Current transformer output in Volts ?

Thread Starter

Externet

Joined Nov 29, 2005
1,873
Hi.
The norm is 100/1 or 100/5 Amperes. But if reading Volts is desired; how does it work?
Example : 100 Amperes trough a current transformer is wanted to show as 10.0VAC on a voltmeter. How ? Tailored ohms load ?
 

Ramussons

Joined May 3, 2013
1,274
A conductor carrying Alternating Current will have an Alternating Magnetic field around it.
The coils of the current transformer will generate a AC voltage from this Magnetic field which is measured by a Voltmeter. The Ratio of the Current - Voltage depends on the Physical dimensions and Turns in the current transformer.

The coils of the current transformer do not carry any current. If the coil of the current transformer carries any current, then it will experience a force to physically move it - aka AC motor. This is the principle used in Linear Electric Motors.
 

Ian0

Joined Aug 7, 2020
5,527
You put a resistor across the output (which is known as the "burden"). Voltage output is output current x resistance.
If you have a 100:1 current transformer, you will get 1A output for 100A input. You need a 10Ω resistor across the output to give 10V.
But don't do it that way, because the resistor will dissipate 10W.
Use a small value of resistor (say 1Ω) then use a 1V voltmeter, or an op-amp with a gain of 10.
Or find a non-industrial type current transformer with a smaller output current, so you need a larger value of resistance to get 10V but dissipate less power.
 

crutschow

Joined Mar 14, 2008
30,116
You need to keep the burden resistor low enough to keep the output voltage below the transformer's value specified in its data sheet.
Above that voltage the output may become inaccurate as the transformer can saturate.
 

Ian0

Joined Aug 7, 2020
5,527
The original intention of those 100/1 and 100/5 current transformers was to connect the secondary to an industrial type panel meter
https://www.hobut.co.uk/range-technical-specification/
with a 1A or 5A movement, and very low internal resistance.
Nothing like the fine moving coil 50uA sensitivity meters that used to be in precision analogue multimeters!
Not only would the magnetics saturate at a high output voltage, but it is unlikely that the transformer could deliver the power for any length of time.
If you are connecting to an electronics input (microcontroller etc) don’t forget that the CT can deliver a large current for a short period of time before it saturates. If the main supply is starting a motor or powering up a transformer, there may be several volts at very low impedance on the output of the CT, enough power to do a lot of damage if it gets where it shouldn’t be (I found out the hard way).
 
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