audiophiles about which system is best.  Sometimes brought into those discussions are talk about the phase shifting we talked about earlier.

The whole audio field is a very interesting one.  In the musical instrument area there are lots and lots of effects pedals that modify the sound from an instrument such as an electric guitar.  Often these filter out some frequencies and pass others.  Sometimes they change the phase of some frequencies with respect to others.  It is now often done with digital effects, but in the past it was done with analog electronics with much use of capacitors.

A decoupling capacitor.

A decoupling capacitor.

Another very important use for capacitors is to pass an AC signal to ground when it is riding on a DC voltage.  This is commonly used in digital circuits.  Because the circuits are switching off-to-on quickly little power surges occur in the power supply wires.  These surges can cause other circuits on the printed circuit board to trigger unexpectedly and create a troubleshooting nightmare.  Normally a capacitor is attached to to the power input to each I.C. chip as shown in the picture.   This can be thought of two ways.  First, it shorts the high frequency change to ground while keeping the DC voltage constant.  Second, it acts like the spring on an automobile to absorb the shock of the bump caused by the voltage dip or voltage spike.  The second is my preferred way to think of the action.  In this example, the capacitor is acting as a low pass filter.

A very important example of capacitors filtering DC and smoothing the DC is capacitors being used as filters in power supplies.  We need to talk a little more about diodes and then we will go into depth about this use of capacitors.

A final use of capacitors is to create a timing delay.  This really goes back to capacitors in DC with the time constant equation, but it is a very important use.  An example of a circuit that uses that property is the 555 timer chip.   I am linking a data sheet for one of those for your review.   I believe I have provided you all the information you need to understand the data sheet, but if you have questions please e-mail me.    The one math symbol we have not talked about used on that data sheet is the symbol Δ ,(delta).  Delta means change, so for example Δt/ΔT in their usage means the change in time divided by change in temperature.   You can contact me at my e-mail address garyfox@create-and-make.com.

Other news:  I have played with the Arduino controller and determined how I will interface it with the thermistor circuit.   I will talk the basics about the Arduino but mostly I will point the way for you to learn it on your own.

One of the things I bought while I bought the Arduino was a power supply circuit.  We are at a very good point to discuss how that circuit operates, so there will be a post about that.

I also bought a couple of books about solar energy.  I will being doing a post about those books.

Finally, I made a cardboard “water wheel” for my Granddaughter and the marbles.  I will probably do one post on that just because it is a very inexpensive item to make and can get children past all the manufactured toys.  My Granddaughter was too young to construct it for herself, but I can easily imagine older children enjoying the construction as well as playing with it.


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