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Winding a transformer (picture included) - Questions

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Russlk said:
It is common practice to put the primary first and the secondary on top, I don't know why, it should not make a difference, performance wise.

I believe that's because most transformers are of the stepdown kind, where the primary has higher voltage (and therefore more turns) than the secondary. Winding on the outside always uses up more wire length than winding on the inside for the same number of turns. If the primary were on the outside, you would need a much longer wire than if you wound it inside. If the secondary were outside, the need for additional wire length would be less since the secondary has fewer turns. Of course, the opposite would be true in a step-up transformer.
 
I don't know audioguru but that certianly looks like a ferrite core to me.
 
Sorry I didn't know there had been replies when I mad the previous post.

FusionITR said:
BTW, the transformer seems to be working perfectly now. It just seems like when the frequency is set too low it causes a a short circuit on the primary windings, probably due to the inductance of the primary winding being too low.
That's called core saturation. The core becomes fully magnetised (saturated) and the inductance drops allowing a large current to flow. To avoid this you need to use a larger core or a lower voltage and more turns.
 
berniedd said:
I believe that's because most transformers are of the stepdown kind, where the primary has higher voltage (and therefore more turns) than the secondary. Winding on the outside always uses up more wire length than winding on the inside for the same number of turns. If the primary were on the outside, you would need a much longer wire than if you wound it inside. If the secondary were outside, the need for additional wire length would be less since the secondary has fewer turns. Of course, the opposite would be true in a step-up transformer.

Why would a step-up transformer changes all that to the opposite? One still got one low voltage winding and one high voltage winding and your reasoning for length of wire still applies.

Therefore voltage cannot be the only reason of where the windings are placed. There are other factors involved.
 
Hi,

When i do transformers, which is not very often these days, i put the
primary on first.
I have always thought this is to check the turns-per-volt is as hoped.
I prefer to put windings side by side where i can, but this is not
always practical.

berniedd,
raises a good point about the length of wire used, I had never
considered that before. I suppose it is of little matter to a home constructor,
but to a transformer manufacturer it might well make a noticeable difference
to the yearly costs.

Here is a pic of my last home made transformer. It's about two years
old now, its job is to light a small filament light when an associated
water heater is heating the water.

Regards, John :)
 

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I've wound switchmode transformers on the cores of inductors from computer powersupplies, with no issues...

wait, maybe I drempt that :p

let me go see if I can find one... haha
 
Eblc1388, I believe the idea is to wind the one with the most number of turns on the inside to save the most on wire length. That would be the secondary in a step-up transformer.
 
I normally wind the secondary on first when I'm making a fluorescent tube inverter transformer. Nornally mains transformer are wound on a double section bobbin, one for the primar and another for the secondary, this improves isolation by reducing the coupling capacitance and increases dialectric strength of the insulation between the primary and secondary this improves safety.
 
No-one seems to have mentioned the magnetic circuit! The pictures all show the core halves slopping about loose. It is VITALLY IMPORTANT that they be clamped, taped or glued securely together. A gap the size of the one displayed will reduce the inductance by a factor of a few hundred!
 
berniedd said:
Eblc1388, I believe the idea is to wind the one with the most number of turns on the inside to save the most on wire length. That would be the secondary in a step-up transformer.

Not according to the many step-up transformers I have seen. All of them have the excitation winding(i.e. low voltage winding) on the inside and then the high voltage winding on the outside.

As I said, there are other factors to take into account regarding placement of the windings. Leakage reactance would certainly be one of them.

spuffock said:
A gap the size of the one displayed will reduce the inductance by a factor of a few hundred!

A gap between cores is often required and is of vital importance in a switching power supply transformer. Do not close any gaps in existing working transformers to try to make the magnetic circuit "more efficient".
 
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Depends on what the transformer is for. To use for 100Khz with no DC component, the self inductance should be high, so there should be no gap. For applications where there is a substantial DC component, such as switchmode supplies, the gap is required to prevent saturation disasters, and is fun to adjust by trial and error.
But, as just mentioned by eblc, dont adjust anything already working!
 
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