Why does the multimeter adjust to zero every time when measuring resistance?
Multimeter zeroing refers to the mechanical meter. It must be zeroed every time when measuring resistance. If it is not zeroed, the measured resistance value will be inaccurate. There is no need to adjust the voltage and current to zero. If it cannot be adjusted to zero, it means that the hairspring in the meter head is somewhat aged and the rebound of the hairspring is not good. At this time, there is a zero-adjusting twist in the center of the meter head. You can gently adjust the zero-setting twist.
A multimeter is indispensable for every electronic hobbyist. The digital multimeter you mentioned relies on a built-in 1.5V battery to limit the current through a resistor. I dismantled and installed several pointer meters and There are many meter heads. The root of the pointer is connected to the center of the meter. There is a magnet with a diameter of several millimeters in the center. A coil is wound around the magnetic column. Use oil wire to connect the root of the pointer. The other end is fixed. When the knob points to the R position, red When the black test leads touch each other, a few microamps of current are supplied to the coil. When the coil is energized, it cuts the magnetic lines and rotates like a motor. It can be pulled by the oil wire like a "mosquito coil" and can only rotate at a fixed 90-degree angle. By engraving a mark on the running angle of the watch hand, you can use various designs to adjust one hand to measure AC, DC, voltage, current, resistance, transistor amplification and other functions.
What is the reason why there is voltage when measuring three-phase electricity using a pen but not using a multimeter?
First, let’s understand the three-phase voltage. The three-phase voltage is different from the single-phase electricity commonly used in our homes. The line voltage between phases is 380V. It is often used to drive three-phase equipment such as motors. If one of the three-phase voltages A lack of phase will cause the equipment to fail to operate or even burn down.
So if we feel that there is a problem with the three-phase voltage, we must immediately use a multimeter to measure the line voltage. If it is 380V and there is no short circuit between line and ground, it can be used normally. It is not recommended to use an electric pen to measure 380V. First of all, the quality of electric pens varies. Not all, some electric pens can't even measure the voltage of 380V.
Some electric pens have a relatively small range. If they exceed their range, they will not display or display an alarm. We mistakenly think that the phase line has no power or is faulty, which plays a misleading role. Therefore, we try to use Take measurements with a multimeter.
In addition to having problems with the electric pen and not being able to measure the voltage, there is also the possibility that this phase is short-circuited to the ground. If it is really short-circuited to the ground, sometimes the electric pen will not measure the familiar voltage, but when the multimeter is used to measure the line voltage, it will be The voltage value can be displayed. The voltage value displayed at this time is most likely not 380V, but 220V.
Of course, there is another possibility that we misoperated and the measurement points are inaccurate. For example, for a three-phase AC contactor, it is very likely that one of the three main contacts in the three-phase AC contactor has a problem, causing the line to be blocked. However, what we are measuring is the lower end of the AC contactor, so one of the main contacts in the lower section must have no power.
We mistakenly thought that one of the three phases had no power. In fact, it is normal to measure the three phases above the main contact, but we did not measure it comprehensively. As a maintenance electrician, we must consider carefully and use appropriate methods to troubleshoot the circuit, so that To ensure the operation of electrical safety equipment.
