Good day. When troubleshooting and repairing electronic equipment, the first thing to do is always to discharge the capacitors available in the circuit. Otherwise, a negligent repairman risks getting a boost of vigor ...
In the past, tube receivers and amplifiers could be found in every home. In their design, they used large capacitors that continued to hold a dangerous level of charge for a long time even after they were disconnected from the network. After that came the era of cathode ray tube televisions. Thanks to technological advances, TVs are now equipped with flat LED screens and it may seem that all 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 by relatively safe linear power supplies (hereinafter - LIP). They are effective, light, but it is in them that the main danger lies. In other words, "a wolf in sheep's clothing."
The LIP rectifies the mains voltage, providing a constant voltage of about 330 V (for the mains voltage of 230 V and 170 V for the mains voltage of 120 V), after which it can be used to power one section or another of the circuit component. It turns out an oil painting. Small, neat black boxes through which laptops, monitors and other devices are connected, in fact, have large amounts of voltage, which can be deadly.
The filter capacitors in the power source are charged with a high constant voltage and retain their charge for a long period of time after the plug is removed from the outlet. It is for this reason that stickers with safety warnings are glued to the cases: “Do not open the box”.
The circuit shown in the article works with potentially dangerous voltage. Do not try to assemble it in iron if you do not fully understand the principle of its operation and / or you do not have experience with high voltage. In any case, all the actions you perform at your own peril and risk.
Step 1: How the discharge chain works
On the Internet, you can find quite a few articles / videos in which people discharge capacitors, simply by simply shorting their terminals, using a screwdriver for this purpose. In common people there is a saying “Neither method nor method is important, the result is important”, so in our case not only the result is important, but also how it was obtained. I actually what it is, this method works. It completely discharges the capacitor. But is it 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 right direction, it is necessary to divert 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. And how long to wait, we will figure it out now.
A relatively safe residual charge level is considered 5% of the original. In order for the charge level to drop to the desired mark, it is necessary that a time equal to 3RC passes (C is the capacity of the conder, R is the value of the resistance of the resistor). Pay attention to the “relatively safe” residual charge of 5%, it can be different. For example, for 10 kV, 5% - 500 V. For a voltage of 500V, 5% - 25V.
Unfortunately, we can’t just connect the resistor (it is through the resistor that the discharge will occur) to the capacitor and wait. Why? Sitting with a stopwatch and controlling the time is not very convenient, is it?
It would be much more convenient to have a visual clue that would notify us that the discharge process is “finished” and that the voltage has dropped to a safe level.
On the Internet you can find a small, simple circuit for discharging capacitors with an external indication. We will try to figure out the principle of its work, make changes by increasing the number of diodes and collect the finished craft.
Use a chain of three standard 1N4007 diodes connected in series (D1, D2, D3) to set the correct fixation point, where we can connect the LED with its current-limiting resistor. 3 series-connected diodes will provide a voltage of about 1.6V, which is enough to turn on the LED. The LED will light until the voltage at the anode D3 drops below the combined forward voltage of the circuit.
We will use a red low-current LED (Kingbright WP710A10LID), which has the usual 1.7V direct voltage and turns on already with a direct current of 0.5 mA, which allows us to use only 3 diodes. In accordance with the small current flowing through the LED, the value of the current-limiting resistor will be relatively high 2700 Ohms 1/4 watts.
The capacitor discharge resistor is a 3 W resistor and a resistance of 2200 Ohms, which is designed for a maximum input voltage of 400 V. This is enough to work with standard power supplies. Please note that if you look at the datasheet for the 1N4007 diode, you will see a nominal forward voltage of 1 V, so you might think that two diodes will be enough to turn on the LED. Not quite so, since the direct voltage of 1 V for 1N4007 is designed for a direct current of 1 A, a value that we will never reach (I hope), since this would mean that we applied a voltage of 2200 V to the input of the circuit. The direct current in our operating range is about 500-600 mV, so we need three diodes.
Always consider the conditions for which the parameters in the datasheet are indicated. Are they used in your circuit? Maybe you should not stop on the first page and you should continue to view characteristic curves!
Step 2: Proper Unloading Scheme
The above diagram is useful for illustrating the principle of operation, but it should not be repeated and used in practice, because it is quite dangerous. The danger lies in the method of connecting the capacitor (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 circuit D1-D2-D3! Therefore, if you accidentally connect the capacitor incorrectly, the current will not flow and the full input voltage will go to the terminals of LED1 as the reverse voltage. If the applied reverse voltage is higher than a few volts, LED1 will burn and remain off. This may make you believe that the capacitor is not charged, although it is still ...
To make the circuit safe, it is necessary to provide a symmetrical path for the current when discharging the capacitor 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. Thus, regardless of the polarity used, we will always know if the capacitor is charged and when the voltage drops to a safe level.
Step 3: Housing
Now that we’ve figured out 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, which is convenient to hold in the workplace and connect to the capacitor using probes.
Let's make a small round box of two halves with plastic discs. The landing was very tight, so the screws were not needed.
The hole in the upper part of the case should correspond to the size of the aluminum “button”, which will help in cooling the discharge resistor. The “button” was machined from an aluminum rod, and then milled from 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 additional external radiator.
It is important to make a good fit between the “button” and the body. As you will see in the next step, the button also helps to keep all components in place. Case dimensions 19 mm by 50 mm.
Step 4: Putting It All Together
It remains to assemble, special attention should be paid to insulation. They don’t joke with such tension! A few points:
- Pay attention to the aluminum “button”, which is a conductor to the outside of the box. The “button” must be isolated from the circuit. It is recommended that you use silicon-based sealant or epoxy to secure the components in the enclosure after you have tested the assembly.
- The copper mesh around the resistor helps to hold it securely in place in the groove and increase the heat transfer by the “button”.
- Use special wires that are designed for a voltage of 600V. Do not try to grab the first wire you get that is designed for an unknown voltage.
That's all. Successful and most importantly safe discharge!
Methods for discharging main flash capacitors
A capacitor to discharge a capacitor costs much less than an IGBT transistor killed by an incandescent lamp.
And this is not someone's whim, this is the recommendation of flash manufacturers.
Categorically, you should not discharge the capacitor with metal objects, such as a screwdriver, prince, etc. .. At the forums of photographic equipment repairmen you can still find messages that such a barbaric method led to damage to the sensitive elements of the electrical circuit, or even the whole camera or flash.
Also, when working with an open flash, you should occasionally check the voltage on the main capacitor and discharge it as necessary. The fact is that even on a discharged capacitor during storage, the voltage can be restored to dangerous limits.
This is a requirement for safe operation. Accidental contact with live parts may result in electric shock. Sometimes even relatively low, at a level of 20-50 V, the voltage causes reflex contractions of the muscles and, as a result, fright and shock or bruises. Do not neglect safety rules.
Colleagues, please describe here those tools that you use to discharge the main capacitors of flash units and cameras and also to control voltage.