convert voltage transformer into current transformer

[paridhi jain]

I am using voltage transformer and I want to use this as current transformer and transformer gives 9 volt ac and i want 470mv as output. what is combination of resistance

 

[spec]

I am using voltage transformer and I want to use this as current transformer and transformer gives 9 volt ac and i want 470mv as output. what is combination of resistance

Hello paridhi,

As underlined in your post above you say you want current but then you say you want mV This does not make sense.

Please tell what you want to do, give as much information as you can, however trivial. Are you simply reducing a 9V signal to 450mV if so we need to know what the load is.

 

[paridhi jain]

i want 470mv as output

 

[spec]

 

[crutschow]

So you want a 470mV output from a current transformer.
Do you mind telling us at what input current?

 

[spec]

So you want a 470mV output from a current transformer.
Do you mind telling us at what input current?

That would make it far too easy cruts.

 

[schmitt trigger]

The design of a current transformer is different from that of a voltage transformer. They are not interchangeable.

On a current transformer, the output current is converted into a small voltage (with emphasis on small) using a low value resistor.
If you increase the resistor value from the manufacturer's recommendation, linearity will suffer and will saturate at higher current levels.

 

[Diver300]

The design of a current transformer is different from that of a voltage transformer. They are not interchangeable.

On a current transformer, the output current is converted into a small voltage (with emphasis on small) using a low value resistor.
If you increase the resistor value from the manufacturer's recommendation, linearity will suffer and will saturate at higher current levels.

It's possible to use a voltage transformer as a current transformer, as long as none of the ratings are exceeded. I've converted a couple of voltage transformers into current transformers. On the last one, I cut away the secondary windings, and replaced them with a single turn, which became the primary. The winding that had been the primary, and was now the secondary, had about 2000 turns, so for 12 A input , I got about 6 mA output, to light an LED. As the secondary had been rated to 230 V, and I was running it at around 3 V, the core was far from saturated. The transformer had been rated at about 6 W, so would have taken around 25 mA input current, so 6 mA on that winding wouldn't trouble it.

The lack of answers from this thread is because the OP hasn't told us enough information, like input current, output current or the specification of the transformer being used.

 

[spec]

............

The lack of answers from this thread is because the OP hasn't told us enough information, like input current, output current or the specification of the transformer being used.

Post #4. Nobody reads me any more

 

[spec]

It's possible to use a voltage transformer as a current transformer, as long as none of the ratings are exceeded. I've converted a couple of voltage transformers into current transformers. On the last one, I cut away the secondary windings, and replaced them with a single turn, which became the primary. The winding that had been the primary, and was now the secondary, had about 2000 turns, so for 12 A input , I got about 6 mA output, to light an LED. As the secondary had been rated to 230 V, and I was running it at around 3 V, the core was far from saturated. The transformer had been rated at about 6 W, so would have taken around 25 mA input current, so 6 mA on that winding wouldn't trouble it.

Didn't know you could do that. You can get a standard current transformer from Ebay for next-to-nothing:

**broken link removed**

I used one in a recent design to turn on a vacuum extractor automatically when a 3D printer operated. https://www.electro-tech-online.com/articles/main-slave-controller.759/

 

[schmitt trigger]

Driver:
You are correct, one can MODIFY a voltage transformer into a current one as long as one respects the resulting flux density.

But the OP wanted to use a voltage transformer "as is"......or at least that is what I understood.

 

[MrAl]

Hi,

I have converted voltage transformers to current transformers also for various purposes. I used to use one a long time ago to monitor line current into an appliance. That was made from a wall wart with one or two turns making an extra winding for the current sense winding. The only requirement really is that there is room in the core window or else you've got to either remove the secondary or if current is low enough tap the secondary.

If the secondary voltage is low enough however i dont see why you could not use it as a current transformer depending on the level of the current, because after all transformers have a turns ratio that applies to both voltage and current.

An extreme example would be a 120v primary with 1.2v secondary. The step down voltage is 100:1 and the step down current (in reverse) is also 100 to 1, so a 1 amp current would lead to about a 10ma output current. Maybe that works, maybe it doesnt, depending on the application. But here we dont know enough to figure out exactly what is needed because not enough information was given as of yet.

 

[Tony Stewart]

The other reason Voltage transformers cannot be used for current transformers is input impedance.
You know about the N square relationship on V or I turns ratio but the inductance and mutual coupling also play a role.

Voltage transformers require the high impedance such that reactive current, also referred to as excitation current of the core is about 10% for rated
current for practical values. Voltage Turns ratio tends to be lower such as 230:9 , I am guessing in your case.

Current transformers require low impedance such that voltage drop on the primary is near zero and thus require a much higher turns ratio, typically of 1k to 5k depending on current range . So a single primary loop or straight thru wire is normally used.


For any current sensor, you need to always specify input V, I & f with min and max
This applies to AC or DC with pulses.