# What burden resistor value to use for current transformer?

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

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 ?

• 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. – user133493 Nov 9 '17 at 2:46

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.