[Radio Wrangler]

Both matter.

n a normal transformer, the turns ratio sets the voltage ratio, and it's easy to just leave it at that, but the inductance of the primary can be thought of as shunting the primary of a perfect transformer.

Get enough inductance and the magnetising current is trivial, you just see whatever current the load on the secondary transformed by the turns ratio.

Get too little inductance and it starts to look like a short across the input. So you need to design to get enough inductance for the lowest frequency in something like an audio output transformer or a microphone transformer.

Go for too much inductance and you run into increased problems with internal capacitance in the windings and self-resonance.

So, for an ordinary transformer, there is a sweet spot, but it's wide enough to leave you some wiggle room.

HOWEVER, this application is not an ordinary transformer. It is the inductor of a boost converter, and then on top of that, it has an overwind to increase the output voltage further.

Primarily (oops, pun!) you have to get the primary inductance right for the boost converter to be able to run the current up to the right value in the time set by the choice of operating frequency. If you have the right cores and get any gapping called for, the inductance should come out OK. If you're uncertain about the cores you have and not sure about gap spacer thickness, then you need to measure inductance. This sort of inductor often foxes those little LCR meters. They're OK on C and R, but L is a bit harder to measure. So the sig gen, resistor and scope approach is less likely to be confusing.

The turns ratio to the secondary will have a bit of leeway.

If the core chosen has an inductance factor of 5815 nH/turn, multiply this value by the number of turns squared, and that's the inductance you should have had without the gapping spacer. That spacer will bring down the inductance rather dramatically, but not as far as 10nH. That has to be a mixture of shorted turns, which bring inductance crashing down, plus the errors in measuring such low inductance.

There is a root cause of all this difficulty. Designers of magazine construction articles really ought to describe the design choices and figures for any custom transformers involved and take people through the numbers of turns and the calculations of the magnetic effects of adding gapping shim. It's all well and good having kits available, but their article loses its value when the kits go out of production and people may still be interested in the thing. Besides, doing such work out in the open might just allow people to LEARN something from the article. I don't suppose it's trying to profit from a captive audience for kits, I think it's publishers wanting to fit things in a given page count, even if it ruins the article for many people... besides, all that magnetics stuff is boring... just calculations and explanations and it takes up the space they could put that DIY tamagochi article in!

Pet rant!

David