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new guy
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Post by new guy » Wed Jul 05, 2006 2:33 pm

what is the general purpose of capacitors and why are there so many in every electronic schematic? AND what is the proper test for capacitors of any size?

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Chris Smith
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Post by Chris Smith » Wed Jul 05, 2006 4:09 pm

Big question, bigger answer?

Caps do many things.
They deliver instantaneous power, unlike a power supply, a line or a feed battery system.

Caps stop or block DC, letting AC pass. [almost like a diode]

Caps store energy for stabilization, delivery, or shock absorbing.

This would just be the beginning, all depending on the usage and need of the cap.

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Post by k7elp60 » Wed Jul 05, 2006 5:37 pm

caps(short for capacitors) or in the old days when I started out in electronics they were called condensers.
In simple terms a capacitor opposes any change in voltage. It does that by charging to the new level when the voltage in the circuit tries to rise. It discharges when the voltage in the circuit tries to decrease.
There are two tests for a capacitor. 1. for capacity in portion of a farad.
2. test for the integrity of the dielectric. There are several terms for that two of them are ESR and some portion of ohms, generally in megohms.

There are many of them in a circuit as they serve many functions. In simple terms any two conductors that are separated and are parallel to each other exhibits capacitance. There are times when this is not wanted
and steps are taken to reduce the effects of this capacitance.

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Post by stevech » Wed Jul 05, 2006 7:38 pm

I'll throw in this...

In very general terms. Capacitors are the heart of "filters".
Filters reduce the strength (amplitude) of voltages that vary rapidly, e.g., alternating current. The wall current varies at a frequency of 60 cycles or Hertz (Hz) per second. Your AM radio stations vary their transmitted "carrier" at 640,000 Hz to about 1,200,000 Hz - this being the limits of the AM broadcast radio band.

Low-pass filters and high-pass filters using capacitors and resistors are the most common use of capacitors. The large capacitors we find in most power supplies for computers and radios and so on are part of a "low pass" filter, to eliminate the power line's 60Hz in the direct current power derived from the power line (mains).

High pass filters are found in electronic circuits where the desired signal is high frequency (Hz) as compared to the undesired signal, and to eliminate any direct current (that is "DC") - and where the desired signal can be superimposed on DC.

Better stop now.

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Post by bearing01 » Thu Jul 06, 2006 12:06 pm

There are meters out there that can test capacitors. One of the cheap hand-held multimeters I have allows you to plug in the cap and it shows the value on the display. I guess you could make one of these if you researched it. Another way to test a cap to see if it has failed is by using a multimeter... preferably an analog meter with a needle display. FIrst you short out the cap leads and you then set the meter to measure resistance. Then measure the resistance of the capacitor. The meter's needle should deflect to zero ohms and then work it's way back to infinite ohms (open). The faster the needle moves back to infinity the smaller the capacitance value.

Here's a capacitance meter for you.
Go: Projects > Digital Capacitance Meter

There are many flavours of capacitors. The differences are determined by how they're constructed and what the insulator (dielectric) is made of. Some capacitors will be made with lose tolerance, meaning if you were to go into production then if your design is using a 1.0uF cap then one batch of caps could be as high as 1.2uF while another comes in at 0.8uF. If you're designing filters then some of them won't work at the frequency you want! IF you're building power supplies then it won't make that much difference. Also, capacitors tend to have a value that will vary with frequency of operation. For eg: a cap may have 1uF value at 100kHz but at 10MHz it's 0.5uF or it may even look like an inductor. Different capacitors types will have a different value-vs-frequency characteristic. Some caps are made to work from low frequency up to high frequency (in GHz range) while others are made with only the intention of operating at low frequency. Capacitors also have a value that may change or vary with temperature or voltage applied. If you were to design a filter with a temperature varying capacitor, at room temp the filter may be fine. In extreme heat (100*F or 40*C) or in freezing temperatures your filter may no longer work at the frequency you want.

Also, some capacitors are polarized meaning you have a +ve and a -ve terminal. Some aren't. This is another attribute determined by how the capacitor is made.

There are many webpages that talk all about the different types of caps. I'm not an expert on all of them. But basically, here's some caps that are commonly used.

Ceramic caps - can be non-linear and create distortion in Audio Circuits, but good for AC coupling (DC blocking) and be used for few MHz.

Film / Foil caps - (Polyester, Polypropylene) good for Audio circuits

Mica caps - good up to radio frequency

Electrolytic caps (these are polarized, Aluminum and Tantalum) - Good for low frequency in 10's of kHz, for power supplys. Tend to be "leaky" where they self discharge over time. Their cap values tend to vary alot from part to part and they have a shelf life where after a couple years the part could be defective.

Another application that wasn't mentioned was oscillators. Capacitors have to be charged up, which if you're doing it through a resistor will take some time period. If you design a circuit to trigger from 0V to 5V after the input voltage is a certain amount then you could use a capacitor's voltage as the input source. The charging voltage of the capacitor will (after some time) be high enough to trigger your circuit's output from 0V to 5V. Now, if you take your circuit's output and use it to turn off the capacitor's charging device (when output is 5V) the capacitor voltage will start to drop. When it drops a certain amount your circuit's output can drop to 0V and this turns back on the capacitor's charging circuit again. This continues and the cyclic pattern is what makes an oscillator. That's where the sine wave or square wave comes from in your signal generator. Another type of oscillator is used at radio frequencies, often called VCO. The VCO requires an inductor and capacitor. If you take an inductor and capacitor and hook them in parallel it forms an energy tank. If you punch it with a blast of current (a current spike) the cap's electric field throws charge into the inductor, which reacts / kicks back to the capacitor with a voltage spike to charge the capacitor, which sucks the current out of the inductor to make it kick its voltage the other way... which again cycles over and over to creat a periodic voltage. This voltage will decay over time becauses of losses and the fact you want to draw power out of the tank to sense this voltage. Therefore, you need an energy supplying device like a transistor (hooked up with positive feedback) to supply that energy dissipated from the tank. Then you have a VCO.

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Post by ku7485 » Thu Jul 06, 2006 1:32 pm

During circuitry designing, how would you determine if a capacitor is needed and where it should be placed?

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Post by bearing01 » Thu Jul 06, 2006 1:50 pm

Some circuits, the capacitor is an integral part of the design. Like building a house... how do you know if it needs a roof.... well it's not a house without the roof!

Besides those types of circuits, if you build a circuit you have to give it a power supply. Alot of times you want an AC ground and you can use a capacitor for that. Biggest cap value you can find as long as it's still reliable as a capacitor. Your power supply should ideally look like an AC ground. Depending on the type of supply or what your circuit is doing, you may get voltage ripple on your power supply. If you put a big capacitor from +V to ground then that will filter out any voltage ripple or noise that may be present on the power supply.

In other cases you may have to throw a capacitor into a circuit where you didn't anticipate. For example, lets say you design an amplifier and it's operating at a couple MHz or higher. Parasitic capacitance in the design (say in the PCB or in the transistor itself) will feed the output signal back to the input. If there is sufficient delay from output to input such that what's appearing at the input is at the same phase as the input... that will make the amplifier amplify itself. It will oscillate. To prevent this you can add a capacitor on a critical (high impedance) node to shift the phase of any signal that is making its way back to the input. That will improve the amplifier's stability and prevent it from oscillating.

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