Transformer design / construction

Thread Starter


Joined Jan 3, 2011
I'm going to try making a power supply similar to the example in the LT1509 datasheet (different output voltage) and I have some questions about the transformer and the way they describe it. The full schematic is attached.

First: the primary shows two coils in series labeled as 17 turns tri-filar.

Screen Shot 2021-01-01 at 3.32.28 PM.png

Question 1: any idea why it's shown as two different coils? You wouldn't layer half of the primary winding on top of the other half, would you? I don't know why else it would be drawn that way.
Question 2: Tri-filar means the three wires are in series, right? So 17 turns of tri-filar is really like 51 turns of wire? This online calculator gives me about 109 turns when designing for 3.1mH (noted in the datasheet) and using an AL value of 262 nH (which I got from a datasheet for a core of the type they note). If I say two coils of 17 turns each of tri-filar is really 102 total turns, that gets pretty close to the calculated turns.

Second: the secondary says 7 turns 0.9" x 0.005" Cu.

Screen Shot 2021-01-01 at 3.56.09 PM.png

Question 3: what does this mean? It makes me picture a 0.9" wide sheet of enameled, 5-mil thick copper, which seems unlikely.