Network Probe for electricians

In the work of every day, electricians often use simple measuring instruments.

To perform most of the work, it takes only two devices that work with the principle yes / no, which are owned by the network voltage and short circuit indicators. Traditionally, the mains voltage indicator, which allows to determine whether the monitored circuit is excited and find the phase wire, is made of a neon lamp, and a short-circuit indicator, which allows to determine the integrity of the circuit, it is constituted by a battery connected in series and a compact lamp.

These probes have several drawbacks, in particular, if a short-circuit indicator is accidentally connected to energized circuits, the lamp burns. The presence of two devices due to drawbacks in their use, before checking the integrity of the circuit, you must ensure that the same there is no tension. These defects are eliminated in the proposed probe, whose scheme is shown in the figure below. Both indicators are combined into a single project, and the use of LED brightness varies as the display elements allows to evaluate unambiguously the state of the circuit to be tested and to work comfortably at any level of illumination. This probe is designed for use during the installation, putting into service and repair of electrical circuits and systems with a voltage of 110 … 400 Frequency V 50 e 60 Hz to make it universal in almost all over the world.

The mains voltage indicator is built on the red LED LR. The resistors R1 – R5 limits the current through the LED to a safe level. The use of 4 resistors connected in series with the same nominal value (R2 and R5) It has allowed the use of the indicator in circuits with voltage up to 400 V. The diode D1 protects the LED LR from the reverse polarity voltage. The resistor R1, in addition to the main function, which will be discussed later, It performs the function of a fuse and burns in the event of a possible electrical breakage of the probe elements.

The composition of the electrical circuits in the probe, in contrast with the traditional circuits, It is made not from the direct current, but from the current high frequency pulses. This has allowed to untie the short circuit indicator from the circuit by the presence of a voltage of 230 V with a frequency of 50 Hz at the entrance of the probe. The short circuit indicator operates at a frequency of about 50 kHz. The effect of inductance and distributed capacitance of the wiring is insignificant for various wireless type up to 200 m length

The transistor T1 and the transformer B1 is connected according to the scheme of jule thief. It is powered by a stack P1 that has allowed to ensure a stable operation of the generator until the battery is not completely discharged;.

The indicator of a short circuit connection to the probes is carried out via the capacitors C1 and C2, which represent a significant resistance for a current to 50 Hz. The active and inductive impedance of the transformer T1 windings, connected in series with the capacitors C1 and C2, It is very small and the voltage drop on them when it is connected to the network does not exceed a few millivolts, bearable for the transistor T1. The resistor R1 reduces the amplitude of the charging current of the capacitors C1 and C2, increasing the reliability of the probe. The discharge of these capacitors when you disconnect the probe from the measured circuit under tension takes place via the elements b1, Dl, LR, and from the series R2 – R5, which enables to prevent electric shock, although small, when you touch the probe has been disconnected from the network 230 V

The short circuit indicator works as follows. When you press the button, the generator is powered, always is the speech so that you can not lit scordarlselo . The blue LED LB, connected to the output of the generator, begins to shine. If you close the probes or are connected to a circuit with a resistance not exceeding 100 Ohm, a negative feedback signal will be sent through the elements C1, C2, R1 and the circuit controlled by the T1 collector. Under its influence, the amplitude of the pulses at the output of the generator will decrease and it will become insufficient so that the LED lights LB. If the button is pressed while applying the mains voltage to the probes (LED LR is on), the negative feedback loop will be closed through a small internal resistance network and the LED will not turn on LB.

Therefore, with a good electrical wiring, LB and LR LED always shine separately, LR between the probes only if there is mains voltage and LB only with high resistance between the probes. When you are making repairs after a problem, you may encounter a situation that both LEDs light up simultaneously. This indicates that the wiring is defective or has an unacceptably high resistance or a low insulation resistance. Such electrical wiring requires repairs or replacements.

There are no special requirements for the device details. The resistors are all 1 / 4W, C1-C2 capacitors must have a rated voltage of at least 400 V. A ferrite ring with a diameter of 10 mm recovered from an energy-saving lamp is used for the transformer T1. Both its windings are wound simultaneously with enameled wire to form 16 coils each.

The beginning of a winding is connected to the end of the second and connected to the power line. For LEDs recommend finding a high brightness low operating current views. The transistor is a low-power pnp with a gain of at least 100. If you reverse the polarity of the LED lighting Lb, of the diode D2 and the battery B1, you can use a npn transistor, but I have many PNP purchased at trade shows at a cost of 1 per cent piece and somehow I'll have to use them.

The circuit can be assembled on a Stripboard. The battery is attached to the card with several rounds of insulating tape, if you did not have a holder it must be replaced no more than once a year.

The probe is located in the adjustment value selection of the resistor R1, which sets the depth of the negative feedback, normally the schema value is optimal but, sometimes this value must vary within the 30%, This increase is mainly due to T1 and the type of ferrite ring that you are able to find. When the probes are closed, LED LB must be switched off, when it connected to a resistance of 100 ohms or more, the light must be switched on. When you are working with the probe, you must meet the safety requirements. Always provide adequate insulation between the circuit and the exterior incorporating all in a suitable container isolated, although to be honest this is expected to go into the hands of those who are aware of what he is doing and what are the risks associated with tensions at play. Anyway better always abound in protections because a distraction is always around the corner for those who work under unfavorable conditions or for excess security.

The following is an example of a possible realization even though having regard to the simplicity of the circuit could assemble quietly in a base cutout breadboard.