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# Use of multimeter

2020-06-23
Before measuring, first check whether the dial hand stops at the "0" position on the left end. If it does not stop at the "0" position, use a small screwdriver to gently turn the middle positioning screw under the dial to make the pointer point to zero, usually called mechanical zero adjustment . Then insert the red and black test leads into the positive (+) and negative (-) test pen jacks, respectively.

1. Use a multimeter to measure current and voltage.

When measuring current or voltage, the selector switch should be turned to the corresponding measurement item and range. The current in the circuit should flow in from the red test lead and out of the black test lead. When reading, pay attention to the selected range.

Reading: Measured value = (Dial pointer indication ÷ Dial full deviation indication) × magnification.

2. Use a multimeter to measure resistance

A. Principle of measuring resistance
The ohmmeter is made according to the closed circuit Ohm's law. Its principle is shown in the following figure. G is an ammeter (header), the internal resistance is Rg, the full bias current is Ig, the battery's electromotive force is E, and the internal resistance is r. Resistor R is a variable resistor, also called zero-adjusting resistor.

1. When the red and black test leads are connected, it is equivalent to the measured resistance Rx=0, adjust the resistance of R, so that the pointer of the meter head points to full scale, so the pointer points at the full deviation of the current and is set as the resistance The zero point of the scale. Rg+r+R is the internal resistance of the ohmmeter.

2. When the red and black test leads are not in contact, it is equivalent to the measured resistance Rx=∞, there is no current in the ammeter, the pointer of the meter does not deflect, and the position pointed by the pointer at this time is set as the ∞ point of the resistance scale.

3. When the measured resistance Rx is connected between the red and black test leads, the current through the meter changes Rx, and the current I changes with each change.Each Rx value corresponds to a current value, and the corresponding value of I is directly marked on the dial Rx value, you can directly read the resistance value of the measured resistance from the dial.

special reminder:
Because I and Rx are not in a linear relationship, the scale of the ohmmeter is uneven. From the dial, "left dense and right", the resistance zero scale is the maximum current scale, and the resistance "∞" scale is the current zero scale.

B. Operation steps for measuring resistance:
(1) Gear selection: Rotate the selector switch to the ohmic gear, and select the range of the selector switch according to the estimated resistance.

(2) Zero adjustment: Touch the two test leads, adjust the zero adjustment knob of the ohm gear, so that the pointer points on the zero scale of the resistance scale. (Note: The zero point of the electrical block is at the right end of the scale).

(3) Measurement and reading: Connect the two test leads to the two ends of the resistance to be measured respectively to measure.
Reading: measured value = dial pointer indication × magnification.

(4) After the experiment is completed, the two test leads should be pulled out of the jack, and the selector switch should be placed in the "OFF" block or the highest AC voltage block. If the ohmmeter is not used for a long time, the battery in the meter should be removed.

special reminder:
(1) When measuring resistance, the resistance to be measured should be disconnected from other components, and do not touch the test pen with your hands;

(2) Choose the range of the ohm gear reasonably, so that the pointer points as close to the center of the dial as possible; if the pointer angle is too large, the low gear should be changed; if the pointer angle is too small, the high gear should be changed. Multimeter is different when measuring current and voltage

(3) When using an ohmmeter to measure resistance, reset the magnification every time.