No.24 Cautionary notes of non-isolated input circuit (prevention of the fuse blown)

DS-0364, 4ch input unit is a non-insulated input circuit.
The ground side of the signal input connector (0 V of the DS-3000 circuit) is connected to the earth.
If there is a potential difference between the ground to be measured and the ground of the DS-3000, large current flows through the ground line and the protective fuse of the input circuit may be blown.
The following are examples of occurrences:

 

1. The measurement target is in the middle of the circuit, and one side is not 0 V.

In Figure 1, a current detection resistor R is inserted to check the frequency response characteristic of the current. By examining the voltage across resistor R, the current can also be examined.
The measurement points are necessary to be considered.
When measuring the potential difference between (A) and (B), the voltage of (B) is not 0 V as it is placed after load. So the current flows to the ground of DS-3000 via (B). Since the resistance value for current detection is small, large amount of current flows at once and the fuse is blown.

 

Figure 1

 

 

As the countermeasure, the resistor is inserted closer to DS-3000 or it inputs to DS-3000 via isolation amplifier.

 

Figure 2

 

2. There is a potential difference between the earth of DS-3000 and the earth of the measured target.

 

Basically, the ground must be the same voltage (0 V) everywhere, however, there may be voltage differences due to the arrangement of the power supply and the ground. There is potential difference shown in Figure 3 as the earth on the equipment side and the earth on the DS-3000 side are not properly grounded.

The equipment itself, DS-3000 itself has the own reference of 0V. However, they are not same. A current may flow through the case of the accelerometer (the case connected to the ground), it may blow the fuse.

Figure 3

 

The countermeasures are;

- to check the voltage between the surface of the equipment where the sensor is mounted and the earth on the DS-3000 side, and confirm that there is no voltage difference, then the sensor is mounted

- to connect the earth properly so that there is no potential difference, if there is a voltage difference

- to insulate the mounting section of the acceleration sensor

Please take note that the potential difference is considered when inputting non-insulated measurement instruments.

We provide isolated input circuit as well.
DS-0366 2ch 100 kHz band for servo analysis, impedance measurement of electric circuit.

(H.K)