Good day. When troubleshooting and repairing electronic equipment, the first step is always to discharge the capacitors present in the circuit. Otherwise, a careless repairman risks getting a boost of energy...

In the past, tube receivers and amplifiers could be found in every home. They used capacitors in their design large capacity that continued to hold a dangerous charge level for a long time even after they were disconnected from the network. After this came the era of televisions with cathode ray tubes. Thanks to technical progress Nowadays TVs are equipped with flat LED screens and you may get the impression that everything modern devices are switching to low voltage digital circuits, but what is the problem then?

In fact, the answer lies on the surface. Low-voltage devices are powered from relatively safe linear power supplies (hereinafter referred to as LPS). They are effective, lightweight, but it is in them that the main danger lies. In other words, “a wolf in sheep’s clothing.”

LIP rectifies the mains voltage, providing a constant voltage of about 330 V (for mains voltage 230 V and 170 V for a mains voltage of 120 V), after which it can be used to power one or another section/component of the circuit. It turns out to be an oil painting. Small, neat black boxes through which laptops, monitors and other devices are connected, in fact have quite high voltage levels, which can be deadly.

The filter capacitors in the power supply are charged at high DC voltage and retain their charge for a long period of time after the plug is removed from the socket. It is for this reason that there are stickers on the cases with safety warnings: “Do not open the box.”

The circuit presented in the article works with potentially dangerous voltage. Do not try to assemble it into hardware if you do not fully understand the principle of its operation and/or you do not have experience working with high voltage. In any case, you perform all actions at your own peril and risk.

Step 1: Working principle of unloading chain

On the Internet you can find quite a lot of articles/videos in which people discharge capacitors by simply short-circuiting their terminals, using a screwdriver for this purpose. The common people have a saying: “Neither the method nor the method is important, but the result is important,” so in our case, not only the result is important, but also how it was obtained. This is exactly what I mean - this method works. It completely discharges the capacitor. But is this right or wrong...? Of course not. This type of discharge can damage the capacitor, damage the screwdriver and cause irreparable harm to your health.

In order for the discharge to be carried out in the correct direction, it is necessary to remove the accumulated charge gradually. In principle, we do not need to wait until the discharge is complete; it is enough to wait a certain period of time for the voltage to become sufficiently low. Now we’ll figure out how long to wait.

A relatively safe residual charge level is considered to be 5% of the original. In order for the charge level to drop to the desired level, it is necessary for a time equal to 3RC to pass (C is the capacitance of the conductor; R is the value of the resistor resistance). Note "relatively safe" residual charge at 5%, it may vary. For example, for 10 kV, 5% - 500 V. For voltage 500V, 5% - 25V.

Unfortunately, we cannot simply connect a resistor (it is through the resistor that the discharge will occur) to the capacitor and wait. Why? Sitting with a stopwatch and monitoring time is not very convenient, is it?

It would be much more convenient to have a visual cue that notifies us that the discharge process is “over” and the voltage has dropped to a safe level.

On the Internet you can find a small simple diagram for discharge of capacitors with external indication. We will try to understand the principle of its operation, make changes by increasing the number of diodes and assemble the finished craft.

Use a chain of three standard 1N4007 diodes connected in series (D1, D2, D3) to set the correct fixing point where we can connect the LED with its current limiting resistor. 3 diodes connected in series will provide a voltage of about 1.6V, which is enough to turn on the LED. The LED will remain lit until the voltage at the D3 anode drops below the combined forward voltage of the string.

We will use a low current red LED (Kingbright WP710A10LID), which has a normal 1.7V forward voltage and turns on already at a forward current of 0.5 mA, which allows us to use only 3 diodes. According to the small current flowing through the LED, the value of the current limiting resistor will be relatively high 2700 ohms 1/4 W.

The capacitor discharge resistor is a 3 W, 2200 ohm resistor that is rated for a maximum input voltage of 400 V. This is sufficient to operate with standard blocks nutrition. Note that if you look at the datasheet for the 1N4007 diode, you will see a nominal forward voltage of 1V, so you would think that two diodes would be enough to turn on the LED. Not really, since the 1V forward voltage for the 1N4007 is designed to carry 1A forward current, a value we will never reach (hopefully) since that would mean we would be applying 2200V to the circuit's input. The forward current in our operating range is about 500-600 mV, so we need three diodes.

Always consider the conditions for which the parameters are specified in the datasheet. Are they used in your circuit? Maybe you shouldn't stop at the first page and continue looking at the characteristic curves!

Step 2: Correct unloading pattern

The above diagram is useful to illustrate the principle of operation, but it should not be repeated or used in practice because it is quite dangerous. The danger lies in the way the capacitor is connected (or rather, in the correct polarity) (the Vcc terminal must be positive relative to the GND terminal), otherwise the current will not flow through the diode chain D1-D2-D3! Therefore, if you accidentally connect the capacitor incorrectly, no current will flow and the full input voltage will flow to the LED1 pins as reverse voltage. If the applied reverse voltage is higher than a few volts, LED1 will burn out and remain off. This may lead you to believe that the capacitor is not charged when it still is...

To make the circuit safe, it is necessary to provide a symmetrical path for the current when the capacitor discharges when Vcc-GND is negative. This can be easily done by adding D4-D5-D6 and LED2 as shown in the diagram. When Vcc - GND is positive, current will only flow through D1-D2-D3 and LED1. When Vcc-GND is negative, current will only flow through D4-D5-D6 and LED2. This way, regardless of the polarity used, we will always know whether the capacitor is charged and when the voltage drops to a safe level.

Step 3: Housing

Now that we understand how the circuit works, it's time to think about the case. All this could be arranged either in the form of a probe or in the form of a small box that is convenient to keep at the workplace and connect to the capacitor using probes.

Let's make a small round box from two halves with plastic blanks. The fit was very tight, so no screws were needed.

The hole in the top of the case should be the size of the aluminum “button” that will help cool the discharge resistor. The "button" was machined from an aluminum rod and then milled on one end to hold the resistor in place and ensure good heat transfer. There is also a small hole that can be used to attach an optional external heatsink.

It is important to make a good fit between the “button” and the body. As you'll see in the next step, the button also helps hold all the components in place. Case dimensions 19 mm by 50 mm.

Step 4: Putting it all together

All that remains is to assemble, special attention should be paid to insulation. This kind of tension is no joke! A few points:

• Note the aluminum “button” that is the conductor to the outside of the box. The “button” must be isolated from the circuit. It is recommended to use a silicon-based sealant or epoxy resin to secure the components into the case after you have tested the assembly.
• The copper mesh around the resistor helps hold it securely in place in the slot and increases heat transfer to the “button.”
• Use special wires that are designed for a voltage of 600V. Don’t even think about grabbing the first wire you come across that is designed for an unknown voltage.

That's all. Successful and most importantly safe discharge!

Capacitors are widely used in household electrical appliances and electronic equipment. When connected to an energy source, they store electric charge, after which they can be used to power various devices and devices or simply as a charge source. Before disassembling or repairing household appliance or electronic device, you need to discharge its capacitor. This can often be done safely with a regular insulating screwdriver. However, in the case of larger capacitors, which are not typically used in electronic devices, and in household appliances, it is better to assemble a special discharge device and use it. First check if the capacitor is charged and, if necessary, choose an appropriate way to discharge it.

Attention: The information in this article is for informational purposes only.

Steps

Check if the capacitor is charged

Disconnect the capacitor from the power source. If the capacitor is still connected to the circuit, disconnect it from all power sources. Usually, this is enough to unplug the household appliance or disconnect the battery contacts in the car.

• If you're dealing with a car, locate the battery in the hood and use a wrench or socket wrench to loosen the nut that holds the cable to the negative (-) terminal. After this, remove the cable from the terminal to disconnect the battery.
• At home, it is usually enough to unplug the appliance from the socket, but if you cannot do this, find the distribution panel and turn off those fuses or circuit breakers, which control the supply of electricity to the room you need.

1. Select the maximum DC voltage range on your multimeter ( direct current). The maximum voltage depends on the brand of the multimeter. Turn the knob in the center of the multimeter so that it points to the highest voltage possible.

   • The maximum voltage value should be selected in order to obtain correct readings regardless of the amount of charge on the capacitor.

2. Connect the multimeter leads to the terminals of the capacitor. There should be two rods protruding from the capacitor cover. Simply touch the red probe of the multimeter to one terminal and the black probe to the second terminal of the capacitor. Press the test leads against the terminals until a reading appears on the multimeter display.

   • You may have to open the device or remove some parts from it to get to the capacitor. If you can't find or reach the capacitor, check your owner's manual.
   • Do not touch both probes of the multimeter to one terminal, as this will give incorrect readings.
   • It does not matter which probe is pressed to which terminal, since in any case the current value will be the same.

3. Pay attention to readings that exceed 10 volts. Depending on what you're dealing with, a multimeter can read anywhere from a few volts to hundreds of volts. Generally speaking, voltages above 10 volts are considered quite dangerous, as they can cause electric shock.

   • If the multimeter shows less than 10 volts, there is no need to discharge the capacitor.
   • If the multimeter reading is between 10 and 99 volts, discharge the capacitor with a screwdriver.
   • If the voltage across the capacitor is higher than 100 volts, it is safer to use a shock device rather than a screwdriver.

   Discharge the capacitor with a screwdriver

   1. Keep your hands away from the terminals. A charged capacitor is very dangerous and its terminals should never be touched. Handle the capacitor only by the sides.

      • If you touch two terminals or accidentally short them with a tool, you may receive a painful electric shock or burn.

   2. Select an insulating screwdriver. Typically, these screwdrivers have a rubber or plastic handle that creates an insulating barrier between your hands and the metal part of the screwdriver. If you don't have an insulating screwdriver, purchase one that clearly states on the packaging that it is non-conductive. Many screwdrivers even indicate what voltages they are designed for.

      • If you are not sure whether your screwdriver is insulating, it is better to purchase a new screwdriver.
      • An insulating screwdriver can be purchased at the store. household goods or goods for the car.
      • You can use either a flathead or Phillips screwdriver.

   3. Check the screwdriver handle for any signs of damage. Do not use a screwdriver with a rubber or plastic handle if it is broken, chipped, or cracked. Through such damage, current can reach your hands when you discharge the capacitor.

      • If your screwdriver handle is damaged, purchase a new insulating screwdriver.
      • It is not necessary to throw away a screwdriver with a damaged handle, just do not use it to discharge a capacitor or for other work on electrical parts and devices.

   4. Hold the capacitor with one hand at the base. When discharging a capacitor, you need to hold it firmly, so grab it by the cylindrical sides near the base with your non-dominant hand. Bend your fingers into a “C” shape and wrap them around the capacitor. Keep your fingers away from the top of the capacitor where the terminals are located.

      • Hold the capacitor in a way that is comfortable for you. There is no need to squeeze it too hard.
      • Hold the capacitor near the base to prevent sparks from getting on your fingers as it discharges.

   5. Place a screwdriver on both terminals. Hold the capacitor vertically, with the terminals facing the ceiling, and use your other hand to hold a screwdriver and press it against both terminals at the same time.

      • At the same time, you will hear the sound of an electrical discharge and see a spark.
      • Make sure that the screwdriver touches both terminals, otherwise the capacitor will not discharge.

   6. Touch the capacitor again to check that it is discharged. Before handling the capacitor loosely, remove the screwdriver and then touch both terminals again and check for spark. This will not cause any discharge if you have completely discharged the capacitor.

      • This step is a precautionary measure.
      • Once you are sure that the capacitor is discharged, it is safe to continue working with it.
      • If you wish, you can also check whether the capacitor is discharged using a multimeter.

   Make and use a discharge device

   1. Purchase copper wire with a diameter of 2 millimeters, a resistor with a nominal resistance of 20 kOhm and a dissipation voltage of 5 W and 2 alligator clips. The discharge device is just a resistor and some wire to connect it to the capacitor. All this can be purchased at a hardware or electrical supply store.

      • Using clamps, you can easily connect the wire to the capacitor terminals.
      • You will also need insulating tape or film and a soldering iron.

   2. Cut two pieces of wire about 15 centimeters long. The exact length is not important as long as you can connect the resistor to the capacitor. In most cases, 15 centimeters should be enough, although sometimes more may be needed.

      • The pieces of wire should be long enough to connect the resistor and capacitor terminals.
      • Cut the wire with a small margin to make your work easier.

   3. Remove the insulating coating from both ends of each piece of wire by about 0.5 centimeters. Take a wire stripper and strip the insulating coating from the wire, making sure not to damage the middle of the wire. If you don't have these pliers, use a knife or razor blade to score the covering and then pull the wire out with your fingers.

      • There should be clean metal on both ends of the wire.
      • Remove enough insulating coating so that you can solder the stripped ends to the terminals and clamps.

   4. Solder one end of each piece of wire to the resistor terminal. One wire sticks out from both ends of the resistor. Wrap the end of one piece of wire around the first terminal of the resistor and solder it. Then wrap one end of the second piece of wire around the second resistor terminal and solder it as well.

      • The result is a resistor with long wires at each end.
      • For now, leave the other ends of the wires free.

   5. Wrap the solder joints with insulating tape or shrink film. Simply wrap the solder joints with tape. This way you will fix them more tightly and isolate them from external contacts. If you are going to use this device again, put a plastic insulating tube on the end of the wire and slide it over the soldering area.

For widespread use in everyday life microwave ovens Microwaves occur and a large number of disruptions and breakdowns occur in their operation. Many people who have encountered this are interested in how to check the microwave capacitor on their own. Here you can find out the answer to this question.

Microwave capacitor

Device principle

A capacitor is a device that has the ability to store a certain charge of electricity. It consists of two metal plates installed in parallel, between which there is a dielectric. Increasing the plate area increases the accumulated charge in the device.

There are 2 types of capacitors: polar and non-polar. All polar devices are electrolytic. Their capacity is from 0.1 ÷ 100000 µF.

When checking a polar device, it is important to observe the polarity, when the positive terminal is connected to the positive terminal, and the negative terminal to the negative terminal.

It is polar capacitors that are high-voltage, while non-polar capacitors have low capacitance.

Microwave showing the location of the capacitor

The power supply circuit of the microwave magnetron includes a diode, transformer, and capacitor. Through them up to 2, 3 kilovolts goes to the cathode.

The capacitor is a large part weighing up to 100 grams. A diode lead is connected to it, the second one on the body. A cylinder is also located near the block. This particular cylinder is a high voltage fuse. It should not allow the magnetron to overheat.

Capacitor location

How to discharge a capacitor in a microwave

You can discharge it in the following ways:

Having disconnected from the power supply, the capacitor is discharged by carefully closing its terminals with a screwdriver. A good discharge indicates its good condition. This method of discharge is the most common, although some consider it dangerous and can cause harm and destroy the device.

Discharging a capacitor with screwdrivers

The high voltage capacitor has an integrated resistor. It works to discharge the part. The device is located under highest voltage(2 kV), and therefore there is a need to discharge it mainly to the housing. It is better to discharge parts with a capacity of more than 100 uF and a voltage of 63V through a resistor of 5-20 kiloOhms and 1 - 2 W. For this purpose, the ends of the resistor are combined with the terminals of the device for a certain number of seconds to remove the charge. This is necessary to prevent the occurrence of a strong spark. Therefore, you need to worry about personal safety.

How to check a high voltage microwave capacitor

The high-voltage capacitor is checked by connecting it together with a 15 W X 220 V lamp. Next, turn off the combined capacitor and the lamp from the socket. When the part is in working condition, the lamp will glow 2 times less than usual. If there is a malfunction, the light bulb shines brightly or does not glow at all.

Checking with a light bulb

The microwave capacitor has a capacity of 1.07 mF, 2200 V, so testing it with the support of a multimeter is quite simple:

1. It is necessary to connect the multimeter so as to measure resistance, namely the highest resistance. Make up to 2000k on your device.

2. Then you need to connect the uncharged device to the terminals of the multimeter without touching them. In operating condition, the readings will become 10 kOhm, going to infinity (on monitor 1).

3. Then you need to change the terminals.

4. When, when you connect it to the device, nothing changes on the multimeter monitor, this means that the device is broken; when there is zero, it means that there is a breakdown in it. If there is a constant resistance reading in the device, even a small value, it means there is a leak in the device. It needs to be changed.

Checking with a multimeter

Checking with a multimeter

These tests are done at low voltage. Often faulty devices do not show problems at low voltage. Therefore, for testing you need to use either a megohmmeter with a voltage equal to the voltage of the capacitor, or you will need an external high voltage source.

It is simply impossible to test it with a multimeter. It will only demonstrate that there is no cliff and short circuit. To do this, you need to connect it to the part in ohmmeter mode - in good condition it will demonstrate a low resistance, which will increase indefinitely over a certain number of seconds.

A faulty capacitor has an electrolyte leak. It is not difficult to make the determination of capacity with a special device. You need to connect it, set it to a higher value, and touch the terminals to the terminals. Check with regulations. When the differences are small (± 15%), the part is serviceable, but when there are none or are significantly lower than normal, it means that it has become unusable.

To test a part with an ohmmeter:

1. It is necessary to remove the outer cover and terminals.

2. Discharge it.

3. Switch the multimeter to test the resistance of 2000 kiloohms.

4. Examine the terminals for mechanical defects. Poor contact will negatively affect the quality of the measurement.

5. Connect the terminals to the ends of the device and observe the numerical measurements. When the numbers start changing like this: 1…10…102.1, it means that the part is in working condition. When the values ​​do not change or zero appears, the device is not working.

6. For another test, the device must be discharged and confirmed again.

Checking with an ohmmeter

Checking with an ohmmeter

It is also possible to test the capacitor to detect malfunctions with a tester. To do this, you need to set up measurements in kiloohms and watch the test. When the terminals touch, the resistance should drop to almost zero, and in a few seconds increase to the reading on display 1. This process will be slowest when you include measurements of tens and hundreds of kiloOhms.

Capacitor Test Job

The feed-through capacitors of the magnetron in the microwave are also tested by a tester. It is necessary to touch the terminal of the magnetron and its housing with the terminals of the device. When the display shows 1, the capacitors are working. When a resistance reading appears, it means that one of them is broken or leaking. They need to be replaced with new parts.

Checking the serviceability of feed-through capacitors

One of the reasons for malfunctions of the capacitor is the loss of part of the capacitance. It becomes different, not like on the body.

It is difficult to find this violation with the support of an ohmmeter. You need a sensor, which not every multimeter has. Breakage in a part does not happen very often due to mechanical stress. Violations due to breakdown and loss of capacity occur much more often.

The microwave does not produce microwave heating due to the fact that there is a leak in the part that is not detected by an ordinary ohmmeter. Therefore, it is necessary to purposefully test the part with the support of a megger using high voltage.

The test steps will be as follows:

1. You need to set the maximum measurement limit in ohmmeter mode.
2. Using the probes of the measuring device, we touch the pins of the part.
3. When “1” is reflected on the display, it shows us that the resistance is more than 2 megaohms, therefore, in working condition; in another version, the multimeter will show a lower value, which means that the part is inoperative and has become unusable.

Before you start repairing all electrical devices, you need to make sure that there is no power.

After checking the parts, measures must be taken to replace those that are not in working condition with new, more advanced ones.

Capacitor discharge to housing

Constant voltage and set the voltage on his crocodiles to 12 Volts. We also take a 12 Volt light bulb. Now we insert a capacitor between one probe of the power supply and the light bulb:

Nope, it doesn't burn.

But if you do it directly, it lights up:

This begs the conclusion: DC current does not flow through the capacitor!

To be honest, at the very initial moment of applying voltage, the current still flows for a fraction of a second. It all depends on the capacitance of the capacitor.

Capacitor in AC circuit

So, to find out whether AC current is flowing through the capacitor, we need an alternator. I think this frequency generator will do just fine:

Since my Chinese generator is very weak, instead of a light bulb load we will use a simple 100 Ohm one. Let’s also take a capacitor with a capacity of 1 microfarad:

We solder something like this and send a signal from the frequency generator:

Then he gets down to business. What is an oscilloscope and what is used with it, read here. We will use two channels at once. Two signals will be displayed on one screen at once. Here on the screen you can already see interference from the 220 Volt network. Do not pay attention.

We will apply alternating voltage and watch the signals, as professional electronics engineers say, at the input and output. Simultaneously.

It will all look something like this:

So, if our frequency is zero, then this means constant current. As we have already seen, the capacitor does not allow direct current to pass through. This seems to have been sorted out. But what happens if you apply a sinusoid with a frequency of 100 Hertz?

On the oscilloscope display I displayed parameters such as signal frequency and amplitude: F is the frequency Ma – amplitude (these parameters are marked with a white arrow). The first channel is marked in red, and the second channel in yellow, for ease of perception.

The red sine wave shows the signal that the Chinese frequency generator gives us. The yellow sine wave is what we already get at the load. In our case, the load is a resistor. Well, that's all.

As you can see in the oscillogram above, I supply a sinusoidal signal from the generator with a frequency of 100 Hertz and an amplitude of 2 Volts. On the resistor we already see a signal with the same frequency (yellow signal), but its amplitude is some 136 millivolts. Moreover, the signal turned out to be somewhat “shaggy”. This is due to the so-called ““. Noise is a signal with small amplitude and random voltage changes. It can be caused by the radio elements themselves, or it can also be interference that is caught from the surrounding space. For example, a resistor “makes noise” very well. This means that the “shaggyness” of the signal is the sum of a sinusoid and noise.

The amplitude of the yellow signal has become smaller, and even the graph of the yellow signal shifts to the left, that is, it is ahead of the red signal, or in scientific language, it appears phase shift. It is the phase that is ahead, not the signal itself. If the signal itself was ahead, then we would have the signal on the resistor appear in time earlier than the signal applied to it through the capacitor. The result would be some kind of time travel :-), which, of course, is impossible.

Phase shift- This difference between the initial phases of two measured quantities. In this case, tension. In order to measure the phase shift, there must be a condition that these signals same frequency. The amplitude can be any. The figure below shows this very phase shift or, as it is also called, phase difference:

Let's increase the frequency on the generator to 500 Hertz

The resistor has already received 560 millivolts. The phase shift decreases.

We increase the frequency to 1 KiloHertz

At the output we already have 1 Volt.

Set the frequency to 5 Kilohertz

The amplitude is 1.84 Volts and the phase shift is clearly smaller

Increase to 10 Kilohertz

The amplitude is almost the same as at the input. The phase shift is less noticeable.

We set 100 Kilohertz:

There is almost no phase shift. The amplitude is almost the same as at the input, that is, 2 Volts.

From here we draw profound conclusions:

The higher the frequency, the less resistance the capacitor has alternating current. The phase shift decreases with increasing frequency to almost zero. On indefinitely low frequencies its value is 90 degrees orπ/2 .

If you plot a slice of the graph, you will get something like this:

I plotted voltage vertically and frequency horizontally.

So, we have learned that the resistance of a capacitor depends on frequency. But does it only depend on frequency? Let's take a capacitor with a capacity of 0.1 microfarad, that is, a nominal value 10 times less than the previous one, and run it again at the same frequencies.

Let's look and analyze the values:

Carefully compare the amplitude values ​​of the yellow signal at the same frequency, but with different capacitor values. For example, at a frequency of 100 Hertz and a capacitor value of 1 μF, the amplitude of the yellow signal was 136 millivolts, and at the same frequency, the amplitude of the yellow signal, but with a capacitor of 0.1 μF, was already 101 millivolts (in reality, even less due to interference ). At a frequency of 500 Hertz - 560 millivolts and 106 millivolts, respectively, at a frequency of 1 Kilohertz - 1 Volt and 136 millivolts, and so on.

From here the conclusion suggests itself: As the value of a capacitor decreases, its resistance increases.

Using physical and mathematical transformations, physicists and mathematicians have derived a formula for calculating the resistance of a capacitor. Please love and respect:

Where, X C is the resistance of the capacitor, Ohm

P - constant and equals approximately 3.14

F– frequency, measured in Hertz

WITH– capacitance, measured in Farads

So, put the frequency in this formula at zero Hertz. A frequency of zero Hertz is direct current. What will happen? 1/0=infinity or very high resistance. In short, a broken circuit.

Conclusion

Looking ahead, I can say that in this experiment we obtained (high-pass filter). Using a simple capacitor and resistor, and applying such a filter to the speaker somewhere in the audio equipment, we will only hear squeaky high tones in the speaker. But the bass frequency will be dampened by such a filter. The dependence of capacitor resistance on frequency is very widely used in radio electronics, especially in various filters where it is necessary to suppress one frequency and pass another.