Measure burden resistor of a current transformer - Page 1
Forum section: electronics / projects

author: heinthetaung
author_rank: Newbie
timestamp: July 01, 2016, 02:13:49 am
content: HiI have a current transformer with burden resistor fixed to it and it is connected to differential amplifier circuit then connects to ADC of a microcontroller. Is it possible to measure the resistance of the burden resistor using the connected ADC? I have different current transformer with different burden resistor, I want to know which current transformer is connected to the circuit.Since the CT is ac current source and burden resistor is a load resistor connected to it, how can I measure the source impedance of the CT?Thanks.

author: Keicar
author_rank: Regular Contributor
timestamp: July 01, 2016, 02:36:07 am
content: Without disconnecting the burden resistor from the transformer, surely you can only ever measure the burden resistance in parallel with the DCR of the transformer secondary? Would that be sufficient for your purposes? If so, I would imagine you could apply an external current source and measure the voltage drop across the resistor/transformer combination with your ADC.

author: Circlotron
author_rank: Super Contributor
timestamp: July 01, 2016, 02:44:49 am
content: Maybe put an external HF ac current through the burden resistor and watch the voltage across it with a scope. As the frequency goes higher and higher the reactance of the CT secondary might get high enough that the voltage across the resistor is almost entirely because of the resistor.

author: heinthetaung
author_rank: Newbie
timestamp: July 01, 2016, 02:56:41 am
content: Quote from: Circlotron on July 01, 2016, 02:44:49 amYou are saying that by applying the resonance frequency to the secondary of CT(inductor itself), I can cancel out the reactance and measure the resistance of burden resistor. What if the primary has current flowing through it (please assume constant current)?

author: heinthetaung
author_rank: Newbie
timestamp: July 01, 2016, 02:57:51 am
content: Quote from: Keicar on July 01, 2016, 02:36:07 amMy intention is to identify the different current transformer connected to the circuit, so your advice is useful for me. I understand I can apply basic ohm's law to calculate the resistance. Can you also imagine what can happen if there is current following through the primary coil?

author: Keicar
author_rank: Regular Contributor
timestamp: July 01, 2016, 02:59:25 am
content: Quote from: heinthetaung on July 01, 2016, 02:44:07 amTricky if you want to perform this transformer ID operation while you're actually sensing current. I guess your induced current will be AC, though - and assuming the waveform is sufficiently symmetrical, will average to zero - so maybe you could still detect the effects of an applied DC current source by averaging out your ADC readings over a long enough time. Applying a DC bias current to the transformer isn't ideal, though - as you're pushing the core closer to saturation by doing that.

author: Circlotron
author_rank: Super Contributor
timestamp: July 01, 2016, 03:08:12 am
content: Quote from: heinthetaung on July 01, 2016, 02:56:41 amAlmost. Apply a high enough frequency so that the reactance will be much greater than the resistance. Then the value of the resistor will be the biggest effect on the circuit so it should be easy to measure.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2016, 09:31:50 am
content: Current transformers are usually made to work at mains frequency. So this can be used to do the measurement. If accessible one could measure an AC current, once in the original configuration and than with an additional parallel shunt. Otherwise one cold do an independent current measurement and calculate back with the current transfer ratio.