Make and use your own current transformer

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I have seen a number of topics on current transformers, but I would like to make one myself.

 

for a 1 off project I want to make a current transformer that can measure up to 20A on the 50Hz mains network.

initially I was looking at a 50mA output with 20A input, but then I end up with 400 windings which is a lot of work.

 

I would if anybody here has ever tried a 2 step aproach.

and if someone has ever tried it this way with what result.

I was thinking of 2 steps. first 20windings on the primary. that would give me 1 Amp on the secondary output.

first question now would be, can I short circuit this winding set or do I need a resistor ? for instance 10E that would give me 10V.

now I want to take that output and feed that again through 20 windings.

this then should result in 50mA current with 20A input. putting a 10 Ohm resistor over that would give me 5V with 20Amp primary.

 

Now I have never worked with current transformers, but what will be the output of the current transformer?

My initial guess is a 50Hz sinewave.

 

Can I feed this through a normal diode bridge rectifier?

As these should pass the current only in one direction I guess that I should be able to use a full bridge rectifier to convert the AC signal to DC.

Then add a capacitor parallel with the 10 ohm resistor to get an average current.

 

Has anybody had experience with doing it this way?

 

regards

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Seems that any build or buy decision is based on price and specifications. Only reason to build a custom unit is if a commercial off the shelf unit is not available with the desired spec. It is probably never cheaper to build something that is cots. Get a current transformer from digikey/mouser/farnell. Experiment. Only Artists hand build one of something at already exists.

 

Imagecraft compiler user

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Below is a web hit-list of several PDF documents that might be a great starting place for gaining an understanding of current transformers.

 

Just remember this...

 

If the primary is say, one turn of wire and the secondary is say, 400 turns of wire, the secondary voltage will be quite high.

 

Having said that, you will need to also design in some form of protection so that if the load resistor connected across the secondary ever opens up, it doesn't snuff the circuitry performing the scaling and measuring of the apparatus.

 

So then, if the primary is 1 turn, the secondary is 400 turns and, if the primary voltage is say, 5.0 VDC, then the secondary voltage will be something on the order of 4,000 volts - open circuit.  Needless to say, you should also employ good personal safety precautions when working with potentially high voltages.

 

The good stuff:

https://www.google.com/webhp?sou...

 

EDIT TO ADD:

Personally, I would consider using a LEM or a HALL effect current transducer, before considering an actual wire-wound ferite core current transformer.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

Last Edited: Sat. Oct 10, 2015 - 07:41 PM
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You can make a current transformer from a 'normal' transformer by passing the current carrying conductor through the core. The primary becomes the secondary in this instance. Depending on the actual transformer, you would have to infer the number of primary turns.

I can't see the problem with hand winding 400 turns of very thin wire. You only need a plastic bobbin - no iron or ferrite core needed.

As Carl mentions, you always need to ensure the 'burden' resistor is in circuit otherwise you can get high voltages.

You do realise you can buy these current transformers out of China for a couple of dollars? There's also hall effect sensors that give you a 0-5V output for a few more dollars.

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Thanks for the feedback.

I have been playing a bit with some inductors I has lying around and it indeed seems to be a hassle to get it all right.

so that experiment is over ( was interesting though)

 

Just before it was sleepy tuck tuck time I found a potential intersting chip the "acs712"

It seems to be mains connectable (2KV+ isolation with mains being 230V overhere)

Just strangely it does 66mV/A with 30Amps max, then I get to 2V. So it starts to look like it is an AC output.

As boards seem to be cheaply available this look slike an interesting chip.

hope to have some time to dig into the spec a bit further to see if it full fills the need.

 

thanks so far for all the feedback.

 

 

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Yes Marcel, modules are available quite cheaply.

 

http://www.ebay.com/itm/ACS712-2...

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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I know Ross,

just need to check that I can actually connect it to mains (being 230V ) without the fuses blowing or more that magical smoke (or worst) starts happening when I do so.

 

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oh yes... or no. The creepage distances look a "touch" too small for my liking.

 

Ross McKenzie ValuSoft Melbourne Australia

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Ross, that was also what I am a bit worried about.

Interestingly enough the datasheet states that there is a 354Vpk isolation resistance.

If I take 240V and take the max from that I get to 340V so that should be enough, but just.

 

just stumbled up on the ACS758 just looking at the package it is in makes me think it suits my needs a bit better.

Only disadvantage I see at the moment is that it goes to 50Amp and the load I tend to check is never more than 16 (or the fuse will pop) , but potentially even far less, more like 1A and even that seems high.

 

 

 

 

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With the acs devices, they are quite noisy, so resolving small currents on a 50A device may be challenging.

There's these available from a number of Chinese suppliers and there's some Arduino apps around on the interwebs.
http://www.electrodragon.com/product/yhdc-non-invasive-ac-current-sensor-100a30a/

Last Edited: Mon. Oct 12, 2015 - 08:38 AM
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valusoft wrote:

Yes Marcel, modules are available quite cheaply.

 

http://www.ebay.com/itm/ACS712-2...

 

Cheers,

 

Ross

 

Have a look at the tracks on the bottomside of the PCB, the via's, the terminals ...... and imagine then what will happen @ 20A. crying

A GIF is worth a thousend words   She is called Sylvia (2018), lives at Mint18.3 https://www.linuxmint.com/

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Hi Nard,

 

you might be right.

need to confirm, but the tracks are on both bottom and top, so about double the with what you think to see, but running 30Amp through a PCB track might indeed call for much wider tracks than they have put on that little board.

This is getting more interesting by the day :)

 

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I have a current transformer. Its the one I am using right now. I had one last week, but that is my former transformer.

 

Imagecraft compiler user

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Bah-DUM-bum---tsssshhh!!

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

Last Edited: Tue. Oct 13, 2015 - 05:14 PM
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Isn't the veal worth mentioning?

 

Ross McKenzie ValuSoft Melbourne Australia

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Tip your waitress!

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Thanks! I'll be here all week!

 

Imagecraft compiler user

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Vicious!