What is an Ohmmeter?

An ohmmeter is a meter that directly measures resistance. It is based on Ohm's law of closed circuits.

Chinese name: Ohmmeter
Definition: Instrument for measuring resistance value

Range: The measurement range is from 0
Measurement basis: Ohm's law of closed circuits

Introduction to ohmmeter

A meter that directly measures resistance. It is based on Ohm's law of closed circuits. As shown in the figure, Rx is the measured resistance, G is the meter head, internal resistance is Rg, is a dry battery, its terminal voltage is U, and R is the resistance. When the measured resistance Rx is connected at both ends of A and B, the meter head current is I. When U, R, and Rg are constant, I only changes with Rz. There is always a certain I value corresponding to each Rz. The value can be calculated as Rz. When the value of Rz is engraved on the corresponding position of the scale, the ohmic number of the measured resistance can be directly read. The characteristics of the scale of the ohmmeter are: The larger the resistance value, the more the corresponding scale on the scale is to the left. This is because the larger Rz is, the smaller I is. The division of the scale is not uniform, and it becomes denser toward the left. This is because the relationship between I and Rz is not proportional. When Rz = R + Rg, I / Ig = 1/2 (Ig is the range of the meter head). At this time, the pointer is exactly in the middle of the scale. (R + Rg) is called the median resistance of the ohmmeter. The median resistance uniquely determines the scale of the ohmmeter. With different median resistances, the ohmmeter has different ranges. In a multimeter, the part that measures resistance is called the ohm range, and generally has 3 to 4 ranges. The ohmmeter also has a zeroing resistor. ^[1]

Ohmmeter The value of the resistance and scale Rx of the ohmmeter corresponds to a value of I / Ig. The actual meaning of this value is the only thing that determines the position of the hand. When I / Ig = 1, the hand indicates the rightmost end, when I / Ig = 1/2, the hand indicates the center of the dial, and so on. That is, each Rx determines an I / Ig value, and each I / Ig determines a needle position. If two ohmmeters have different (R + Rg) values, the same Rx corresponds to different I / Ig , Which corresponds to the position of different hands. Their scales are different, and vice versa; only two ohmmeters (R + Rg) have the same value. Their scales are exactly the same. (One scale can be shared.) The (R + Rg) of the ohmmeter is called its median resistance, that is, the median resistance uniquely determines the scale of the ohmmeter. Once the median resistance is determined, the scale of the dial will be finalized. When the value of Rx is 2 times, 3 times, 4 times ... The current I in the ammeter is 1/3, 1/4, 1/5 of the full-scale current Ix, respectively. When the values of Rx are 1/2, 1/3, and 1/4 in R, respectively, the deflection angles of the meter pointer are 2/3, 3/4, and 4/5 when full offset ... The dial scale is uneven.

Ohmmeter characteristics

The ohmmeter's scale characteristics are different from the ammeter and voltmeter. The ohmmeter has the following significant features:

(1) As the scale of the ammeter and voltmeter increases to the right, the larger the value, the ohmmeter is opposite. This is because the smaller Rx is, the larger I is. When Rx = (open circuit), I = 0, it is at the leftmost end; when Rx = 0 (the two test leads are shorted), I is Ig, and the resistance at the full scale of the ammeter is 0 at the rightmost end.

(2) The scale of the ammeter and voltmeter is even. The scale of the ohmmeter is very uneven. This is because after zero adjustment, E, R, and Rx are all constant, and I changes with Rx. But they are not simple linear proportional relations. So the dial scale is uneven.

(3) The scale of the ammeter and voltmeter are from 0 to a certain value, so each meter has a certain range. The scale of the ohmmeter is always from 0 . Does this mean that all ohmmeters have the same scale? Does the ohmmeter have no range problem? No. These two issues are analyzed separately below.

(4) Both the ammeter and voltmeter require a circuit connection (ie, series or parallel) when used, but the ohmmeter can directly read its display without connecting the circuit.

Ohmmeter range

Although the measurement range of any ohmmeter is from 0 , the scale becomes denser as it goes to the left. An ohmmeter with a median resistance of 100 is difficult to read when Rx is above 200, and when Rx is 1000. No more readings. To accurately measure the large resistance, you should use an ohmmeter with a larger median resistance (that is, shifting gears). In order to share a scale for each block of the ohmmeter, it is generally based on the R × 1 median resistance, which is enlarged by 10 times. For example, R × 1 block median resistance R = 10, R × 10 block is 100, R × 100 block is 1000, etc., and so on. To extend the range of an ohmmeter is to increase the total internal resistance of the ohmmeter. To implement another additional circuit.

Ohmmeter error

The indication error of the ohmmeter is the current meter and voltmeter with uniform division. The larger the displayed value, the smaller the relative error. When the total internal resistance of the ohmmeter is equal to the median resistance R of the scale, the differential method can be used to derive R / R = ( x / L) [(R -R) 2 / (R ××) +4]. (2) (2) The meaning of the letters in the formula is shown in Figure 2. The error is the smallest when R = R, so it is a more accurate result. Use the middle of the ruler whenever possible.

Use of ohmmeter

Ohmmeter operation steps

(1) Selecting the gear: Turn the selection switch to the ohmic gear, and select the range of the selection switch according to the estimated resistance.

(2) Zero adjustment: Touch the two test leads and adjust the adjustment knob of the ohmic block so that the pointer points to the zero position of the resistance scale. (Note that the zero position of the electrical block is at the right end of the scale)

(3) Measurement (reading): Connect the two test leads to the two ends of the resistance to be measured, and perform the measurement. Multiply the pointer indication by the multiplication factor to obtain the resistance value of the resistance to be measured.

(4) After the experiment, the two test leads should be pulled out from the jack, and the selection switch should be set to the "OFF" position or the highest AC voltage position. If the ohmmeter is not used for a long time, remove the battery from the meter.

(5) The electromotive force E and the internal resistance r of the battery in the ohmmeter will change the value after using for a long time, which will cause the measurement error to increase. At this time, the resistance measurement is not accurate enough, and the circuit method should be used for more accurate resistance measurement. ^[2]

Notes on using the ohmmeter

(1) Measure the resistance with an ohmmeter, and retry after each shift and before measurement.

LU-DP4 smart current, voltage, ohmmeter Zero adjustment.

(2) When measuring the resistance, the resistance to be measured must not only be disconnected from the power supply, but also from other components.

(3) Be careful not to touch the metal part of the test lead when measuring, otherwise the resistance of the human body will be connected in parallel, affecting the measurement result.

(4) Reasonably select the range so that the pointer is as close to the middle scale as possible, and refer to the range of the pointer deflection in R / 5 / 55R (or the ammeter pointer deflection full-scale current 1/3 ~ 2/3). If the deflection angle of the pointer is too large, it should be changed to the low gear, and vice versa. When reading, the pointer indication should be multiplied by the gear multiple.

(5) To prevent over-range in practical applications, do not measure the resistance of appliances with a very small rated current (such as the internal resistance of a sensitive ammeter).

(6) After the measurement is completed, the test lead should be pulled out, and the selection switch should be set to the OFF position, or the highest AC voltage. When not in use for a long time, the battery should be removed to prevent battery leakage.

(7) Ohmmeter function: measuring resistance, measuring diode positive and negative.

(8) Usage: It is best to hit the pointer in the middle to reduce the error.

(9) Selection of ohmmeter:

In theory, the ohmmeter can directly measure the resistance of any resistance value from zero to infinity, but due to the unevenness of the panel scale (that is, the nonlinear relationship between I and Rx), it makes the value near zero and infinite. It is difficult to accurately read the value of the measured resistance nearby (the measurement error is very large). When Rx = R, that is, near the median, the relationship between the angle of the pointer deflection and Rx is closer to linear, and the scale is more uniform. Therefore, it is best to position the pointer near the center when measuring specifically. This is the basis for selecting gears.

Before using the ohmmeter, you should first estimate the size of the resistance to be measured, and select the appropriate gear (usually in the order of large range to small range). If the selected gear is not suitable, it should be reselected. That is, if the angle of the pointer is small, you should choose a gear with a higher magnification and vice versa. Each shift must be re-zeroed before measurement. ^[2]