DIY transformer

[vindi93]

Hi, has anyone produced their own transformer at all?
i have the specs of a transformer but i am a little confused with it and i hope some one can clear it up.
i have attached the specs of the transformer its says ratio is 25:1, primary winding is <300ohms and secondary <0.75ohms. Is the resistance the wire resistance?? if so the primary being 300ohms i thought at 25:1 that would mean the secondary would measure 12ohms i.e 300 divided by 25. Or am i doing something wrong hope someone can shed some light

[TowerRadio]

This is the turns ratio, for every 25 turns on the primary there is 1 on the secondary.You should look at one of the many explanations available on Google. The resistance of the secondary will always be rather low while the primary resistance could be hundreds of ohms. Les

[Syrinx1]

The wire guage of primary and secondary are not the same, so the turns ratio will differ from the resistance ratio between primary and secondary windings. In addition, windings use a greater length of wire as they are wound concentrically from inner to outer winding.

[GrimJosef]

There are two types of 'resistance' to consider here.

The first, which is the one in the transformer spec, is simply the Ohmic resistance of the wire itself. They specify that the primary must be less than 300ohm and the secondary less than 0.75ohm. This resistance will be set by the lengths of the wires and their cross-sectional areas. The lengths will be determined by the number of turns, the bobbin dimensions and the way you lay down the windings (whether you start with the primary or the secondary closest to the core and whether you divide either or both windings into sections).

The second type of resistance is the 'reflected' resistance. This is the resistance 'seen' by the circuit connected to one side of the transformer which is caused by the actual physical resistance connected to the other side. The relationship between these two, neglecting stray effects, is actually the turns ratio squared, not simply the turns ratio. Taking a concrete example, if this were an output transformer and you connected an 8ohm load resistor across the secondary then the circuit on the primary side would 'see' 8ohm multiplied by 25 squared which would be 5kohm. Transformers only transform AC, so the primary circuit would only see 5kohm for any AC signals. Any DC there would just see the (less than) 300ohm Ohmic resistance.

There's no direct connection between the Ohmic resistances and the reflected ones. We can change the Ohmic ones by changing the wire cross-sections, but doing that will have only a negligible effect on the reflected one which is determined very largely by the turns ratio. People often seem to be misled into thinking that they can learn something about the reflected resistance by measuring the Ohmic resistance with a meter. They really can't.

VB

[Herald1360]

The spec as given is enough to enable a suitable transformer to be built by someone familiar with transformer design and materials properties. It's not, unfortunately, a "how to wind your own" recipe.


There are certainly forum members who could do just that but as already suggested, unless you really want to duplicate the original it would be much easier just to use a cheap modern amplifier module instead or even brew your own version of something that doesn't need any transformers. Historically, transformers in audio circuits were only used when there was no practicable cheaper alternative way to do things.

[PJL]

I assume this is for your Philips car radio and the driver transformer is an impedance matching transformer to drive a transistor output stage. The primary is connected to a valve so the resistance is not critical. Based on the spec of 25:1 and 6H primary inductance, a small valve output transformer should work fine.

The spec is enough to design the transformer, the inductance determines the number of turns, the winding resistance determines the wire thickness. Obviously it's more complicated than that as you need to take into account the core properties, length of average winding and so on.

[ms660]

So far as I can make out, primary impedance = 4.4k, secondary impedance = 7.04 Ohms.

Lawrence.

[vindi93]

Think i will give that a miss then, thanks for your input guys. And yes it was for the Philips radio. Is there a suitable modern transformer I could use like a doorbell transformer or something? Failing that I may just have to use a load resistor attached the anode of the EF98 and then work from there.

[PJL]

As said earlier, a small valve output transformer should work. Put a request in the wanted section.