A capacitor is an electronic device which consists of two plates (electrically conductive material) separated by an insulator. The capacitor's value (its 'capacitance') is largely determined by the total surface area of the plates and the distance between the plates (determined by the insulator's thickness).
A capacitor's value is commonly referred to in microfarads, one millionth of a farad. It is expressed in
microfarads (units, µ
F) because the farad is such a large amount of capacitance that it would be impractical to use in most situations.
This analogy should help you better understand capacity. In the following diagram, you can see 2 tanks (capacitors) of different diameter (different capacitance). You should readily understand that the larger tank can hold more water (if they're fill to the same level
[voltage]). The larger capacitor has more area in which to store water. Just as the larger capacitor's larger plate area would be able to hold/store more electrons.
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When a DC voltage source is applied to a capacitor there is an initial surge of current, when the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is 'charged'. If the DC source is removed from the capacitor, the capacitor will retain a voltage across its terminals (it will remain charged). The capacitor can be discharged by touching the capacitor's external leads together. When using very large capacitors (1/2 farad or more) in your car, the capacitor partially discharges into the amplifier's power supply when the voltage from the alternator or battery starts to fall. Keep in mind that the discharge is only for a fraction of a second. The capacitor can not act like a battery. It only serves to fill in what would otherwise be very small dips in the supply voltage.
Generally, if an AC voltage source is connected to a capacitor, the current will flow through the capacitor until the source is removed. There are exceptions to this situation and the A.C. current flow through any capacitor is dependent on the frequency of the applied A.C. signal and the value of the capacitor.
For most of my time in electronics, most capacitances were designated as either microfarads or picofarads. nanofarads was known but we didn't use it. For a 1000p
F capacitor, we used 1000p
F. Now, people tend to use 1 nanofarad (1n
F) instead of the picofarads designation. If you haven't seen it, the
Milli-? page has a converter. If you enter 1000p
F and 1n
F you will see that they're the same.
There is significantly more information on these types of capacitors on the
Resistors and Capacitors page of the tutorial.
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ESR is the equivalent series resistance of a capacitor. An ideal capacitor would have only capacitance. As you remember, all conductors have resistance. In a capacitor, there are multiple conductors like the wire leads, the foil and the electrolyte. The resistance of all of the conductors contribute to the capacitor's series resistance. It's essentially the same as having a resistor in series with an ideal capacitor. Capacitors with relatively high ESR will have less ability to pass current from its plates to the external circuit (to the amplifiers in the case of stiffening capacitors in car audio). Low ESR is desirable when using a capacitor as a filter.
When choosing replacement capacitors for an amplifier, specifically for the primary side of the power supply, you typically want capacitors with low ESR. Some capacitors are designed to have very low ESR so looking at those as replacements is a good start. If you want to get a bit lower ESR, you can go up in the voltage rating (if they will fit, since they're likely to be larger).
ESL is the equivalent series inductance of a capacitor. Since most electrolytic capacitors are basically a large coil of flat wire, it will have even more inductance than it would have if it were flat. This inductance, along with the small amount of inductance from the wire leads, will make up the ESL of the capacitor. The ESL is essentially the same as having an inductor in series with an ideal capacitor. Low ESL is desirable when using capacitors for filtering purposes.
The Zener represents what you will have (DC current passing when it should not) if you go beyond the working voltage of the capacitor or if you connect it with reverse polarity.
Capacitors designated as low-impedance will typically have lower ESR and lower ESL than capacitors with similar voltage and capacitance rating. These are often used for the primary filter capacitors in an amplifier. The formula for impedance (Z) is given as follows. This was taken from
THIS Cornell Dubilier document.
When choosing replacement capacitors, other than dimensional considerations, you have to select based on capacitance, voltage rating and the maximum operating temperature. In most cases, all of the secondary capacitors will be rated for 85C. In general, you will use capacitors rated for 105C on the primary side of the power supply. The primary caps will be stressed far more than those in the secondary.
Even though a capacitor's plates are insulated from each other, there is a small amount of 'leakage' current between its plates. This current is generally insignificant but will cause a capacitor to slowly discharge with no external circuit path between the capacitor's leads.
If you want to know more about capacitors, in general, read through
THIS Cornell Dubilier document.
Some large capacitors used in car audio systems have a digital voltmeter. Some of these displays will have a remote turn on lead to turn on the LED display. Others will have a timer that will turn the display off after a few minutes. If, in either case, the capacitor's positive lead was removed from the power source (and the display remained on), the capacitor would be discharged by the display. The display acts as a load and isn't the same as the discharge due to leakage current.
Many low value capacitors (less than 1 microfarad) will have a plastic type of insulator (polyethylene, polypropylene...) between the plates. Sometimes the plates are actually a metallized layer bonded onto one side of the plastic film. Multiple layers of the metalized plastic film make up the capacitor. Adding layers or increasing the size of the layers (without increasing the thickness of the layers) will increase capacitance. The following diagram is an incredibly generic film capacitor. You can see the amber insulating film between the white conductive layers that make up the plates. The plates are soldered to one of the terminals on one end of the plates. Half of the plates are soldered to terminal A and the other half of the plates are soldered to terminal B.
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Electrolytic caps are more complex than film capacitors and are generally used for larger capacitance values (0.47 microfarad and higher). The electrolytic capacitor generally consists of 2 layers of aluminum foil (red and green below
[gray in real caps]) with a layer of paper insulating/separating material between the plates. Click the image to see a larger version. Click in new tab to toggle fit/full.
In the graphic above, the foil is extended up for illustration. In reality, terminals (something like that shown) are crimped into the foil at various points along its length. This isn't to scale but gives you an idea of what's used to make the connections. The individual terminals for each plate/foil are combined at the terminal that extends from the body of the capacitor.
The aluminum foil that makes up the plates in the electrolytic capacitor is treated in a few different processes to make it function more efficiently. The most important process is the anodizing of the foil. Anodizing is a process that forms a very thin layer of aluminum oxide on one or both sides of the foil when the foil is immersed in an acidic solution and direct current is applied to the foil (one lead of the DC power supply is connected to the foil and the other is connected to a conductive plate in the acidic solution). This layer of aluminum oxide is the dielectric (insulator) and serves to block the flow of direct current. To increase the surface area on the foil (and ultimately increase capacitance), the foil can be etched by a chemical process. This would be done before the anodizing.
The paper element serves to hold the electrolyte
† in place. The electrolytic solution (generally ethylene glycol and ammonium-borate) can vary in content but must generally serve a couple of purposes. First and foremost, it must be electrically conductive to help pass the electrons from one plate to the other (the glycol part of the solution does this). Secondly it helps to heal any areas of the dielectric that become damaged (the ammonium-borate does this). If the conductive properties of the electrolyte were absent, the capacitor's value would be drastically reduced. If the healing properties were absent and the anodized coating was scratched or otherwise damaged, the capacitor would leak DC from plate to plate. The healing properties greatly increases the useful life of the capacitor.
†The electrolyte is typically a liquid but that's not always the case, there are solid electrolytes (tantalum capacitors and the types used in computer power supplies). One type of these are known as solid conductive polymer capacitors (
AKA solid state capacitors).
Electrolytic capacitors generally have a positive and a negative terminal. As we said earlier, the plates (foil) of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material (it determines which side of the plate is positive and which is negative). We also said that part of the electrolyte was to help heal a damaged plate. Since it has the properties to heal a damaged plate, it has the ability to re-anodize the plate. Since anodizing process can be reversed, the electrolyte has the ability to remove the oxide coating from the foil. This would happen if the capacitor was connected with reverse polarity. Since the electrolyte can conduct electricity, if the aluminum oxide layer is removed, the capacitor would readily pass direct current from one plate to the other (it would basically be a short circuit from one plate to the other). This would, of course, render the cap useless.
All capacitors have a voltage rating. This tells you how much voltage the dielectric (insulator) can withstand before allowing DC to pass between its plates. Sometimes a capacitor has a working voltage (i.e. 50
WVDC - 50
Working
Volts
DC) and a
surge voltage. The working voltage tells you how much voltage the capacitor can withstand long term (for the normal life of the capacitor). The surge voltage is the voltage is can withstand for short periods of time. An example of the voltages can be seen in the
TS-UP datasheet. Generally, if too much voltage is applied to a capacitor, it will fail. In electrolytic capacitors, the forming voltage (voltage used to anodize the plates) and the thickness of the paper element determine the working voltage of the cap. In film type capacitors, the insulating material (polyethylene, polypropylene...) will determine the maximum working voltage.
As you've likely noticed, large 1 farad (and 1/2 farad) capacitors are available in both 16v and 20v versions. As was said above, the voltage rating tells you how much voltage the capacitor can withstand. It does NOT tell you how much voltage it will have when connected to your system. If both a 16v and a 20v capacitor are connected to the electrical system (with a voltage of 14.4 volts), both the 16v capacitor AND the 20v capacitor will have exactly 14.4 volts. The voltage on the capacitor will be the same as the circuit to which it's connected. In this situation, both the 16v and the 20v capacitors (which have identical capacitance ratings) will hold precisely the same amount of energy. If (IF) the capacitors were charged to their maximum working voltage, the 20v capacitor would hold more energy because it can survive higher voltage. As you can see in the diagram below, all of the electrical components have the same level of water in them (they have the same voltage). If you continue this analogy, you'll be able to imagine that the lower voltage capacitor would 'overflow' if the voltage would go too high (above 16 volts). The 20 volt capacitor could accept a higher water level (voltage) before it overflows. You can also see that when the capacitors are fully filled, the 20 volt capacitor can hold more water (energy). But... in this situation (and in a car), they hold the same amount of energy.
As a side note... The volume of water that a cylinder can hold is equal to the surface area of the cross section of the cylinder (which is analogous to the surface area of the capacitor's plates) multiplied by the height of the cylinder (which is analogous to the voltage that the capacitor's dielectric (insulator) can withstand). Increasing the surface area and/or height of the cylinder (the maximum voltage rating) will increase the maximum volume (charge) the cylinder can hold.
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An electrolytic
capacitor will generally
overheat if subjected to adverse conditions such as
overvoltage,
excess ripple or
reverse polarity. This will cause the electrolyte to boil off which will create pressure in the sealed aluminum can that envelopes its internal components. If there were no form of controlled venting, the capacitor would eventually
explode. For safety's sake, capacitor manufacturers employ some sort of pressure relief that will fracture before the capacitor's aluminum enclosure. On relatively small capacitors, the vent is simply a
few stamped lines in the top of the capacitor. The stamping weakens the aluminum casing slightly and allows venting when the capacitor's internal pressure reaches dangerous levels. The other type of vent (used on very large capacitors) is a plug that will blow when pressure reaches dangerous levels.
Capacitors, like any other component, have a particular behavior when connected in groups. The simplest configuration is the parallel configuration.
When connected in parallel (very commonly used for primary capacitor banks and for rail capacitors), the values of the capacitors add together. Five 10µ
F capacitors in parallel will equal 50µ
F. In this configuration, the same rail voltage is applied to all of the capacitors
† and they share the load (filtering/bypassing noise/ripple).
†In some circuits, large electrolytic capacitors are paralleled with smaller ceramic or film capacitors. You can see this configuration
HERE. The smaller capacitors can be more effective at bypassing higher frequencies because they have less ESL and therefore lower impedance at the higher frequencies. For the most part, in this configuration, the larger electrolytic capacitors pass the greatest share of the current and the smaller capacitors, much less current. In most instances, if you pulled the smaller capacitors from the parallel group, there would be no noticeable difference in the operation of the group of capacitors.
Regardless of the AC.switched frequency or if it's DC (in car amp circuits), most of the information above applies.
When capacitors are series, there is a significant difference in the way the voltage is distributed. Below, DC applied with only capacitors in series and all capacitors rated to withstand the total rated voltage, the DC may or may not (likely not) be equally distributed. To get equal distribution, you must parallel a resistor with each capacitor. This forms a voltage divider and forces the DC to be equally distributed (if the divider resistors are of equal value). For AC, the voltage division is likely to be equal across each capacitor if all capacitors are of equal value.
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There is a bit more about this on the
Repair Introduction page of the tutorial.
A capacitor stores energy. To get that energy into the capacitor, you have to charge it. The following shows the similarities between a mechanical system of components and an electrical system. The following demo shows how a capacitor charges quickly at first then, as the difference in the capacitor's voltage and the power supply is reduced (the meter shows the difference in voltage), the capacitor charges more and more slowly. When you push the red button on the demo below and the latch releases the piston (also notice that the switch across the capacitor opens to allow it to charge), you should notice that the piston initially moves up at a fast rate of speed and as the spring exerts less and less force (because its stretched less and less), the piston moves more slowly. You should notice that, initially, the rate of volume of fluid flowing into the cylinder is high. Then as the spring exerts less and less force, the cylinder fills more and more slowly.
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If you need to know how long it will take to charge a capacitor with a given resistor, you can use the following calculator. The output data tells you how long 'one time constant' is (time to reach 63.2% of the difference in voltage between the capacitor's voltage and the supply voltage). It also tells you how much time it will take the capacitor to become charged. Lastly, it tells you the maximum power dissipation across the resistor (use this value as a guide in selecting a resistor).
A capacitor's time constant is the time it takes for the capacitor to charge to 63.2% of the supply voltage when charged through a given resistor. At the end of one time constant, if the supply voltage is 10 volts, the capacitor will have charged to 6.32 volts. A capacitor is considered to be fully charged after 5 time constants. After 5 time constants, the capacitor will have charged to 99.2% of the supply voltage. The following chart shows the charging curve for a 1 farad capacitor and a 50 ohm resistor. As you can see, the capacitor charges more quickly at first and then (as the difference between the capacitor's voltage and the supply's voltage is reduced) the rate of charge slows. As the capacitor reaches full charge, the current flow is reduced to nothing.
The diagram above is for a 1F capacitor and 50 ohm series resistor connected to a 15v power supply. The numbers on the x-axis are the time in seconds divided by 10 to save space. The formula, again:
The formula for 1 time constant is T=RC where T=time in seconds, R=resistance in ohms and C=capacitance in farads.
This means that the voltage across the capacitor will be at 63% of supply voltage (8.72 volts for a 13.8 volt supply) after 50 seconds. Earlier, I said that the time constant is the time to charge 63.2% of the difference in voltage between the capacitor's voltage and the supply voltage. After one time constant the voltage across the cap was 63% of the supply voltage. This means that the voltage across the charging resistor is now only 37% of the supply voltage (instead of 100% of the supply voltage when the capacitor was fully discharged). This will reduce the current flow through the resistor and into the capacitor. For the second time constant, the capacitor's voltage will increase only 63% of the voltage across the resistor. (13.8-8.72=5.08 volts).
For the second time constant...
At the end of the second time constant, the voltage is going to be the voltage at the end of the first TC plus the voltage increase from the second TC. In this case, it was 8.72 volts + 3.21 volts or 11.93 volts. If you look at the chart above you can see that the charging curve crosses the 11.93 volt mark at 100 seconds (2 time constants).
If you want to know the voltage at any given point in time, we can use the following formula. We'll use 65 seconds in this example.
At 125 seconds...
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