What causes excessive AC condensation

Electric capacitance: capacitors and electric fields

Practical event customer
by Herbert Bernstädt, article from the archive of

The antagonist of inductances is capacitance. What technical influence does it have in event technology?


Overview:

Electric field strength - in terms of formulas
Something analogous nostalgia
Component capacitor
Charging in the capacitor
Electronic ballast and function of switching power supplies
Capacitor designs
Capacitor aging
Small electronics appetizer


Analogous to how flowing current creates a magnetic field, an electric field builds up between two potential differences: If there is an excess of electrons on an electrical conductor such as a metal plate, and significantly fewer electrons on another electrical conductor or a second metal plate - Just as we got to know when defining the electrical voltage - there is also a field between the different potentials: the electrical field. Field lines are thus formed between the differently charged plates. The direction of the field line direction has been defined with the direction of the force acting on a positive particle. If the plates are symmetrically opposite, the field is evenly distributed directly between the plates, i.e. field lines with the same density are formed and run parallel to one another. We then speak of a homogeneous (uniform) field. If one pole is designed as a sphere instead of a plate shape, the field lines emerge from the sphere at the surface vertically and bend towards the other pole. The field is consequently inhomogeneous. If the potential difference does not change, we also speak of a static field. Electrically charged parts that are in an electric field are accelerated along the field line.

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Electric field strength - in terms of formulas

The electric field strength is a measure of the force on a charge in the electric field. From this follows the formula:

E = F ÷ Q

With
E = electric field strength (Vm-1) Volts per meter
F = force (N) Newtons
Q = electrical charge (As) ampsecond

as

E = U ÷ d

With
E = electric field strength (Vm-1) Volts per meter
U = voltage (V) volts
d = plate spacing (m) meters

One consequence of the fact that free charge carriers are accelerated by an electric field is the influence. This means that in an electrically conductive body such. B. a hollow sphere, charge carriers are shifted and thus the hollow sphere is also electrically polarized. The strength of the polarization counteracts the field exactly.

The result is that the field within the sphere is therefore equal to zero.

W = F × s

With
W = work (Nm) Newton meter
F = force (N) Newtons
s = distance (m) meters

 

So we have the case of the so-called “Faraday cage”, similar to a car, which is why we are quite safely protected from injuries caused by lightning strikes in the car in a thunderstorm. We encounter this principle in our daily work with all shielding of conductors, the wire mesh between the core insulation and the cable sheath.

In contrast to magnetic fields, electrical fields can be shielded very well - this is how official EMC measurements are carried out in shielded rooms.

EMC measuring room with shielding
Wire mesh for shielding the signal line

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    Something analogous nostalgia

    Modern semiconductor technology has displaced the importance of electric fields from many areas. However, the acceleration of electrons by electric fields was or is the basis of all tube technology that semiconductor technology uses today. Electron beams were generated in the television set and deflected according to their positioning on the phosphor layer (i.e. the screen). The Braun tube, which is also used as the basis for oscilloscopes, is still used today to display the fastest voltage potential shifts over time. Due to their typical properties, tubes can still be found in high-quality amplifier circuits today, used for the "warm, lively" sound. But the electrostatic loudspeakers are also based on the properties of the electric fields.

    Semiconductors also use the electric field, such as B. the field effect transistor. Of course, the field continues to have an influence on all electronic components, which we will look at in more detail in the chapter on capacitors.

    Electric fields are also used in production, for example during electroplating or painting, so that the previously charged pigments are attracted to the correspondingly charged object to be coated. This prevents unnecessary loss and makes it easier to get to places that are otherwise difficult to reach (e.g. around corners). By the way: this process can also be used to filter dust out of the air.

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      Component capacitor

      As we could see from the description of the electric field, two opposing metal plates with different potentials cause an electric field. This field stores energy in the same way as the magnetic field in the coil; If the surface of the plate is enlarged, a larger area is available for the charges. The closer the plates are to each other, the more charge carriers they also attract, since the opposite charges also attract each other.

      The electric field also ensures the alignment of the molecular dipoles in the dielectric, which is called dielectric polarization. Depending on the insulating material, even more charges can be absorbed, which also determines the capacitance of a capacitor. This is the opposite of a magnetic field, in which ferromagnetic materials can conduct magnetic fields more or less well. In the case of a capacitor, the medium between the opposing layers (such as metal foils) an insulator - called a dielectric (in the simplest case air) - is important. Depending on the insulating material, the dielectric polarization binds a certain amount of the originally present capacitor charge. The capacitor plate can therefore take up more charges than without a dielectric. The permittivity number (formerly dielectric number) indicates the ratio of how many times larger the capacity becomes with the substance instead of with air. Hence ɛr for air is 1 and z. B. for hard paper 4-8, Polysterol 2.5 and ceramic 10-10,000.

      Metal-paper capacitor
      Ceramic, electrolytic and metal-paper capacitors

        On the one hand, you want to place the plates as close as possible to one another in order to increase the capacitance, on the other hand, a high voltage or insufficient insulation causes the voltage to break down through the dielectric, which destroys the capacitor. Let's just get ahead of it: There are also self-healing capacitors. At the point where the breakdown occurs, the short circuit on the arc vaporizes the thin metal coating in the area, causing the damaged area to isolate itself again.

         

        C = ɛ0 × ɛr × A ÷ d

        With
        C = capacity (F = As / V) Farad
        A = plate surface (m2)
        d = plate spacing (m)
        ɛ0 = Electric field constant = 8.85 * 10-12 As / Vm
        ɛr = Permittivity number

        and

        W = ½ × C × U2

        With
        W = electrical energy (Ws) watt-second
        C = capacitance (F = As / V) farads
        U = voltage (V) volts

        and

        Q = C × U

        With
        C = capacity (F = As / V) Farad
        Q = electrical charge (As) amps seconds
        U = voltage (V) volts

        and

        τ = R × C

        With
        C = capacity (F = As / V) Farad
        R = resistance (Ω) ohms
        τ = time constant (s) seconds

         

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        Charging in the capacitor

        If the capacitor is connected to a direct voltage, a current flows immediately to flood one surface with electrons, while electrons are sucked out on the other surface. A voltage builds up between the coverings - the electric field. When the voltage is applied, the voltage increases until the potential of the applied voltage is reached. When the same potential is reached, the flow of current also stops. The capacitor blocks the direct current after charging. The capacity that a capacitor takes is defined as 1 Farad if it has been charged by 1 V from the 1 As charge. Colloquially, a capacitor is therefore also called a capacitance.

        The capacitor needs time to charge: At first the current shoots up and then becomes smaller and smaller in an E-function. 63% was defined as the charge or discharge time as a time constant Tau τ. After five time units there is hardly any discharge or charge, because after the E-function the curve approaches the X-axis more and more, but does not come to a standstill there. The time constant τ is the product of resistance and capacitance.

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        Electronic ballast and function of switching power supplies

        If we look at modern switched-mode power supplies or electronic ballasts (EVG), we know that they have a high capacity for your intermediate circuit. That is why the electronic ballasts and switched-mode power supplies are also viewed as capacitive loads (CLast) in contrast to magnetic ballasts and transformers. The high capacitance of the circuit - typically a capacitor - means that the inrush current will be extremely high. The level of the inrush current can be limited by an upstream resistor, but this robs the property of being able to quickly provide a lot of energy in the intermediate circuit after a lot of energy has been called up.

        Let's say we want to replace all halogen lamps with LEDs in the ceiling of the hall. Then when the cleaning light is switched on, all LED ballasts would draw a high current at once. This is why high-quality electronic power supplies - e.g. from headlights or active loudspeakers - also inherently have a kind of random time delay so that several devices can be connected to one phase with one switch when switched on. Therefore, in some instructions you will find the information about the maximum number of devices of this type that can be connected to a supply line, although the total output is still far below the maximum value. Other manufacturers also state how much higher the inrush current is compared to the operating current and sometimes even how long it is drawn.

        In the stage machinery, frequency converters can also be found for controlling the speed of the drives, which work on a very similar principle to the switched-mode power supply. Only here, in the secondary circuit, a new alternating voltage is now generated or switched from the direct voltage energy store in accordance with the desired frequency. With frequency converters, there are of course high energy densities due to the machines to be connected, which accordingly also have to be coped with by the converter capacity. Now another effect comes into play: If we connect the converter as usual on our platform via the sub-distribution with residual current devices, the RCD will trip even though there is no leakage current or insulation leakage. What happened? The converter capacitor first fills its capacitor plates with electrons. An RCD compares the current flowing in with the current flowing back. If the two currents are of different magnitude, an error is deduced - but more on that elsewhere. An RCD with 30 mA is happy to trip when the electrons charge the capacitor, but there is no return flow of current on the other line. This is why the instructions for converters often state that they should not be connected via RCD.

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        Capacitor designs

        Capacitors are available in various designs and capacities. Similar to resistors with your E6 or E12 series, there are also value standardizations from picofarad (pF), that is 10-12 farad, to microfarad (μF), which is 10-6 farad. The values ​​are often given without a unit or can just as easily be applied as color coding.

        Another important key figure when choosing capacitors is the nominal voltage, which is specified at 40 ° C. The voltages here range from 30 to 1000 V. The loss factor is specified for 800 Hz or 1 kHz and is of course only of interest for alternating voltages. In the case of capacitors, too, the ambient temperature has an influence on the capacitance and is specified in the same way (as with resistors) with the temperature coefficient. In principle, capacitors can be operated independently of polarity and with alternating voltage. Correct polarity must only be observed for aluminum electrolytic capacitors and tantalum electrolytic capacitors, and these must not be operated with alternating voltage. Because the positive anode consists of an aluminum foil on which an oxide layer has been applied. Together with the electrolyte, these capacitors have very high capacities due to the very thin oxide layer. The disadvantage here is that if the polarity is incorrect, the oxide layer is broken down and the capacitor is destroyed, which can sometimes be heard with a loud bang and the smallest scraps of paper sink down in the room. The additional advantage of tantalum is its great independence from temperature and voltage.

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        Capacitor aging

        Electrolytic capacitors, in particular, are subject to an aging process with their electrolyte. At higher temperatures, the electrolyte becomes thinner in viscosity and escapes more easily. The internal pressure increases with higher temperatures, so that the electrolyte can also escape. As with halogen lamps, it can be said that an increase in temperature by 10 ° C doubles the service life. Experience shows that the loss of an operating point in a circuit, or in short the switched-mode power supply, very often dies in old age due to the change in capacitance of electrolytic capacitors. Often it is not the LEDs that suffer sudden death, rather death is caused by faulty driver electronics if the switched-mode power supply does not die itself. Because of this, of course, there are also different quality classes: The 85 ° C / 2000 h quality class has a service life of 8,000 hours at an operating temperature of 65 ° C.

        Now everyone can work out how long the component will stay alive for an 8-hour day and five days a week: This is over after 3.8 years, provided that other influences do not ensure faster aging. A guarantee according to VOB (five years) is not responsible for the design.


        "Death lurks in the capacitor class."

        Herbert Bernstädt | about the aging process of capacitors


        With the quality class 105 ° C / 2000 h, that's 32,000 hours. Even better is the 105 ° C / 5000 h class, which then allows 80,000 hours. Now it is safe to ask yourself which quality class of capacitors is built into which type of device. Chances are, the inexpensive devices will include the simplest of capacitors as well. So it is not surprising when after a year the first LED spotlights bless the time, regardless of whether the LED has an average service life of 50,000 hours or not. To make matters worse, the compact design in the housing of many devices very quickly exceeds 65 °, because the fan blows the hot air onto the electrolytic capacitors or the use under the tent cover already entails high ambient temperatures and the air filter are not cleaned in front of the fan and there is no air flow. Death lurks in the capacitor class.

        Ambient temperature

        Grade

        85 ° C / 2,000 hours 105 ° C / 2,000 hours 105 ° C / 5,000 hours
        105 ° C

         –

        2,000 hours5,000 hours
        95 ° C

         –

        4,000 hours

        10,000 hours

        85 ° C

        2,000 hours

        8,000 hours

        20,000 hours

        75 ° C

        4,000 hours

        16,000 hours

        40,000 hours

        65 ° C

        8,000 hours

        32,000 hours80,000 hours
        55 ° C

        16,000 hours

        64,000 h

        160,000 hours

        45 ° C

        32,000 hours128,000 h

        320,000 h

        Overview of the quality classesAt what temperature can how many perfect operating hours can be expected?

        Not using the devices is not necessarily good for the device. Similar to a car that acidifies when it is left standing for a long time, the electrolytic capacitors (ELKO) also have the property that the oxide layer changes when no current flows. Just as you can get a gasoline engine running again, the ELKOs regenerate themselves when they start up again. But after the long break, a very high initial current flows, which is like a short circuit. The ELKO heats up excessively and further electrolyte is lost. The service life is shortened dramatically.In the case of inexpensive types, the device should therefore be switched on after six months at the latest to ensure continued reliability. With good capacitors, however, two years of downtime are not a problem.


        Maybe also interesting?

        Basic article on inductors: Because inductors are late

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        Small electronics appetizer

        A capacitor C1 is charged with a constant current. The current flows to the capacitor via R4 and P1. The level of the current can be set using P1, because the higher the current, the faster the capacitor's voltage has risen to the desired maximum value. However, since the current flow to C1 could also change with a change in the voltage at C1, a constant current flow is required so that the rising voltage forms a straight line. For this purpose, a constant current source is formed with the field effect transistor T1 together with R4 and P1 (PRODUCTION PARTNER reported on the functioning of a constant current source in issue 11 | 16 in the article “Even more resistance”).

        If the capacitor C1 is charged up to the operating voltage by means of constant current, what could induce the capacitor to assume zero volts again, as we need it for a sawtooth (periodically recurring)? To do this, we use an impulse (a) to control transistor T3. When the base of T3 is supplied with the pulse, the collector-emitter is conductive and the capacitor C1 is short-circuited. The capacitor is discharged and the voltage at C1 is 0 again. The pulse (a) at transistor T3 does not last long and C1 begins to charge again. The sawtooth voltage is created (c).

        Now let's look at a sawtooth and recall the above formulas. The energy that the capacitor has absorbed corresponds to the area of ​​the triangle: W = (Q × U) ÷ 2. Instead of Q we now insert C × U and we find our above formula W = ½ × C × U2 again. Now you can also imagine that if the capacitor shorts, very high currents can flow within a very short time - especially if a capacitor with a large capacity is present at high voltage. Short-circuit currents of large capacities can sometimes cause an open-ended wrench to evaporate.

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