In this measurement column, we will address frequently asked questions received by our customer support center.
We are presenting the answers.
The DS-0364, a 4-channel input unit for the DS-3000, features a non-isolated input circuit.
The ground side of the signal input connector (0V in the DS-3000 circuit) is connected to earth.
If there is a potential difference between the ground being measured and the ground of the DS-3000, a large current may flow through the ground line, potentially blowing the protective fuse in the input circuit. Examples of situations in which this may occur are shown below.
1. The object to be measured is located in the middle of the circuit, and one side is not at 0V.
In Figure 1, a current-sensing resistor R is inserted to investigate the frequency characteristics of the current.
By measuring the voltage across resistor R, you can also determine the current.
The problem here lies in the measurement point. If we try to measure the potential difference between 1 and 2, the voltage at 2 is not 0 V because there is a load behind it.
Therefore, current flows from point 2 to the ground of the DS-3000. If the resistance value of R is large, a large current will not flow and the fuse will not blow, but since the resistance value of the current sensing resistor is small, a large current flows and the fuse blows.
-
Figure 1
As for countermeasures,
The resistor value should be placed last, or it should be input to the DS-3000 via an isolation amplifier.
-
Figure 2
2. When a potential difference occurs between the ground of the DS-3000 and the ground of the device being measured.
Ideally, the ground should have the same voltage (0 V) everywhere, but voltage differences can occur due to the routing of the power supply and ground connections.
In Figure 3, the ground on the device side and the ground on the DS-3000 side are not properly connected, resulting in a potential difference. The device itself, and the DS-3000 itself, have a defined 0 V reference.
However, the 0V reference is different between the device and the DS-3000.
In this case, if you install an accelerometer (the type where the case is connected to ground), current may flow through the case of the accelerometer, causing the fuse to blow.
-
Figure 3
As for countermeasures,
Check the voltage between the sensor mounting surface on the device side and the ground on the DS-3000 side.
After confirming that there is no voltage difference, install the sensor.
If there is a voltage difference, connect the ground properly to eliminate the potential difference.
The mounting area for the acceleration sensor will be insulated.
This is what it will be.
In summary, please pay attention to potential differences when using non-isolated input circuits for measuring instruments.
Input units with isolated input circuits are also available.
DS-0366 2-channel 100 kHz bandwidth input unit.
For servo analysis and impedance measurement of electrical circuits, please use this unit.
It's lame.
(Excerpt from the email newsletter issued on March 20, 2019)
