An oscilloscope is a commonly used measuring instrument, primarily used to detect various electrical signals, with applications in fields such as power, electronics, medicine, scientific research, and military. Do we understand the methods and precautions for measuring voltage with an oscilloscope? Below, I will introduce the methods and precautions for measuring voltage with an oscilloscope in detail, hoping to help everyone.
1. Methods for Measuring Voltage with an Oscilloscope
1. Direct Measurement Method
The so-called direct measurement method involves directly measuring the height of the voltage waveform displayed on the screen and converting it into a voltage value. For quantitative voltage testing, the fine-tuning knob of the Y-axis sensitivity switch is generally turned to the “calibration” position, allowing the measured voltage value to be directly calculated from the “V/div” indication and the vertical axis coordinate value occupied by the measured signal. Therefore, the direct measurement method is also known as the ruler method.
(1) Measurement of DC Voltage
Set the Y-axis input coupling switch to the “ground” position and the trigger mode switch to “automatic” position to display a horizontal scanning line on the screen, which represents the zero voltage line.
Set the Y-axis input coupling switch to the “DC” position and apply the measured voltage. At this point, the scanning line will produce a vertical displacement H in the Y-axis direction, and the measured voltage will be the product of the “V/div” switch indication value and H.
Direct measurement is simple and straightforward, but it has a larger error margin. Factors contributing to the error include reading error, parallax, and system errors of the oscilloscope (attenuator, deflection system, edge effects of the oscilloscope tube), etc.
(2) Measurement of AC Voltage
Set the Y-axis input coupling switch to the “AC” position to display the AC component of the input waveform. If the frequency of the AC signal is very low, the Y-axis input coupling switch should be set to the “DC” position.
Move the measured waveform to the center of the oscilloscope screen and adjust the “V/div” switch to keep the measured waveform within the effective working area of the screen. Read the degree H occupied by the entire waveform in the Y-axis direction according to the coordinate scale, then the peak-to-peak value VP-P of the measured voltage can be equal to the product of the “V/div” switch indication value and H. If using a probe for measurement, the attenuation of the probe should be accounted for, i.e., the above calculated value should be multiplied by 10.
For example, if the Y-axis sensitivity switch “V/div” is set to 0.2 and the measured waveform occupies a Y-axis coordinate amplitude H of 5 div, then the peak-to-peak value of this signal voltage is 1V. If measured with a probe, the indicated value remains the same, then the peak-to-peak value of the measured signal voltage would be 10V.
2. Comparative Measurement Method
The comparative measurement method involves comparing a known standard voltage waveform with the measured voltage waveform to obtain the measured voltage value.
Input the measured voltage Vx into the Y-axis channel of the oscilloscope, adjust the Y-axis sensitivity selection switch “V/div” and its fine-tuning knob to display a height Hx that is easy to measure on the screen and record it, while keeping the position of the “V/div” switch and fine-tuning knob unchanged.
Remove the measured voltage and input a known adjustable standard voltage Vs into the Y-axis, adjusting the output amplitude of the standard voltage to display the same amplitude as the measured voltage. At this point, the output amplitude of the standard voltage equals the amplitude of the measured voltage. The comparative method for measuring voltage can avoid errors caused by the vertical system, thus improving measurement accuracy.
2. Precautions for Using an Oscilloscope
To ensure the safety of the instrument operators and the instrument itself, to ensure that the instrument operates within a safe range, and to guarantee accurate measurement waveforms and reliable data, the following precautions should be noted:
1. If users need to measure switch power supplies (primary of switch power supply, control circuit), UPS (uninterruptible power supply), electronic rectifiers, energy-saving lamps, inverters, or other types of products or electronic devices that cannot be isolated from AC220V, a DP100 high-voltage isolated differential probe must be used for floating signal tests.
2. General oscilloscopes should adjust the brightness and focus knobs to minimize the diameter of the light spot for clearer waveforms, reducing measurement errors; do not allow the light spot to remain stationary at one point; otherwise, the electron beam will bombard one point, forming a dark spot on the screen and damaging the phosphor screen.
3. The outer casing of the general oscilloscope, the metal outer ring of the BNC socket for signal input, the ground wire of the probe, and the grounding wire of the AC220V power socket are all interconnected. If the instrument is used without grounding, directly measuring floating signals with the probe will create a potential difference between the instrument and ground; the voltage value will equal the potential difference between the probe ground wire contact point and ground. This poses serious safety hazards to the instrument operators, the oscilloscope, and the measured electronic devices.
4. The grounding wires of the measurement system – such as oscilloscopes, signal sources, printers, computers, etc. – and the measured electronic devices – such as instruments, electronic components, circuit boards, and power supply grounding wires – must be connected to a common ground (earth).
5. TDS200/TDS1000/TDS2000 series digital oscilloscopes, when used with probes, can only measure signals with (measured signal – signal ground is ground, output amplitude of signal less than 300V CAT II). Absolutely do not measure floating signals of AC220V or electronic devices that cannot be isolated from AC220V (floating ground cannot be connected to ground, otherwise it will damage the instrument, such as testing an induction cooker).
