Transformer winding -101

[gearheadgene]

Maybe this is a s silly question, but -

I'm working on transformer, ETD style with bobbin. The pins are numbered like an IC as follows (top view):

8 7 6 5
--------
| |
--------
1 2 3 4

The primary can go to pins 1-2 or 1-8. Both seem reasonable choices to me depending upon where the windings physically end. So, say the first cut assumes the start at pin-1 and end at pin-8 for shortest leads after winding. But, what if later on, it turns out the better choice would have been pin1 and pin2? So now the PCB is complete, and there's no choice but to have the extra lead length.

questions:
1) Is the extra lead length a problem?
2) Any conventional wisdom for pinout?
3) Is it ok to have primary, for example, on pin1 and pin8, then secondary on pin2 and pin3?

thanks

[infinia]

Consider bobbins pins PCB mounted creepage (8mm) between pri-sec and it makes sense to confine them to opposite sides to keep all pins usable.
I usually have 2 pri windings connected in parallel on the PCB, so say Pri-1 goes from 1-3 and Pri-2 goes from 2-4. I've never seen a problem with "extra lead length" on nonfractional windings.

[hesener]

Agree, it is more important to keep the creepage distance pri-sec and also keep the beginning and ending of the (primary) winding apart, as they will see high peak voltages so arcing may or may not be a problem. The extra lead length away from the core is not really an issue.

just my two cents...

[gearheadgene]

Originally Posted by infinia

Consider bobbins pins PCB mounted creepage (8mm) between pri-sec and it makes sense to confine them to opposite sides to keep all pins usable.
I usually have 2 pri windings connected in parallel on the PCB, so say Pri-1 goes from 1-3 and Pri-2 goes from 2-4. I've never seen a problem with "extra lead length" on nonfractional windings.

Oh. That didn't occur to me. But in my case, I don't think it'll work out. The bobbin I have chosen has 14 pins, which I have numbered like:
14 13 12 11 10 9 8

1 2 3 4 5 6 7

Mains side, I have the primary winding + 1 bias winding.
Isolated side, I have a pair of 40 Volt windings, and a pair of 7 volt windings.

In this case, that's 4 pins mains side, and 8 pins secondary isolated side. My plan was to remove pins 3 and 12. I think that makes enough room for the creepage/clearance? So the plan was to use 1 and 14 for primary, 2 and 13 for bias. skip 3 and 12. Then use the remaining pins for all my secondaries.

Based on what you've said here, I can arrange the pins any way that makes sense on the PCB.

Sound okay?

[infinia]

Sometimes you can double up on pins for windings that are connected either in parallel or series on the common connections. Also flying leads are not uncommon esp for higher secondary currents. No I don't see any advantage to your plan, bias s/b located on the same side as isolation IMO cutting bobbin pins is the very last resort for the desperate. Those ETD bobbins and cores have a lot of engineering built in, study it and use it.

[gearheadgene]

here's the bobbin drawing:
http://www.mag-inc.com/File%20Librar.../PCB3434FB.pdf

If I use your recommendation, then 7 pins are consumed by the mains side, and 7 for the secondary side. In this case, I need 4 pins for the mains side so would waste 3 pins. My method (cut pins) would waste 2 pins. Not much of a difference there - unless i really need that extra pin.

But - I also didn't consider combining pins (seems obvious now)! So, my design has at least one common pin and that saves one. If I get creative, I *may* be able to combine one more - but that'll take a bunch more work.

So okay - I'm going to see how this looks on the pcb