Components and technical solutions for power electronics

Calculation guide
Electronic technology current sensors
NCS sensors

1 - Reminder of the key elements

 Formulas:                                                                  Abbreviations 

VM2 = RM2 x IS2

VM1 = RM1 x IS1

with 0Ω < RM1 or Rwith 0Ω < 350Ω
IP  : primary current
IPN : nominal primary current
IPMAX : nominal primary current
IS1 : secondary current at IPN
IS2: secondary current at IPMAX
VS1 : secondary voltage at IPN
VS2 : secondary voltage at IPMAX
VA : supply voltage
VM : measuring voltageRS : resistance of the secondary winding
RM : measuring resistance
RMMIN : minimum measuring resistance
RMMAX : maximal measuring resistance

Reminder of the sensor electrical connection

2 - Measurement circuit calculation (current output)

Example with NCS125-2 sensor

 IPN = 2 000A  IS1 @ IPN = ±20mA
 IPMAX = 10 000A IS2 @ IPMAX = ±20mA
 RM = 0 - 350Ω (IS1 & IS2) VS1 @ IPN = ±10V
 RM > 10kΩ (VS1 & VS2)  VS2 @ IPMAX = ±10V
 VA = ±15V ... ±24V  

 The design of the sensor requires that 2 operating points are respected on the outputs IS1 and IS2:
• A maximum measuring voltage of 7V d.c. (VMMAX ≤ RMMAX x ISMAX)
• A maximum output current of ±20mA d.c..

The supply voltage does not have any influence on the output signals.

2.1 - What load resistance (RM) is required to obtain a 5V measuring (VM = 5V) when the current IP = 6000A peak?

The measured current is greater than IPN (2000A for a NCS125-2), IS2 is therefore used as the measuring signal. Firstly the output current on IS2 must be calculated when IP = 6000A d.c.
IS2 = IP / IPMAX x ISMAX = 6000 /10000 x 20 = 12mA (correct because IS2MAX = ±20mA d.c.)
Now determine the value of the resistance RM
RM = VM / IS2               = 5 / 0,012             = 416,67Ω

The NCS125-2 sensor can measure a 6000A peak on the signal output IS2 with a resistance of 416.67 Ω (greater than 350 Ω) because the output current is smaller than ISMAX i.e. 20mA d.c..The product of RM x ISMAX must always be smaller than or equal to maximum output of 7V d.c..

3 - Measurement circuit calculation (voltage output)

No special calculation needs to be made. This NCS sensor range supplies a voltage directly proportional to the primary current IP between -10V and +10V. A load resistance of a value greater than or equal to 10kΩ adapts the impedance of the measured output (VS1 or VS2) to the acquisition system.