Please teach me a resistance measurement circuit

yaoyong

Please teach me a resistance measurement circuit [Copy link]

The circuit schematic is shown in the figure above.

P16 is connected to the negative plate. This circuit is used to measure the resistance value. When the resistance value of P16 is different, the waveform at T36 is different:

When short-circuited, it is a sine wave with Vmax=1.84, Vmin=-1.84, and when disconnected, it is an oscillating attenuation waveform;

The waveform measured at T33 is approximately 0V when short-circuited and 5.2V when disconnected;

REM_DRV and REM_CLK are both square wave signals, DG419 is a single-pole double-throw switch, and TL052CDR is a FET input amplifier

How does this circuit measure resistance? Can I get the corresponding conversion formula?

呜呼哀哉

What is JW13 for? I don't know if your excitation comes from this place. If the excitation from this place can cause the measured resistor to resonate with those capacitors, a large electrical parameter can be obtained and transmitted from the transformer to the back. Although the capacitance accuracy is not high, it can be corrected with precision resistors when the stability is good. This is a guess. This is neither a four-wire resistance measurement method nor a bridge.

maychang

"How does this circuit measure resistance?"

It doesn't look like this is a resistance measurement circuit.

chunyang

The circuit is seriously incomplete and cannot be judged.

bigbat

If the resistance is a fixed value, a voltage comparator circuit can be used to control it according to the output of the circuit.

yaoyong

chunyang posted on 2022-9-27 12:03 The circuit is seriously incomplete and cannot be judged.

Sorry, this is the complete circuit. After excitation, the relay is closed, which will form the negative pole of the unipolar output;

P16 is used to connect the negative plate, which can be a single-pole plate or a bipolar plate.

I just don't understand the principle of this circuit. I have measured the waveform, but I just can't understand the working principle.

呜呼哀哉

yaoyong posted on 2022-9-27 16:07 Sorry, this is the complete circuit. After excitation, the relay is closed, which will form the negative pole of the unipolar output; P16 is used to connect the negative plate, which can be...

I would like to ask, what is the resistance test accuracy? What is the test range?

呜呼哀哉

yaoyong posted on 2022-9-27 16:07 Sorry, this is the complete circuit. After excitation, the relay is closed, which will form the negative pole of the unipolar output; P16 is used to connect the negative plate, which can be...

From your full picture, U33 should be used as an oscillator circuit (it should be, but I haven't analyzed it), enter U32 trigger shaping, enter U34 AND gate (it seems to be an AND gate), and the AND gate output enters U36 as a control switch. Another U32 output enters U35 (most likely a NOT gate or buff), and comes out of U35 through capacitor coupling and superposition with the transformer signal, and enters U36 as an input signal to simulate the switch. Therefore, it is still speculated that there is an excitation signal at BW9, and under different resistances, the circuit formed by the capacitor can transmit electrical parameters to the transformer back stage.

Gen_X

I think this is an AC "short circuit" detection circuit. When the resistance at T33_T34 is less than a certain value, the "resonance" time that CLK excitation cannot generate in the forward channel is too short, and the energy cannot be transferred to the backward amplification channel through the analog switch, so the output is the lowest "alarm".

If the input resistance is large, the attenuation oscillation will last long and have a large amplitude, and the backward output voltage will be large or no alarm will be given.

This is designed for the measured circuit with a certain cable length or inductive and capacitive characteristics. The current to be detected is too large, so the "resonance" mode is adopted to save excitation energy.

For reference only.

yaoyong

Gen_X published on 2022-9-27 21:23 I think this is an AC "short circuit" detection circuit. When the resistance at T33_T34 is less than a certain value, the CLK excitation cannot be generated in the forward channel...

When P15 is short-circuited, the waveform measured by T36 is shown in the figure

The following is the scaled waveform

When the resistance of P15 increases, the waveform measured by T36 is shown in the figure

Continue to increase the P15 resistance

The scaled waveform is shown in the figure

At the rear pole T33, it is converted into a DC voltage, which increases with the increase of the sine wave amplitude. After the sine wave increases to a certain value, it will no longer increase, and the DC voltage will remain unchanged.

These measurements are static and have nothing to do with excitation. They are measured when the value of the external resistor of P15 is changed.

I just don't quite understand how this conversion process is implemented in the circuit, and what factors determine the voltage limit.

Gen_X

Judging from the waveform and circuit you gave, the right side of TR5 is an LC loop with a fixed center frequency, which is excited by the periodic CLK. If there is no consumption on the left side, the oscillation can maintain a certain amplitude (maximum). If the left side introduces loss until it is short-circuited, the LC will maintain the minimum oscillation amplitude at this time, thereby linearizing the amplifier circuit on the right, and finally obtaining a potential that can characterize the upper input resistance on the left.

From the analysis of the figure, we can see that the resistance of the measured resistor will not be too large.