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kynix
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Join Date: Mar 2017
Location: America
Posts: 39

Does Winding a Choke Inductor Produce a Transformer?

06/01/2017 2:07 AM

Hello all. Thanks for taking up your time to read my post.

I obtained schematics and build instructions for a device which generates low current, high voltage (50-100V) signals. The high voltages are generated from a normal 5V USB supply.

The instructions specify to get a 390µH fixed inductor (the PT12133SL as the datasheet of http://www.kynix.com/uploadfiles/pdf8798/PT12133SL.pdf to be precise), and wind magwire around it 16 times. The ends of the inductor are connected to the high-voltage section, and the ends of the magwire are connected to the input signal (0-5V).

This implies that the inductor is acting as a transformer (assuming the instructions are correct). That makes sense, since in some sense there's a winding on the inside (the inductor's winding) and a winding on the outside of a magnetic core. But I've never seen a transformer constructed this way.

If this is in fact a legitimate way to construct a transformer, then (a) how can I compute the transformer's voltage ratio (given that I don't know how many turns are inside the inductor), and (b) how does this actually work electronically ?

EDIT: Here's the relevant part of the schematic (from EAGLE).

The part in question looks like a transformer, and is labeled TRANSFORMER in the document. The capacitors are on the "high-voltage" side of the circuit, and the "V+" and transistor on the left are on the "low-voltage" side of the circuit.

Can anyone help me ? I am very puzzled about this question. I do need your help.

thanks in advance

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Andrew Westman
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#1

Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 8:19 AM

It works as a transformer - a jury-rigged transformer, to be sure - but a transformer nonetheless. As you have control over of the number of exterior turns, it's easy to find the turns ratio: apply a sine-wave signal of known amplitude to the N exterior turns and measure the output voltage across the inductor's terminals. The ratio of these two voltages will be your effective turns ratio.

Vpri/Vsec = Npri/Nsec = n, the turns ratio

Ex: Npri = 16, Vpri = 5 Vrms

Nsec = ?, Vsec = 50 Vrms

Nsec = Npri·Vsec / Vpri

Nsec = 16*50/5

Nsec = 160 turns
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Andrew Westman
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#3
In reply to #1

Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 9:22 AM

Btw, note that I said 'effective' turns ratio, not 'actual' turns ratio. The reason for this is the degree to which the primary's magnetic flux is coupled to the secondary (inductor) winding.

Off-the-shelf transformers are designed to have a very high degree of coupling between the primary and secondary. You want as much of the magnetic flux generated by the primary to cut across the secondary winding.

However, where the coupling is reduced, only a fraction of that flux will be seen by the secondary. As far as this 'inductor'-style transformer goes, the coupling is reduced and so your effective turns ratio will deviate from the actual, physical turns ratio. The effective turns ratio is what you use because this determines how the transformer will behave in your circuit.

If you really need to know the number of physical turns, get another, identical inductor and take it apart.
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Andrew Westman
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#2

Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 8:42 AM

Now, looking at your circuit diagram: firstly, there is no rectifier following the transformer. If this is supposed to output a DC voltage, you're going to need a rectifier between the transformer and those capacitors because right now those three capacitors are simply shunting the AC current back to the transformer. The higher the AC frequency, the more those capacitors will act like a short-circuit across the transformer's output terminals. I strongly suspect that this is not the result you want.

Secondly, as it is, at some frequency that arrangement will resonate - it's basically an inductively-coupled 'tank' circuit (a parallel-resonant circuit). Depending on the losses in the inductor, if they're low enough the output voltage could go very high at resonance causing insulation breakdown as well as posing a shock hazard. One thing you're not going to get out of it is DC, however.

Now, if you're building a DC supply, here's where you put the rectifier:
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PeterT
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Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 10:10 AM

FYI, it is not unusual for transformers to be wound this way, with the primary and secondary wound concentrically on the same core. In 3-phase transformers, each leg of the core has the primary and secondary of that phase wound one on top of the other, with insulation in between. I don't offhand know which is typically inside or outside, but they are not wound with primary and secondary on separate legs of the core. Just a bit of general knowledge for you.

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kynix
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#5

Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 9:44 PM

thanks for your replies. I am so happy to hear that. thanks a lot.

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Andrew Westman
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#6
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Re: Does Winding a Choke Inductor Produce a Transformer ?

06/01/2017 9:48 PM

"Here's the relevant part of the schematic (from EAGLE)."

Who or what is EAGLE?
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