Install a relay for smooth charging of the capacitor. Smooth capacity charging: what to choose? Sergey Chemezov: Rostec is already one of the ten largest engineering corporations in the world

Let's connect a circuit consisting of an uncharged capacitor with a capacitance C and a resistor with a resistance R to a power source with a constant voltage U (Fig. 16-4).

Since at the moment of switching on the capacitor is not yet charged, the voltage across it. Therefore, in the circuit at the initial moment of time, the voltage drop across the resistance R is equal to U and a current arises, the strength of which

Rice. 16-4. Charging the capacitor.

The passage of current i is accompanied by a gradual accumulation of charge Q on the capacitor, a voltage appears on it and the voltage drop across the resistance R decreases:

as follows from Kirchhoff's second law. Therefore, the current strength

decreases, the rate of charge accumulation Q also decreases, since the current in the circuit

Over time, the capacitor continues to charge, but the charge Q and the voltage on it grow more and more slowly (Fig. 16-5), and the current in the circuit gradually decreases in proportion to the voltage difference

Rice. 16-5. Graph of changes in current and voltage when charging a capacitor.

After a sufficiently large interval of time (theoretically infinitely long), the voltage on the capacitor reaches a value equal to the voltage of the power source, and the current becomes equal to zero - the charging process of the capacitor ends.

The longer the capacitor charging process, the greater the circuit resistance R, which limits the current, and the greater the more capacity capacitor C, since with a large capacitance a larger charge must accumulate. The speed of the process is characterized by the time constant of the circuit

the more , the slower the process.

The time constant of the circuit has the dimension of time, since

After a time interval from the moment the circuit is turned on, equal to , the voltage on the capacitor reaches approximately 63% of the power source voltage, and after the interval, the charging process of the capacitor can be considered completed.

Voltage across the capacitor when charging

i.e., it is equal to the difference between the constant voltage of the power source and the free voltage, which decreases over time according to the law of an exponential function from the value U to zero (Fig. 16-5).

Capacitor charging current

The current from the initial value gradually decreases according to the law of the exponential function (Fig. 16-5).

b) Capacitor discharge

Let us now consider the process of discharging capacitor C, which was charged from the power source to voltage U through a resistor with resistance R (Fig. 16-6, Where the switch is moved from position 1 to position 2).

Rice. 16-6. Discharging a capacitor to a resistor.

Rice. 16-7. Graph of changes in current and voltage when discharging a capacitor.

At the initial moment, a current will arise in the circuit and the capacitor will begin to discharge, and the voltage across it will decrease. As the voltage decreases, the current in the circuit will also decrease (Fig. 16-7). After a time interval, the voltage on the capacitor and the circuit current will decrease to approximately 1% of the initial values ​​and the capacitor discharge process can be considered completed.

Capacitor voltage during discharge

i.e., it decreases according to the law of the exponential function (Fig. 16-7).

Capacitor discharge current

that is, it, like the voltage, decreases according to the same law (Fig. 6-7).

All the energy stored when charging a capacitor in its electric field is released as heat in resistance R during discharge.

The electric field of a charged capacitor, disconnected from the power source, cannot remain unchanged for a long time, since the dielectric of the capacitor and the insulation between its terminals have some conductivity.

The discharge of a capacitor due to imperfection of the dielectric and insulation is called self-discharge. The time constant during self-discharge of a capacitor does not depend on the shape of the plates and the distance between them.

The processes of charging and discharging a capacitor are called transient processes.

A capacitor (cap) is a small "battery" that charges quickly when there is voltage around it and quickly discharges back when there is not enough voltage to hold a charge.

The main characteristic of a capacitor is its capacity. It is indicated by the symbol C, its unit of measurement is Farad. The larger the capacitance, the more charge the capacitor can hold at a given voltage. Also than more capacity, the less charging and discharging speed.

Typical values ​​used in microelectronics: from tens of picofarads (pF, pF = 0.000000000001 F) to tens of microfarads (μF, μF = 0.000001). The most common types of capacitors are ceramic and electrolytic. Ceramic ones are smaller in size and usually have a capacitance of up to 1 µF; they don’t care which of the contacts will be connected to the plus and which to the minus. Electrolytic capacitors have capacitances from 100 pF and they are polar: a specific contact must be connected to the positive. The leg corresponding to the plus is made longer.

A capacitor consists of two plates separated by a dielectric layer. The plates accumulate charge: one is positive, the other is negative; thereby creating tension inside. The insulating dielectric prevents internal voltage from turning into internal current, which would equalize the plates.

Charging and discharging

Consider this diagram:

While the switch is in position 1, voltage is created on the capacitor - it charges. Charge Q on the plate at a certain point in time is calculated by the formula:

C- capacity, e- exponent (constant ≈ 2.71828), t- time from the start of charging. The charge on the second plate is always exactly the same in value, but with the opposite sign. If the resistor R remove, only a small resistance of the wires will remain (this will become the value R) and charging will occur very quickly.

By plotting the function on a graph, we get the following picture:

As you can see, the charge does not grow uniformly, but inversely exponentially. This is due to the fact that as the charge accumulates, it creates more and more reverse voltage V c, which “resists” V in.

It all ends with this V c becomes equal in value V in and the current stops flowing altogether. At this point the capacitor is said to have reached its saturation point (equilibrium). The charge reaches its maximum.

Remembering Ohm's Law, we can depict the dependence of the current in our circuit when charging a capacitor.

Now that the system is in equilibrium, put the switch in position 2.

The capacitor plates have charges of opposite signs, they create voltage - a current appears through the load (Load). The current will flow in the opposite direction compared to the direction of the power source. Discharge will also occur in the opposite way: at first the charge will be lost quickly, then, with a drop in the voltage created by it, slower and slower. If for Q 0 designate the charge that was on the capacitor initially, then:

These values ​​on the graph look like this:

Again, after some time the system will come to a state of rest: all charge will be lost, the voltage will disappear, and the current flow will stop.

If you use the switch again, everything will start in a circle. So the capacitor does nothing other than break the circuit when the voltage is constant; and “works” when the voltage changes suddenly. This property determines when and how it is used in practice.

Application in practice

Among the most common in microelectronics are the following patterns:

Backup capacitor (bypass cap) - to reduce supply voltage ripples

Filter capacitor - to separate the constant and changing voltage components, to isolate the signal

Reserve capacitor

Many circuits are designed to provide constant, stable power. For example, 5 V. The power supply supplies it to them. But ideal systems do not exist, and in the event of a sudden change in the current consumption of the device, for example, when a component is turned on, the power source does not have time to “react” instantly and a short-term voltage drop occurs. In addition, in cases where the wire from the power source to the circuit is long enough, it begins to act as an antenna and also introduces unwanted noise into the voltage level.

Typically, the deviation from the ideal voltage does not exceed a thousandth of a volt, and this phenomenon is absolutely insignificant when it comes to powering, for example, LEDs or an electric motor. But in logic circuits, where the switching of logic zero and logic one occurs based on changes in small voltages, power supply noise can be mistaken for a signal, which will lead to incorrect switching, which, like a domino effect, will put the system in an unpredictable state.

To prevent such failures, a backup capacitor is placed directly in front of the circuit

At moments when the voltage is full, the capacitor is charged to saturation and becomes a reserve charge. As soon as the voltage level on the line drops, the backup capacitor acts as a fast battery, releasing previously accumulated charge to fill the gap until the situation returns to normal. Such assistance to the main power source occurs a huge number of times every second.

If we think from a different point of view: the capacitor extracts the alternating component from the direct voltage and, passing it through itself, takes it from the power line to the ground. This is why the backup capacitor is also called "bypass capacitor".

As a result, the smoothed voltage looks like this:

Typical capacitors that are used for these purposes are ceramic capacitors with a nominal value of 10 or 100 nF. Large electrolytic cells are poorly suited for this role, because they are slower and will not be able to quickly release their charge in these conditions, where the noise is of high frequency.

In one device, backup capacitors can be present in many places: in front of each circuit, which is an independent unit. For example, Arduino already has backup capacitors that ensure stable operation of the processor, but before powering the LCD screen connected to it, you must install your own.

Filter capacitor

A filter capacitor is used to remove the signal from the sensor, which transmits it in the form of a varying voltage. Examples of such sensors are a microphone or an active Wi-Fi antenna.

Let's look at the connection diagram for an electret microphone. The electret microphone is the most common and ubiquitous: this is exactly what is used in mobile phones, in computer accessories, public address systems.

The microphone requires power to operate. In a state of silence, its resistance is high and amounts to tens of kiloohms. When it is exposed to sound, the gate of the field-effect transistor built inside opens and the microphone loses internal resistance. The loss and restoration of resistance occurs many times every second and corresponds to the phase of the sound wave.

At the output, we are only interested in the voltage at those moments when there is sound. If there were no capacitor C, the output would always be additionally affected constant voltage nutrition. C blocks this constant component and allows only deviations that correspond to the sound.

The audible sound, which is of interest to us, is in the low-frequency range: 20 Hz - 20 kHz. In order to isolate the sound signal from the voltage, and not the high-frequency power noise, as C A slow electrolytic capacitor with a nominal value of 10 µF is used. If a fast capacitor, say 10 nF, were used, non-audio signals would pass through to the output.

Note that the output signal is supplied as a negative voltage. That is, when the output is connected to ground, current will flow from the ground to the output. Peak voltage values ​​in the case of a microphone are tens of millivolts. To reverse the voltage and increase its value, the output Vout usually connected to an operational amplifier.

Connection of capacitors

If compared with the connection of resistors, the calculation of the final value of the capacitors looks the other way around.

When connected in parallel, the total capacitance is summed up:

When connected in series, the final capacity is calculated using the formula:

If there are only two capacitors, then with a series connection:

In the particular case of two identical capacitors, the total capacitance of the series connection is equal to half the capacitance of each.

Limit characteristics

The documentation for each capacitor indicates the maximum permissible voltage. Exceeding it can lead to breakdown of the dielectric and explosion of the capacitor. For electrolytic capacitors Polarity must be observed. Otherwise, either the electrolyte will leak out or there will be an explosion again.

The circuit is designed to protect against surge charging current when an uncharged capacitor is connected to on-board network. Anyone who hasn't tried to connect an uncharged capacitor to a network without a limiting resistor - it's better not to... At a minimum, the contacts will burn out.

When the discharged capacitance is connected to the network, capacitance C1 is discharged, T1 (n-MOS switch with low channel resistance) is closed. Capacity C2 (the same farad) is charged through low-resistance R5. T2 opens almost instantly, I shunt C1 to ground and T1 gate. When the potential of the negative terminal of C2 drops below 1V (charging to Ubattery - 1V), T2 closes, C1 smoothly charges to approximately 9/10 Ubattery, opening T1. The time constant R2C1 is large enough so that the current jump T1 (recharging C2 by +1V to Uacb) does not exceed the permissible limit for T1.

In the future, the negative terminal C2 is constantly shorted to ground through T1, INDEPENDENT OF THE DIRECTION OF CURRENT T1 (both in the forward direction - from drain to source, and in the reverse direction). There is nothing wrong with “turning over” an OPEN MOS transistor. When choosing a sufficiently well-conducting transistor, the entire reverse current will flow through the channel, and the built-in reverse diode will not open, since the voltage drop across the channel is several times less than the 0.5-0.8 V required for opening. By the way, there is a whole class of MIS devices (the so-called FETKY) designed specifically for operation in the reverse direction (synchronous rectifiers), their built-in diode is shunted by an additional Schottky power diode.

Calculation: for transistor IRF1010 (Rds=0.012 Ohm), a voltage drop of 0.5 Ohm will be achieved only with a channel current of 40A (P=20W). For four such transistors in parallel and the same discharge current of 40A, each transistor will dissipate 0.012*(40/4)^2 = 1.2 W, i.e. they will not need radiators (especially since 1.2W will be dissipated only when the current consumption changes, but not constantly).

For dense installation (do you have a lot of space for an extra radiator?) - it is advisable to parallel small-sized (TO251, DIP4 package) transistors that do not provide radiators at all, based on the ratio of current (power) consumption of the amplifier - Rds - maximum power dissipation. Since Pds max is usually 1W (800mW for DIP4), the amount n transistors (with Rds each) for an amplifier with output power Pout must be at least n > 1/6 * Pout * sqrt(Rds) at 12V power supply (I omitted the dimensions in the formula). In fact, taking into account the short duration of current pulses, n can be safely halved compared to this formula .

The charging resistor R5 is selected based on a compromise between thermal power and charging time. At the specified 22 Ohms, the charging time is about 1 minute with a power dissipation of 7 W. Instead of R5, you can turn on a 12V light bulb, say, from a turn signal. Resistors R1, R3 are safety resistors (discharge capacitors when disconnected from the network).

To indicate switching on, we connect an additional inverter (reducing R2). Attention! The circuit is operational when using npn transistors T2, T3 with h21e > 200 (KT3102). Depending on the brightness of the LED, select R1 in the range of 200 Ohm - 1 kOhm.

And here is a version of the circuit in which the gate switch is controlled by the REMOTE signal (transistor AND). When REMOTE is not connected or turned off, the key transistor is guaranteed to be closed. LEDs D3-D4 indicate charging of C1, D5-D6 - open state of the key.

Accurate indication of the network voltage threshold is most easily provided by the TL431 (KR142EN19) IC in standard voltage comparator mode (with a corresponding divider in input circuit and current-limiting R in the cathode circuit).

Circuit losses largely depend on installation. It is necessary to ensure a minimum resistance (and wire thicknesses corresponding to the current) in the power circuit (terminal + / C2 / T1 / terminal -). In amateur practice, I think it is not advisable to make external terminals - it is better to immediately solder the short AWG8 wires that connect the circuit to the amplifier terminal block.

When designing amplifier power supplies Often problems arise that have nothing to do with the amplifier itself, or that are a consequence of the used element base. So in power supplies transistor amplifiers high power Often the problem arises of implementing a smooth switch-on of the power supply, that is, ensuring a slow charge of electrolytic capacitors in the smoothing filter, which can have a very significant capacity and, without taking appropriate measures, will simply damage the rectifier diodes at the moment of switching on.

In power supplies for tube amplifiers of any power, it is necessary to provide a feed delay high anode voltage before warming up the lamps, in order to avoid premature depletion of the cathode and, as a result, a significant reduction in the lamp life. Of course, when using a kenotron rectifier, this problem is solved by itself. But in the case of using a conventional bridge rectifier with an LC filter, without additional device can't get by.

Both of the above problems can be solved by a simple device that can be easily built into both a transistor and a tube amplifier.

Device diagram.

Schematic diagram of the device smooth start shown in the figure:

Click to enlarge

The alternating voltage on the secondary winding of transformer TP1 is rectified diode bridge Br1 and is stabilized by the integral stabilizer VR1. Resistor R1 ensures smooth charging of capacitor C3. When the voltage across it reaches a threshold value, transistor T1 will open, causing relay Rel1 to operate. Resistor R2 ensures the discharge of capacitor C3 when the device is turned off.

Inclusion options.

The Rel1 relay contact group is connected depending on the type of amplifier and the organization of the power supply.

For example, to ensure smooth charging of capacitors in the power supply transistor amplifier power, the presented device can be used to bypass the ballast resistor after charging the capacitors in order to eliminate power losses on it. A possible connection option is shown in the diagram:

The values ​​of the fuse and ballast resistor are not indicated, since they are selected based on the power of the amplifier and the capacitance of the smoothing filter capacitors.

IN tube amplifier the presented device will help to organize a delay in delivery high anode voltage before the lamps warm up, which can significantly extend their service life. A possible inclusion option is shown in the figure:

The delay circuit here is turned on simultaneously with the filament transformer. After the lamps have warmed up, relay Rel1 will turn on, as a result of which the mains voltage will be supplied to the anode transformer.

If your amplifier uses one transformer to power both the lamp filament circuits and the anode voltage, then the relay contact group should be moved to the secondary winding circuit anode voltage.

Elements of the switch-on delay circuit (soft start):

• Fuse: 220V 100mA,
• Transformer: any low-power with an output voltage of 12-14V,
• Diode bridge: any small-sized one with parameters 35V/1A and higher,
• Capacitors: C1 - 1000uF 35V, C2 - 100nF 63V, C3 - 100uF 25V,
• Resistors: R1 - 220 kOhm, R2 - 120 kOhm,
• Transistor: IRF510,
• Integral stabilizer: 7809, LM7809, L7809, MC7809 (7812),
• Relay: with operating winding voltage 9V (12V for 7812) and contact group appropriate power.

Due to the low current consumption, the stabilizer chip and field effect transistor can be mounted without radiators.

However, someone may have the idea to abandon the extra, albeit small-sized, transformer and power the delay circuit from the filament voltage. Considering that the standard value of the filament voltage is ~6.3V, you will have to replace the L7809 stabilizer with an L7805 and use a relay with an operating winding voltage of 5V. Such relays usually consume significant current, in which case the microcircuit and transistor will have to be equipped with small radiators.

When using a relay with a 12V winding (somehow more common), the integrated stabilizer chip should be replaced with a 7812 (L7812, LM7812, MC7812).

With the values ​​of resistor R1 and capacitor C3 indicated in the diagram delay time inclusions are of the order 20 seconds. To increase the time interval, it is necessary to increase the capacitance of capacitor C3.

The article was prepared based on materials from the magazine "Audio Express"

Free translation by the Editor-in-Chief of RadioGazeta.