Application Note #108
Current Transformer Applications:
Don't Get Stuck in a Rut

Introduction

If you are tired of large, bulky and expensive current transformers that are not even adjustable, consider the following design criteria.

1. Determine the maximum voltage allowed on the secondary that will not saturate the core. Select a value of Vmax from the chart on the Current Transformer page.
2. Determine the maximum current that you would like to measure and divide this value by the turns in the secondary (assuming a one turn primary). This is the maximum current that will flow in the secondary of the current transformer. Use this value for I_secondary.
3. Using Ohm's law calculate a value for the burden resistor that goes across the output of the current transformer. One of the purposes of this resistor is to prevent the voltage from reaching values that will saturate the transformer.

   R_Burden = V_Max/I_Secondary

4. Connect the output of the current transformer to the input of an operational amplifier with sufficient gain to provide the desired output.

Using an HL-IH250F/ current transformer to measure currents to 100 Ampsat 60Hz.

1. Using the chart on the Current Transformer page, the HL-IH25F/ is shown to saturate at 60Hz at 0.4 Volts. Use this value for V_max.
2. The HL-IH250F/ contains 500 turns on the secondary. Dividing the maximim current of 100 Amps by 500 gives us a value for the secondary current of Secondary = 0.20 amps. I_secondary = 0.20 amps
3. The value of the burden resistor can then be calculated. This should be a precision resistor in order to help make measurements as accurate and stable as possible.

   R_Burden = 0.4Volts/0.2Amps = 2.0 Ohms

4. The output of the current transformer is connected to the operational amplifier as shown in the shematic below. The gain of the amplifier is setto provide the desired output at any given current. In our test we set the output to +5.0 Volts at 100 Amps.
   
   The gain in this case would be:

   Gain = Desired Output at Maximum Current/V_max = 5 volts/0.4 volts = 12.5

Parts List

• T₁ Hurricane Electronincs Lab HL-IH250F/
• R_B 2.0 Ohms (for this application)
• IC₁ LT 1006 CN8
• R₁ 5 Kohm
• R₂ 220 Ohm
• R₃ 100 Kohm
• C₁ 10 uf
• D₁ 1N4967 or equivalent