1
\$\begingroup\$

I'm building a current measuring circuit. The circuit im using is seen below, enter image description here

How do I choose the burden resistor for the CT ?

If say im measuring 8A max, then with a 1:1000 CT, thats 8 mA on the secondary. If I want the voltage to be 3.5 V full scale, then R = V/I = 3.5/8mA = 438 ohms ?

\$\endgroup\$
  • 1
    \$\begingroup\$ CT datasheets generally give recommended burden ranges. It can be better to use a buffer then amplifier topology to get the output scaling you want, rather than try to achieve it with too high a resistor value which sacrifices linearity. \$\endgroup\$ – user133493 Nov 9 '17 at 2:46
4
\$\begingroup\$

Firstly, it may be helpful to remember that voltage transformers and current transformers are complimentary.

  • Voltage transformers are happy when open-circuit and have a minimum load resistance value (which causes the maximum rated current to flow).
  • Current transformers are happy when short-circuited and have a maximum load resistance value (which causes the maximum rated voltage to be generated).

You need to watch a couple of things:

  • Your CT will have a maximum "burden" that it can drive. That's the maximum the resistance mentioned above.
  • The CT will have a primary rating - the maximum number of ampere-turns it can accept on the primary.
  • You need to remember that 8 A is 8 A peak and not 8 A RMS which will result in \$ 8 \sqrt 2 \ \$ A.

If say im (sic) measuring 8 A max, then with a 1:1000 CT, thats (sic) 8 mA on the secondary.

Correct - but note whether it's peak or RMS.

If I want the voltage to be 3.5 V full scale, then R = V/I = 3.5/8mA = 438 ohms?

Correct - again subject to peak or RMS.

You probably want to calculate true RMS current so you have a bit of work to do on your sampling and coding.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.