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Loop resistance technical issues

time:2021/4/8   source:华天电力  reading:758 time

1 High-power resistance standard resistor

High-power resistance standard resistors can be roughly divided into three categories: high-power micro-resistance standard resistors, high-power analog DC standard resistors, and active high-power standard DC resistors.

1.1 High-power micro-resistance standard resistors

The high-power micro-resistance standard resistor is composed of multiple resistors in series, as shown in Figure 1. When calibrating DC resistance as a standard DC resistance, the large output current forms different voltage divisions at each end of the resistor, and thus DC resistances of different resistance values are measured. Although this standard resistance is a physical resistance, it can reflect the real situation measured by a DC resistance measuring instrument. However, because it is an oil-filled resistor, it is large in size, high in heat, and stability is difficult to guarantee. Therefore, it is gradually replaced by high-power analog DC standard resistors.

Micro Ohmmeter.png

1.2 High-power analog DC standard resistors

High-power analog DC standard resistors convert the DC current output by the DC resistance tester into a small current through a modem, and generate a voltage drop from a fixed resistance value as a voltage output to simulate high-power DC resistances of different resistance values. See figure 2. The standard resistor has a wide working range, high accuracy, and good stability; however, there are too many terminals, the wiring is more complicated, and there is always the influence of ripples on the detection in the test.

1.3 Active high-power DC standard resistors

Active high-power standard DC resistor is composed of a fixed resistance high-power DC standard resistor and one precision electronic transformer box. In fact, the precision electronic transformer box and the high-power DC resistance constitute a four-terminal resistance. C1, C: is the current terminal, P. , P2 is the voltage terminal. From C to C: input high-power DC current f, through the high-stability high-power DC fixed value standard resistor rl to produce a voltage drop. That is, the large current is first converted to a small voltage to avoid the heating phenomenon caused by the large current to the measurement. Moreover, the high-precision electronic transformer box uses low-pass filtering to offset the influence of ripples, ensuring the accuracy of the verification. Therefore, active high-power DC standard resistors are currently ideal instruments for calibrating DC resistance testers.

2 Active high-power resistor verification system and error analysis

2.1 Active high-power DC standard resistor verification system design

The circuit of the high-precision electronic transformer box in the active high-power DC standard resistor is mainly composed of analog/digital converter ADC, DSP, digital/analog converter DAC, amplifying, filtering circuit and external interface. From the formula, .=11.0rs/u;=/o-. It can be seen that the key to the accurate equivalent resistance value in the verification is the accuracy of the transformation coefficient of the electronic transformer box. The electronic transformer box can read accurate II through a series of filtering samples of its system, and the error relative to UO, n; can be ignored. The use of high-resolution DAC and a voltage reference source with a temperature coefficient of not less than V/"C ensure the accuracy of ‰. Therefore, the theoretical accuracy can reach 0.05%.

2.2 Error analysis

The DC standard resistance rating of active high-power DC resistors is generally 0.01%, and the error of the high-precision electronic transformer box is mainly derived from the digital/analog converter DAC, and the conversion brings about an error of about 10 V. Therefore, the theoretical error should not be less than 0.05%, but the actual accuracy of the active high-power resistor used in the DC resistance tester in different measurement ranges should be equal to 0.05%.

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