Current transformers (CTs) provide a simple and yet accurate means to sense the current flow in power conductors.
The external load applied to the secondary of a CT is called the burden (or load). Default burden resistors RA1/RA2 connected to the CT are each 1.62Ω (1%) for a total burden resistance of 3.24Ω. As an example, when used with a 2000:1 ratio CT, a 240 Arms primary current would develop the max input voltage of 0.55 Vpk (240 Arms × √2 × 3.24Ω/2000 = 0.55 Vpk).
Attention: This burden resistor is sometimes integrated inside the Current
Transformer.
Warning: An open circuit in a
CT can cause a dangerous overvoltage at the secondary terminals of the CT. Ensure
that the burden resistor is connected (or integrated inside the CT), and do not
remove the protective diodes D12, D13, D14 and D15.
Figure 1. IPx and INx Current Sense
Schematic when Using CT
An anti-aliasing filter is required for band-limiting the input signals. Typically, a
single-pole RC filter is sufficient for metrology applications. For CT operation, the
filter must be tuned with the following component values:
- RA1, RA2 = 1.62Ω, 1%, 25 ppm/ºC
- RB1, RB2 = 0Ω
- RC1, RC2 = 3 kΩ, 1%, 25 ppm/ºC
- CA1, CA2 = 47 pF Y7R
- CB1, CB2 = Do Not Populate
- CC1, CC2 = 3.3 nF NP0/COG
- D1, D2 = Si Junction diodes