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Old 3rd Dec 2018, 10:43 am   #21
kalee20
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Default Re: Current transformer.

Quote:
Originally Posted by Diabolical Artificer View Post
The 90 deg...looking online most commercial current tfmr's sense I with a wire (primary) though the middle, so turns are at 90 deg and not parallel to the primary.
Just imagine a toroidal transformer, but instead of a wire passing straight through the middle (single turn), you want to increase sensitivity five-fold, so you pass that wire 5 times through the middle. You can immediately see that these turns are parallel with the output winding! (I think the bold underlined you mean secondary... otherwise in this text snippet you have two primaries)

Quote:
Originally Posted by Diabolical Artificer View Post
Why the need for a burden R? Is it because if reading with just a voltmeter a current tfmr is effectively shorted?
No, it's because if reading with just a voltmeter a current tfmr is effectively open-circuited.

A ideal transformer - whether voltage transformer or current transformer - has a really big inductance, so primary current is almost all due to secondary current. If the secondary current is zero, primary current is virtually zero too.

If you open-circuit a 1:100 current transformer, secondary current is zero, so primary current is also zero. This means that if you are trying to use your current transformer to measure the current to, say, an electric fire from 240V mains, the current to the fire will be nearly zero so virtually all the 240V will appear across the CT primary. So secondary voltage will be 100x bigger, ie 24kV. Something will give up.

If you short-circuit your CT, or connect an ammeter or burden resistor dropping say 2V, then primary voltage will be 100x less ie 20mV. So with the mains at 240V, the fire will see whatever is left, ie 239.98V.

TrevorG3VLF made a good suggestion - use a toroidal mains transformer, with secondary rated to carry the current you want to measure - 'backwards.' Unless you are looking for accuracy better than 1%, you don't need exotic cores. You can fine-tune accuracy by adding or removing a few turns.
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Old 3rd Dec 2018, 10:43 am   #22
TrevorG3VLF
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Default Re: Current transformer.

If you are measuring mains frequency, then ferrite is not good. As I say in #4, use a normal mains transformer with a sufficient current capability. You do not need to wind one.
A 6v 6A transformer would be suitable. The burden resistor would be reflected into the primary with a reducton of 40 * 40. The rectifier will add some drop so will give a little non linearity.

Kaylee20 #21 has reiterated this in a better way than I can manage.
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Old 3rd Dec 2018, 11:33 am   #23
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Default Re: Current transformer.

If your meter is 10mA FSD and your max current is 5A you simply need a current transformer (on a suitable core) with 5/0.01=500 turns on it. Connect the current TX to the ac terminals of a bridge and the meter to the dc output. Nothing else required. The current transformer will develop whatever secondary voltage is needed to drive 1/500th of its primary current in its secondary circuit (hence the need for a load of some sort to stop the secondary voltage going high- in principle it could go as high as 500 times the available primary voltage!).

To calibrate everything to make sense of the scale on the meter, start with a long tail on the end of the winding and add or remove turns until FSD corresponds with 5A as measured directly with a believable DMM in series with the wire from the variac going through the TX core as its single turn primary. Don't forget to switch off before you break the TX secondary circuit to make adjustments!

Primary/secondary wires parallel or right angles? If you use a toroid, where the primary wire passes through the toroid it will be parallel with the secondary (or near enough).

To get optimum magnetic field coupling between wires they need to be parallel to each other- if they just cross at right angles there is in principle no coupling.

Edit: Kalee20 beat me to it- took me a while to compose my missive!
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Last edited by Herald1360; 3rd Dec 2018 at 11:38 am.
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Old 5th Dec 2018, 1:42 pm   #24
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Default Re: Current transformer.

Thanks chaps, will see what I have in the sheds.

Andy.
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