Transformer coils in contact?

[antknee]

I was always under the impression that transformer primary and secondary coils had to be physically separated. I just pulled one apart and the first coild was simply wrapped under the second.

How can mutual induction take place if it looks like current could just pass from the first to the second coil like a continuous circuit?

Thanks.

 

[MikeMl]

If you think primary and secondary need to be separated, check out Bifilar windings

 

[antknee]

*Scratches head*. I don't think I even want to understand how that can happen lol. So they don't need to be separated. Thanks.

 

[mneary]

They don't need to be on separate bobbins, if that's what you mean by 'separated'. To function as a transformer they only need to be insulated from one another, but depending on the voltage this insulation may only need to be a thousandth of an inch or less. In general, the closer they are, the better the coupling.

Coils are often placed on separate bobbins to achieve a physical isolation required to meet safety requirements.

 

[antknee]

Ah i see, thanks. The transformer had very thin wires and was very small, both inductors were on the same bobbin. The 10uH primary had a couple of dozen of turns and must have been separated from the secondary, I didn't put it under a microscope or take a pic to zoom in so I couldn't see that.

I'm also thinking the wire gauge was too thin for my purposes. It is too thin to get anything reliable with some calipers even.

If i needed a primary of up to 0.5A and secondary up to 50mA what gauge would you recommend?

Thanks.

Antknee.

 

[Hero999]

It depends on the power and frequency requirements?

 

[transistor495]

The 10uH primary had a couple of dozen of turns and must have been separated from the secondary, I didn't put it under a microscope or take a pic to zoom in so I couldn't see that.

Antknee.

It seems to me like you're not aware of the enamelled copper wire that is used for winding. Thin layer of enamel is painted on the copper wire so that they're normally isolated on each layer of winding, so an explicit separation between primary and secondary is not actually necessary, but they do.

 

[antknee]

It depends on the power and frequency requirements?

I'd be looking for 200KHz maximum and minimum 20KHz. I have read that I need to consider the minimum frequency I want to pass through the primary. So I do the calculations

Impedance = inductance x 2 x Π x F to get the minimum inductance

I'm not sure of the power requirements I'd need. I was thinking the current was the main determinant of gauge size, too thin and its a fuse. So 0.5A and 50mA would be the maximums i'd need to work with.

I was hoping to use this transformer with an audio amp output 4 ohms, with the above calculation it looks like I need inductance minimum of around 20uH. So this one with 10uH primary and 1900uH secondary looked ok. Until I pulled it apart. Now the wires look too thin to be of much use. I've probably missed something else, I can kind of tell its there but I'm not sure what it is! I won't be able to test it for a few days as I'm waiting on a power supply.

I have taken a pic of the transformers I have. They are impedance matching transformers for a piezo buzzer of some kind, rated at up to 125KHz. The datasheet doesn't mention the power requirements. It simply says, 125KHz, 2.2mH minimum, Q = 80 and turns ration 1:20.

It seems to me like you're not aware of the enamelled copper wire that is used for winding. Thin layer of enamel is painted on the copper wire so that they're normally isolated on each layer of winding, so an explicit separation between primary and secondary is not actually necessary, but they do.

I wasn't aware of that no! It's very clever, and very simple. In this case it is I think just plain wire, could be wrong, i've taken a pic.

Thanks for your comments.

 

[Hero999]

Use Ohm's law to work out the power.

If the primary impedance is 4Ω and the current is 0.5A the power will be I²R = 0.5²×4 = 1W.

That transformer looks an IF can from a radio which won't be large enough, you need a ferrite E core used in a SMPS.

Is 50mA the secondary current? If so, the turns ratio should be 1:10.

 

[transistor495]

In this case it is I think just plain wire, could be wrong, i've taken a pic.

No, they're still insulated copper wires! You can do an image search on google for enamelled copper wire - Google Search

 

[antknee]

Hi Hero,

Yes I can see this transformer isn't up to the job. It was logged on Farnell as a piezo impedance matching transformer without much detail, it was worth a look for £1.40, but not much else.

Hi transistor495. I took a picture of the wire and zoomed in. It is indeed insulated copper. Very thin stuff!

Thanks both.

 

[Hero999]

It looks like a piezo microphone transformer.

By the way is the 0.5A RMS or the peak value?

 

[bychon]

Audio frequencies are not 20 khz to 200 khz. They are 20 hz to 20 khz.
At 1000 circular mils per amp,
Proper wire size for .05 amps and .5 amps are...AWG 33 and AWG 23. AWG 30 is the size my transformer guy said would not be a nuisance with breaking. AWG 22 might be easier to find and buy than AWG 23. The point is, do not use smaller wire than suggested. In American Wire Gauge, bigger number means smaller wire.