The method of judging the phase line of power supply by digital multimeter
Turn the digital multimeter to the 20V (or 200V) AC voltage block, do not connect the black test lead, hold the insulating rod of the red test lead with your hand, and touch the two holes of the power socket (or the exposed parts of the two power lines) with the tip of the pen in turn. , where the one with the larger value is the phase line, and the one with the smaller number is the neutral line.
1. If you touch the two power jacks with the red test pen, the two display values are close and the reading is larger, indicating that the neutral wire is open to the ground, and the AC voltage is also induced because the neutral wire is close to the phase wire.
2. Checking the phase wire with a digital voltmeter also has the advantage of being able to judge accurately without damaging the insulation sheath.
As long as the red test pen is used to touch the outer skins of the two wires in turn, the wire with the larger reading is the phase wire.
Since the tip of the pen does not touch the internal wires, the induced voltage is relatively weak, so it is better to choose the 2V AC voltage range of the digital multimeter.
How to Repair and Replace Parts of Multimeter
1. DC current part
The installation of the moving coil should be correct, and there should be no ferromagnetic substances in the moving gap. If the shunt resistor is burnt out, it should be replaced according to the marked value on the circuit diagram.
If the sensitivity of the watch head is high, it can be demagnetized, the hairspring can also be replaced, or the brake ring can be rewound.
Clean the movable contacts of the transfer switch, and re-solder for desoldering or weak soldering.
2. DC voltage part
The adjustment and repair of the DC voltage circuit can only be carried out after the current circuit is adjusted. During the adjustment, it is carried out step by step from the small limit to the large limit. The public resistance related to the DC current cannot be readjusted.
3. AC voltage part
The adjustment of the AC voltage part is mainly on the rectifier element, such as rectifier element breakdown, damage, and reverse resistance reduction. When replacing, select a tube with characteristics close to the original. Similarly, anything related to DC current and DC voltage cannot be readjusted.
4. Resistance part
Replace the battery if it is low. If there is rust on the contact point, clean it. If necessary, lightly sand it with fine sandpaper and apply a thin layer of Vaseline.
The electromagnetic clip should have sufficient elasticity, and it should be adjusted from the R*1 gear to the maximum gear, and any resistance related to the current and voltage loops is not allowed to be adjusted again.
