Checking the voltage in the circuit is a procedure necessary when performing various types of work related to electricity. Some amateur electricians, and sometimes professionals, use a homemade “control” for this - a socket with a light bulb to which wires are connected. Although this method is prohibited by the “Rules for the Safe Operation of Consumer Electrical Installations,” it is quite effective when used correctly. But still, for these purposes it is better to use LED identifiers - probes. You can buy them in a store, or you can make them yourself. In this article we will tell you what these devices are needed for, what principle they work on, and how to make an LED voltage indicator with your own hands.
Purpose of elements and principle of operation of the circuit
Many readers have LED light switches installed in their homes. The LED backlight circuit looks like this:
- A chain consisting of a quenching resistor, an LED and a simple silicon diode is switched on parallel to the switch contact.
- When the switch is open, electric current flows through a quenching (current-limiting) resistor, back-to-back LEDs and an incandescent lamp.
- During one of the half-waves, when a positive voltage is applied to the anode of the LED, the light-emitting diode glows. This not only provides illumination of the switch, but also provides LED voltage indication.
If we remove the switch, light bulb and wires from the circuit, we are left with a chain consisting of a resistor and two diodes. This chain is the simplest indicator (pointer) of 220 V alternating current.
Let us dwell in more detail on the purpose of the circuit elements. We indicated above that the operating current of the signal LED is about 10-15 mA. It is clear that when a light-emitting diode is directly connected to a 220 V network, a current will flow through it many times greater than the maximum permissible value. In order to limit the current of the LED, a quenching resistor is connected in series with it. You can calculate the resistor value using the formula:
R = (U max – U led) / I led
In it:
- U max – maximum measured voltage;
- U led – voltage drop across the LED;
- I led – operating current of the light-emitting diode.
Having performed the simplest calculation, for a 240 V network we get the value of resistor R1 equal to 15-18 kOhm. For a 380 V network, you need to use a resistor with a resistance of 27 kOhm.
The silicon diode performs the function of overvoltage protection. If it is missing, with a negative half-wave U, the locked LED will drop 220 V or 380 V. Most light-emitting diodes are not designed for such reverse voltage. Because of this, breakdown of the pn junction of the LED may occur. When a silicon diode is connected back-to-back, during the negative half-wave it will be open and U on the LED will not exceed 0.7 V. The LED will be reliably protected from high reverse voltage.
Based on the considered circuit, you can make a voltage indicator of 220/380 V. It is enough to supplement the radio elements with two probes and place them in a suitable housing. To make the indicator body, a large marker or thick felt-tip pen is suitable. You can place radio components on a homemade printed circuit board or make connections using a hinged method.
A hole is made in the marker into which an LED is inserted. A metal probe is attached to one end of the body. A wire is passed through the second end of the housing, going to the second probe or an insulated alligator clip.
Despite the simplicity of the design, the device will allow you to check the presence of voltage at the output of a circuit breaker or in a socket, and find a blown fuse in the distribution board. Note that the above indicator diagram is also used in industrial products.
Manufacturing Guide
You can assemble a circuit using a light or sound indicator for your car at home. If you have experience in electrical engineering, this task will not take much time. But even if you have never done such work before, there is nothing difficult about it. The main thing is to correctly connect all the elements of the circuit and connect them to the on-board network.
Let's look at an example of assembling an indicator to determine the voltage of a car battery. Instead of the ten individual diode elements that are marked in the diagram, we will use a solid indicator since it does not take up much space.
What will you need?
What to prepare before starting the process:
- the circuit itself, in our example we use LM 3914;
- diode strip designed for 10 segments, you can use Kingbright DC-763HWA;
- power supply adjustable from 10 to 15 volts;
- resistors.
Stages
Let's briefly look at the instructions for making the device:
- First of all, the printed circuit board must be cleaned of dust. Make sure that this component is clean and there should be no signs of burning, otherwise this may lead to the device not working in the future.
- On a ready-to-use board, all components must be assembled in accordance with the diagram shown. To solder elements, use a soldering iron with consumables. Without exception, all components of the device must be fixed as firmly as possible. If the fastening of the resistors and diode strip is weak, then the structure may become loose over time as a result of vibrations, and accordingly, the performance of the device will be impaired.
- For greater convenience and to ensure a more compact assembly of the device, the right resistor element must be cut off.
- After all components on the board are installed, the system is configured. To do this, you need to apply a voltage of 10.5 volts to the board and adjust the right trimmer. You need to ensure that the first diode strip on the device turns on.
- Then you need to apply 15-volt voltage to the device and adjust the device so that the last strip on the board starts to light. Remember that not all strips should light up, but only one of them.
- Next, all you have to do is install the manufactured device in any place convenient for you and connect it to the on-board network. In particular, if you were making a device to determine the battery charge, then it would be better to connect it to a section of the circuit that is connected directly to the battery.
- To make sure that the device is functioning correctly, you need to use a multimeter to check the charge of the battery itself. Then compare these numbers with the division on the scale of the 10-segment indicator. If the battery charge is full, then the last bar should light up, if it’s medium, then the diode light in the middle of the indicator, and if the charge is minimal, then the first light.
Board for assembling the device
Price issue
If you want to install a voltage indicator in your car, you can buy a ready-made digital device. The cost of a more or less high-quality device will start from 250 rubles. On the market you can find options whose price is 1,500 thousand rubles, but such digital devices are additionally equipped with various regulators, for example, temperature in the cabin.
DC Voltage Check
Often there is a need to ring the low-voltage circuit of household appliances, or check the integrity of a connection, for example, a wire from headphones.
As a current limiter, you can use a low-power incandescent lamp or a 50-100 Ohm resistor. Depending on the polarity of the connection, the corresponding diode lights up. This option is suitable for circuits up to 12V. For higher voltages, you will need to increase the limiting resistor.
Indicator circuit
The operation of the device is based on the initial turn-on voltage of the LED. Any LED is a semiconductor device that has a voltage limit point, only exceeding which it begins to work (shine). Unlike an incandescent lamp, which has almost linear current-voltage characteristics, the LED is very close to the characteristics of a zener diode, with a sharp slope of the current as the voltage increases. If you connect LEDs in a circuit in series with resistors, then each LED will start to turn on only after the voltage exceeds the sum of the LEDs in the circuit for each section of the circuit separately. The voltage threshold for opening or starting to light an LED can range from 1.8 V to 2.6 V. It all depends on the specific brand. As a result, each LED lights up only after the previous one lights up.
AC and DC voltage indicator up to 600 V
The next option is a slightly more complex system, due to the presence in the circuit, in addition to the elements already known to us, of two transistors and a capacitance. But the versatility of this indicator will pleasantly surprise you. It can safely check the presence of voltage from 5 to 600 V, both direct and alternating.
The main element of the voltage indicator circuit is a field-effect transistor (VT2). The threshold voltage value that will allow the indicator to operate is fixed by the gate-source potential difference, and the maximum possible voltage determines the drop at the drain-source. It functions as a current stabilizer. Feedback is provided through a bipolar transistor (VT1) to maintain the set value.
The operating principle of the LED indicator is as follows. When a potential difference is applied to the input, a current will arise in the circuit, the value of which is determined by the resistance (R2) and the voltage of the base-emitter junction of the bipolar transistor (VT1). In order for a weak LED to light up, a stabilization current of 100 μA is sufficient. To do this, the resistance (R2) should be 500-600 Ohms, if the base-emitter voltage is approximately 0.5 V. A capacitor (C) is required to be non-polar, with a capacity of 0.1 μF, it serves as protection for the LED from current surges.
We select a resistor (R1) with a value of 1 MOhm; it acts as a load for the bipolar transistor (VT1). The functions of the diode (VD) in the case of DC voltage indication are pole testing and protection. And to check the alternating voltage, it plays the role of a rectifier, cutting off the negative half-wave. Its reverse voltage must be at least 600 V. As for the LED (HL), choose a super-bright one so that its glow at minimal currents is noticeable.
A brief excursion into theory
Today we will not delve too deeply into the theoretical foundations of electrical engineering, but will try to briefly explain the essence of the problem. For those who wish to become more familiar with this issue, we recommend reading a series of articles on the physics of alternating electric current on our website.
Standard switch installation.
Let us take as an example a fragment of a household electrical network, where the connection of an electric lamp and a plug connector (socket) is organized.
Fragment of a household network with connection of a lamp and socket
Designations:
As is known, in single-phase circuits the electric current (Ì) flows from phase to zero. In the above figure, the switch SW is in the open position, therefore the lamp will be de-energized, which can be verified by measuring the voltage U2. In this case, the operating potential U1 corresponding to the phase voltage will remain on the plug connector and the part of the network up to the switch (marked in red). This is the normal operating mode for this circuit, where the switch opens the phase wire.
Please note that if you take measurements with a voltage indicator, it will show the presence of a phase on one of the contacts of the plug connector and its absence on both contacts of the lamp socket
Setting the switch to zero
Now let's see what happens if you swap the phase and zero, or, what is more common in practice, set the switch to zero, and not the phase wire.
Switch installed incorrectly
Outwardly, such a change will not manifest itself in any way. The lamp will turn on and off in the same way as in the previous example, and a potential difference will be present at the contacts of the socket. But, certain nuances arise, which manifest themselves in the presence of voltage at the contacts of the socket and part of the zero line between the lamp and the switch. This is easy to verify using an electrical probe.
This connection option carries a potential risk of electric shock when attempting to replace or repair the lamp.
It is typical that measuring the presence of voltage between the contacts of a lighting fixture socket with a voltmeter will not bring results. The device will show “0”, since the contacts will have the same phase potential level.
Summarizing the results of the chapter, we can state that incorrect connection of switch contacts in the distribution box does not have a significant effect on the operation of electrical appliances connected to the outlet. In addition, we found out about the need for the combined use of measuring instruments (voltmeter and probe).
Indicator for microcircuits - logic probe
Having learned how to create a simple electrician's probe with your own hands, you can also make a simple logic probe based on LED, which will help you find faults in digital devices.
Logic probes have been around since the early days of computing. With their help, specialists analyzed the logical levels at the inputs and outputs of digital microcircuits. The high level (voltage) at the output of the logic element is assigned the value of logical “one”, and the low level is assigned the value of logical “zero”. By comparing the levels at the input and output of a digital microcircuit, you can judge its serviceability.
Types of car battery charge indicators
Not many car enthusiasts know the digital values of the battery voltage sufficient to reliably start the car engine. This problem is especially relevant for beginners. On-board computers of modern cars provide the consumer with a large amount of necessary information, including the open circuit voltage of the battery. Analog voltmeters on older cars have a scale that gives information about the battery voltage.
Therefore, there is a need to have indicators that can assess the readiness of the battery to start the engine and communicate the results to the driver in the form of a visual message. The following types of indicators can be distinguished:
- built-in, showing the state of the battery, located directly on the battery case;
- Battery charging indicators produced by third-party manufacturers, which have a scale of permissible and prohibited battery voltage values for starting the start, the charge level expressed as a percentage of its full value.
Built-in batteries have medium and high price scales, mostly of a maintenance-free type. To use indicators from third-party manufacturers, it is necessary to carry out additional work to install them inside the car (in a visible place) and connect them to the battery bus of the car electrical wiring.
How to make an electrician's tester with your own hands?
Some thrifty hobbyists can find many useful things in their “arsenal,” including an earphone (capsule) for the TK-67-NT telephone.
Another similar device, equipped with a metal membrane, inside which there is a pair of series-connected coils, is also suitable.
Based on such a part, a simple sound probe can be assembled.
First of all, you need to disassemble the telephone capsule and disconnect the coils from each other. This is necessary in order to free their conclusions. The elements are placed in the earphone under the sound membrane, near the coils. After assembling the electrical circuit, we will receive a completely working identifier with sound indication, which can be used, for example, to check the tracks of printed circuits for mutual bridging.
The base of such a probe is an electric generator with an inductive opposite relationship, the main parts of which are a telephone and a low-power transistor (preferably germanium). If you do not have such a transistor, then you can use another one with NPN conductivity, but in this case the polarity of the power supply should be changed. If you cannot turn on the generator, the terminals of one (any) coil must be swapped with each other.
You can increase the sound volume by choosing the frequency of the electric generator so that it is as close as possible to the resonant frequency of the earphone. To do this, the membrane and the core must be placed at an appropriate distance, changing the interval between them until the desired result is obtained. Now you know how to make a voltage indicator based on a telephone earphone.
The manufacture and use of a simple voltage probe is clearly shown in the video:
This concept includes a number of devices and instruments of the following names:
- so-called phase indicators or, more simply put, indicator screwdrivers;
- bipolar voltage indicators;
- universal probes;
- control devices (type “Arkashka”).
It should also be noted that most of the devices listed in the list do not, as a rule, take up much space in the repair kit. Some of their samples are generally carried directly in the pockets of work equipment, where they are, figuratively speaking, “always at hand.” The last statement especially applies to such well-known devices as an indicator screwdriver and a homemade control device. It should be especially emphasized that all these devices are quite reliable and easy to use and are a good replacement (supplement) for a relatively large and not always easy-to-use tester. With their help, you can always figure out the electrical wiring in the house.
Voltage indicator on two-color LED
Another popular indication scheme is one that uses a two-color LED to display the state of charge of the battery or to signal when a lamp is turned on or off in another room. This can be very convenient, for example, if the light switch in the basement is located before the stairs leading down (by the way, do not forget to read an interesting article on how to illuminate the stairs with LED strip).
Before you go down there, you turn on the light and the indicator lights up red, when off you see a green glow on the key. In this case, you don’t have to go into a dark room and feel for the switch there. When you leave the basement, you know by the color of the LED whether the light in the basement is on or not. At the same time, you monitor the health of the light bulb, because if it burns out, the red LED will not glow. Here is a diagram of a voltage indicator on a two-color LED.
The ideal indicator for a trader
The question arises, if the EMA is already working at its limit and shows the most relevant price changes, then why do traders not use the EMA all the time? The fact is that for EMA it does not matter at all how long the trend being studied is. Any more or less significant price surge radically changes the direction of the forecast, which can be very misleading and lead to losses.
It turns out that traders do not need to have indicators with low lag - we need some balance between short reaction time and filtering out false emissions. The most famous of these developments is the adaptive moving average - AMA. There are also quite a lot of variations.
Option for car
A simple circuit for indicating the vehicle's on-board voltage and battery charge. The zener diode limits the battery current to 5V to power the logic chip.
Variable resistors allow you to set the voltage level to trigger the LEDs. It is better to carry out the setup from a network stabilized power source.
Wire colors and designations
In order to find the phase, neutral and ground wires of electrical wiring without instruments, they, in accordance with the rules of the Electrical Installation Code, are covered with insulation of different colors.
The photo shows the color marking of the electrical cable for single-phase electrical wiring with an alternating current voltage of 220 V.
This photo shows the color coding of an electrical cable for 380V AC three-phase wiring.
According to the presented diagrams, wires began to be marked in Russia in 2011. In the USSR, the color marking was different, which must be taken into account when searching for phase and zero when connecting installation electrical products to old electrical wiring.
What is the difference between N and PE wires in electrical wiring
According to modern requirements of the PUE, in addition to the phase and neutral wires, a yellow-green ground wire must also be supplied to the apartment.
The neutral N and grounding PE wires are connected to one grounded bus of the panel in the entrance of the house. But they perform different functions. The neutral wire is intended for electrical wiring, and the grounding wire is intended to protect people from electric shock and is connected to the housings of electrical appliances through the third contact of the electrical plug. If an insulation breakdown occurs and a phase gets into the body of an electrical device, then all the current will flow through the grounding wire, the fuse links will burn out or the circuit breaker will trip, and no person will be harmed.
If the electrical wiring is laid indoors with a cable without color marking, it is impossible to determine where the neutral conductor is and where the grounding conductor is, since the resistance between the wires is hundredths of an ohm. The only clue can be the fact that the neutral wire is inserted into the electric meter, and the grounding wire passes by the meter.
Attention! Touching exposed parts of a circuit connected to an electrical outlet may result in electric shock.
Life-threatening voltage detector, manufacturing
The device is made with three transistors, without a board for surface mounting.
Please note that the circuit uses transistors of different structures. There are no special requirements for them; almost any will do. An LED and a buzzer are used as signaling elements. The role of the antenna is played by a piece of wire 5 cm long.
The detector is powered by two little finger elements.
The body is a transparent plastic tube.
After assembly, if all elements of the circuit are in working order, the detector starts working immediately and does not need adjustment.
How the tool is checked
Like any device with measuring qualities, our alarm screwdriver sometimes needs to be checked for its functionality.
Working indicator screwdriver
To ensure that our signal instrument is always ready to measure voltage, pay attention to the following nuances:
The body of the instrument must always remain intact and clean. There should be no cracks on it;
- Before starting work, you should always make sure that the tool is working properly. You can check it simply by touching the conductor in which there is voltage;
- If the batteries are low, you need to replace them. To do this, carefully unscrew the fixing cap and replace the old batteries with new ones. The main thing here is not to confuse their polarity. Otherwise the screwdriver will not function.
Do-it-yourself repair of the alarm screw, if damage is detected, will be irrational - it is cheaper to buy a new tool.
Nuances in the operation of the voltage indicator
A self-assembled LED indicator, just like industrial devices of this type, can be used to check the presence of voltage. It is not a measuring device, but only indicates the presence or absence of voltage. Having gained some experience with the pointer, you can determine the voltage between two conductors by the brightness of the light-emitting diode. However, for accurate measurements you need to use pointer or digital voltmeters.
Unlike indicators with gas-discharge lamps, the LED indicator cannot be used to find the “phase” by touching one of the probes with your finger. The device has low internal resistance, and this method of searching for a phase conductor can result in electric shock.
Working with a 220V network
Let's consider the simplest option - phase checking.
This circuit is a current indicator light found on some screwdrivers. Such a device does not even require external power, since the potential difference between the phase wire and the air or hand is sufficient for the diode to glow.
To display the mains voltage, for example, to check the presence of current in the socket connector, the circuit is even simpler.
The simplest current indicator on 220V LEDs is assembled using capacitance to limit the current of the LED and a diode to protect against reverse half-wave.
Why is control dangerous?
An incandescent lamp is an electrical product that requires careful handling.
Therefore, it is installed in lamps that are permanently attached to building structures or have a stable base, for example, in a tabletop version. The control is portable and easily damaged. She can:
- explode from increased tension;
- slip out of the hands of an electrician, fall and break (often while connected);
- make a short circuit;
- cause a person to be exposed to electric current.
Experienced electricians have tried to take these risks into account and adopted various technical tricks to increase their own safety. But they are all prohibited by modern rules.
Possibility of explosion
Technical background
When connected to voltage, the resistance of the filament changes according to a nonlinear law. At the initial moment, transient processes occur briefly, and then the nominal operating mode is established.
This is explained by the fact that tungsten in a cold and heated state has different electrical resistance. Let's consider this using the example of a popular 60-watt light bulb. It consumes a current of 0.27 amperes, and its filament has a resistance of 815 ohms.
If you measure its resistance in a cold state, the ohmmeter will show about 59 Ohms. The difference is almost 14 times. This property of tungsten eliminates the need for complex ballast control equipment for the lamp, simplifies the design, but requires consideration during operation.
When the voltage exceeds a linear value of 380, the thread most often burns out, but in a worn-out structure it can explode. There are many examples of such damage. They arise where apartment owners save on protection such as voltage control relays.
Human factor
Electricians who use controls at enterprises worked not only on 220-volt networks, but also on 36-volt networks, which are used to illuminate hazardous areas.
The design of the socket and the shape of the bulbs are interchangeable: when working in control, the bulbs were simply twisted to the appropriate voltage. If, when changing workplaces in a 220-volt network, they forgot about this, then the flask exploded. And for some reason small fragments fly straight into your eyes.
Mechanical damage
The glass of the flask is fragile and breaks easily, especially in a portable design. If a stationary lamp has a lamp screwed in and secured, then the control is usually held in the hands. She might slip out.
And a person does not always follow safety rules when working with a tool; he can slip and drop it from his hands or fall with it and cut himself on the glass.
A particular danger is falling with a lamp to which voltage is applied. The filament will break, and the electrodes for its attachment may short-circuit through a random conductive object or the human body. A short circuit immediately occurs with all aggravating circumstances.
Possibility of touching live parts
To create an electrical contact when connecting a control, they usually leave the bare end of the metal on the wire or solder a simple tip with an alligator clip.
This point is under mains voltage and poses a danger.
Homemade warning lamp protection
Considering the risks of working with the controller, experienced electricians tried in every possible way to protect its design:
- put a tin or other lampshade on the cartridge:
- wrapped the flask with tape or rags;
- adapted a hook for suspension;
- a fuse was installed in front of the cartridge to protect against short circuits;
- used to connect wires with a high degree of insulation protection;
- used for connection were probes with safety limiting rings from measuring instruments designed to operate under voltage.
However, even a complete set of all these measures does not allow you to safely perform work with a control lamp. It is more reliable to work with an indicator and a voltmeter.
Main conclusions
They make their own indicators using simple schemes. No other expensive parts are required. To make a probe, you can use the body of a dried marker or a faulty mobile phone. A probe in the form of a pin can be placed on the front part, and a cable equipped with an alligator clip or probe can be placed on the end.
Category: Miscellaneous
Signal LEDs (in English literature - LED, light-emitting diode) consume a current of 10-15 mA. Depending on the color, the forward voltage drop across the light-emitting diode ranges from 1.5 to 2.5 V. The small size, low current consumption and low operating voltage of LEDs allow radio amateurs to make many useful devices.
Using a minimal set of parts, you can make an LED voltage indicator with your own hands.
