In this section:
|Part / PCB|
|47pF||47p||470 or 47|
|220pF 50V||n22||221 50|
|1200pF or 0.0012uF||1n2||122|
|micro (u)||.000,000,1F||1uF||1u0||105 or 1uF|
|milli (m)||.001F||1000uF 16V||1m0||1000uF 60V|
For some strange reason, the nano multiplier is rarely used in schematics; values in that range are expressed as thousands of pico farads or fractional milli farads. In Europe, and sometimes elsewhere, on PCBs but also sometimes on schematics, the multiplier may be used in place of the decimal point: For example: 4n7 means 4.7nF or 4700pF. By selecting the correct range, any value can be written with 3 symbols using this method.
On the component itself, the value may be written out if there is room, or smaller devices will use the number system:
###A |||'-tolerance (optional) ||`--number of zeros (optional if value is 0) |`---second digit `----first digit
e.g. 234M = 230,000pF or 0.23uF with ±20% tolerance
Very small value capacitors may have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be! The number system is often used on PCB's and often on schematics. If there is a letter following the numbers, it specifies the Tolerance. If there are two numbers, the second one is probably the voltage rating.
Older small tantalums may have a stripe and dot system: Two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF rather than pF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V).
Capacitors store electrical potential energy by providing a large surface area in a set of conductive "plates" for the electrons to pile onto or fly away from. The space available for electrons to be stored in the capacitor is indicated by its capacitance measured in "farads" after British physicist Michael Faraday. When electrons are made to flow onto one plate of the capacitor by applying a voltage, they try to jump over to the other plate where electrons are being removed. This electric charge or "field" builds up a pressure or voltage which eventually exactly counteracts the voltage applied to the capacitor, unless it reaches a point where the separation between the plates, which is called the "dielectric" breaks down and the capacitor is destroyed. The voltage required to break down the dielectric is indicated by the voltage rating of the capacitor.
In a variable capacitor, used for tuning radios or adjusting other circuits where the capacitance must change, the plates are actually visible.
So, capacitors allow electrical current to flow until a charge equal but opposite to the applied voltage is reached. Then the current stops flowing until the applied voltage is changed. You can see this by connecting a multi-meter set to resistance measuring mode to the leads of an 5 to 100uF capacitor. In order to measure resistance, the meter must provide a small voltage through it's leads in order to detect the amount of current which is able to flow through a resistor being tested. By connecting it to the capacitor, you will see a spike on the meter as the capacitor initially allows all the meters current to flow, then builds up an opposing voltage and stop the flow of current entirely. Reversing the leads will show an even bigger spike as the accumulated voltage AND the applied voltage cause a larger flow of current, but again, the capacitor will charge, oppose the voltage that is charging it, and stop the current flow. You can discharge the capacitor and see the original spike by connecting the leads of the capacitor together through a resistor.
Warning: Do not short the leads of a capacitor, especially a very large one, as the resulting flow of current will be so fast that serious damage to the leads, internal structure of the capacitor, and (if the capacitor explodes) your hands, face and other body parts can quite easily result.
The time required to discharge and the remaining voltage (and therefore amperes of current that will flow) when a resistor is connected between the leads of a capacitor can be calculated. The voltage will drop by 1/e (where e is the natural log or about 2.718) in t seconds were t = R * C. R is the resistance in ohms and C is the capacitance in farads. So, for a 1uF cap and a 1K resistor, the voltage will reach about 37% (~1/e) of it's original value in 1 second (.001F * 1000Ohm). In the next second, it will reach about 37% of the remaining voltage or about 13.5% of the original charge. And so on; notice that the cap will never actually be totally discharged.
On electrolytic capacitors (and generally on all polarity sensitive radial lead devices), the longer lead is positive, the shorter lead negative. On aluminum electrolytic capacitors the marked lead is the negative lead. If it's tantalum, the the marked lead is Positive or "when the spot is in sight, the positive is to the right"
Axial lead electrolytic capacitors typically have an indented ring at the positive end. On aluminum electrolytics, the crimp holds the seal in place. It's not explicitly for indicating polarity, but the can is the negative terminal so for axial caps the open end where the positive lead emerges is always the crimped end. For radial caps the can may not be part of the circuit at all. The process of oxide formation on the foil is called "anodizing", i.e., it occurs on the positive electrode. This process is applied to the foil but of course cannot be performed on the casing, so the casing must either be isolated, or be made the cathode.
To check electrolytic capacitor (metal can version only) polarity in circuit:
Properly polarised caps will typically show a voltage of from about 0.1 to 0.5 volts. Reverse biased caps will have a can potential much closer to supply. ie the can is not at but is NEAR the potential of the true negative terminal. For a reversed cap in the 10uF - 1000uF range on a 5 volt supply the can is typically at 2 or 3 volts but anything over about 0.5 is suspect. Bit of a problem here, in that in a very short time if the cap is in the wrong way around you could have some rather spectacular self destruction of the cap occurring. TOUCH BRIEFLY
Martin McCormick WB5AGZ Stillwater, OK
One quick and dirty solution I have for testing electrolytic capacitor polarity is to connect a 15-volt supply such as a little wall wart through a 10 or was it a 20-Kohm resistor. The actual value does not matter as long as it is high enough so as not to draw more than a milliamp or less if completely shorted to ground.
I then pick off the voltage at the positive end of the capacitor and low end of the current-limiting resistor and feed that to a voltage-controlled oscillator such as a NE566 whose own RC constant is set to deliver a tone of about 1,000 HZ when the modulation input is near ground. This modulation input is relatively high impedance so it just monitors the voltage at the capacitor.
When I connect a test capacitor, the tone from the oscillator goes high and then sweeps down as the voltage rises.
This little circuit works fine for many electrolytic capacitors. If you are testing a big filter capacitor, the time constant created with the series resistor means that you have to wait 20 or 30 seconds to see if you are going to get a charge. If the tone starts to slide down, then it is correctly polarized. If it stays high, then it is either shorted or backwards.
Of course, remember to discharge the capacitor after testing or you will zap something later when installing it or touching it against some other part of the circuit. It will eventually charge to near VCC if left in the tester long enough so keep that in mind.
Any other VCO with a high-impedance modulation input should work as well or better than the NE566 which is kind of an old chip that I just happened to have on hand. The square-wave output of the 566 will drive a pass transistor and small speaker just fine.
For a given amount of capacitance, generally tantalum tends to be smallest (highest capacitance per volume), then aluminum electrolytic, then ceramic, then film.
Generally speaking, Tantalum's will be small thru-hole or SMD types. Whereas, most Aluminum electrolytics will be physically large thru-hole types. Aluminum Electrolytics sometimes have metal tabs to use for connections instead of wires. Thru-hole Tantalum's will generally have a teardrop or upside down pear shape. The SMD versions will typically be rectangular, although I have seen round can types too. It's really just a matter of experience when it comes to identifying electrolytics as to whether they are Aluminum or Tantalum. But generally, if a given capacitor is Large, High Capacitance, has tabs or wires for connection into the circuit, it probably will be Aluminum. On the other hand, if the capacitor is relatively small, with a larger than expected capacitance, has short leads or is an SMD type package, then it probably is a Tantalum.
Most standard electrolytics are large enough to have the value printed right on the can. Other possibilites include the number system (### = first, second, number of zeros: e.g. 234 = 230,000pF) or the same thing with the numbers in color codes. Older small tantalums may have a stripe and dot system: Two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF rather than pF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V).
Generally poly film has the lowest (best) impedance, followed by ceramic caps, then tantalum caps, then electrolytic caps. However, the equivalent series inductance (ESL) often depends much more on the packaging of the capacitor than on its capacity or on the internal materials used to construct the capacitor. At high frequencies, generally small SMD caps have the lowest (best) impedance, followed by large SMD caps, followed by small through-hole caps, followed by large through-hole caps. "A bypass-capacitor dialogue peels back the layers"
A 1 uF tantalum has a flat impedance curve below 1 ohm from 100 kHz up to 10+ MHz.
A 0.1 uF monolithic ceramic in a lead type package has sharpness (Q) in its impedance curve, so its impedance is below 1 ohm only between 2 and 10 MHz. It acts as a notch filter at about 1 MHz. SMD ceramic caps have a tiny inductance and are good for far more than 10MHz
A 1000 pF poly film cap has < 1 ohm impedance only from 50 MHz to 1 GHz, and acts as a notch filter at about 100 MHz.
The decoupling capacitor article has some more details.
Tantalums have their uses, but in general they aren't nice components. They have very little, if any overload resistance. A small voltage surge exceeding the working voltage will likely cause the device to go short circuit, or at least reduce it's life. If you have any requirements to build a circuit with very high MTBF's then avoid tants. The main problem is the failure mode. Tantalums fail short circuit with a great deal of heat. They sit there burning with a hot flame. So when they do go, they take out the rest of your circuit and make a mess of your board. Other types of electrolytics have some degree of self healing although they will go bang if you hit them hard enough.
I have been told by Tom Santrizos that Tantalum caps, direct from the mfgr, OR after sitting for years, will become "unformed" and need a few milliseconds to "form up." If they are stressed at initial powerup, after not being powered on reciently, they are MUCH more likely to fail. If your design must always work, first time, or after sitting for years, insure a slow inrush by adding a resistor. Or use another type.
William K. Borsum says:
Watch out for leakage, too. I cannot use Tantalum caps on any of my designs because the leakage (1-5 uA) severely impacts battery life on a device that draws an average of 32-65 uA total. Digikey now carries large value (to 10uF at 6.3V) caps in 1206 surface mount ceramic (X7R and X5R)--be sure to avoid the Z5U temperature tolerance devices like the plague! Most of the data sheets on the Z5U imply +/-20%--but its really +20 -80%--that -80% is a killer!
Common electrolytic caps are rated for only 2000 to 7000 hours of operation at high temperatures. That's usually 4% failures at that life. Usually consider life to double for every 10'C lower. Don't forget that ordinary electrolytic capacitors are filled with a liquid which does eventually dry out. It is because they have a liquid in them that they are called electrolytic. Temperature affects evaporation.... It is the heat generated by the ripple current through the effective series resistance which is the main killer. This heat causes the electrolyte to "boil off" and evaporate.
If a capacitor is used at too low a voltage below its rated voltage, the electrolyte fails to maintain the oxide film on the surface of the metal plates, and the effective leakage resistance drops alarmingly, making the device look more like a resistor than a capacitor.
The "rehealing" feature with electrolytics relates to the working electrolyte in that any faults in the oxide layer will be repaired by further anodization. This is a factor is the design of the cap.
"Self-Healing" refers to metallized caps where a partial discharge results in a localized failure of the dielectric which burns away the metallized electrode effectively isolating the fault.
"Conventional aluminum electrolytic capacitors which have gone 6 months or more without voltage applied may have to be reformed." A DC source at the rated voltage with a 1.5K internal resistance for caps with a rated voltage exceeding 100V or 150 ohms for <=100V must be applied for one hour after the cap reaches it's rated voltage +/- 3%. The cap is then discharged through a resistor of 1ohm/V.
Small value capacitors are unpolarised and may be connected either way round.
It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!
Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!
For example 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.
A number code is often used on small capacitors where printing is difficult: the 1st number is the 1st digit, the 2nd number is the 2nd digit, the 3rd number is the number of zeros to give the capacitance in pF. Letters indicate tolerance and voltage rating.
For example: 102 means 1000pF = 1nF (not 102pF!)
For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).
These numbers may also be represented by the standard color codes just like resistors
Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF).
PICList post "Most 'All-Purpose' Capacitor?"
I am seeking someone with tantalum capacitor manufacturing experience to help me with a market and cost study. Any ideas where I can find someone interested in a short term consulting opportunity?
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