Never though transformers would be so confusing to many - I found about a dozen similar questions here. Adding mine to the pile.

Here are the datasheets for 3 Transformers: 750313975, 750313445 and 750311088. transformers They all have 2 secondary coils, but the drawings are different. Is that supposed to reflect the physical coil arrangement on the core somehow?

The second says it "allows" to parallel two secondaries. But the pinout is exactly as the first one, so what's the difference?

More than that, the third one says "customer to tie terminals on PCB", which I understand as we must tie them together. A rather weird requirement.

Finally, the wire diameter or current rating is nowhere to be found. Should I assume that it would be at least close to saturation current?


To clarify the question: Is there any specific convention for drawing coils I am not aware of?

All my life I used to draw transformers as the one on the first picture, with primary and aux coils on the left, secondaries on the right, all lined up along the core lines.

  • \$\begingroup\$ The middle one, judging only by the picture in your question, does not appear to have two secondary windings. \$\endgroup\$ – mkeith May 28 '18 at 23:30
  • \$\begingroup\$ That's what threw me off. But if you click at the datasheet link I provided you'll find separate turn ratios for two coils. \$\endgroup\$ – Maple May 28 '18 at 23:33
  • \$\begingroup\$ I see what you mean. OK, watching this question with interest. \$\endgroup\$ – mkeith May 29 '18 at 2:05
  • \$\begingroup\$ well, the terminal numbering on the second one implies that the single coil as drawn is either two coils already in parallel (they just simplified the drawing a bit), or possibly one coil with 2 sets of terminals. (As these appear to be some kind of signal/data isolation device it could also be that they are allowing for a single primary to drive two isolated loads, albeit of higher impedance?) \$\endgroup\$ – dmb May 29 '18 at 4:42
  • \$\begingroup\$ @dmb: according to manufacturer this transformer is specifically designed for LT8302 flyback converter. They have entire MID-FLYLT line of them. But since they provide separate turn ratios for secondary coils I don't think they already tied. My guess was it reflects winding method somehow, e.g. one coil after the other vs two wound simultaneously. \$\endgroup\$ – Maple May 29 '18 at 6:21

If secondaries have the same output voltage (i.e. number of turns) you can either parallel them (for twice current/half output Z - which boils down to the same thing) or put them in series (twice the voltage, twice the output Z).

The only thing is to take care of phase - if you parallel them so that the phases are fighting each other, they are likely to get hot (on a power transformer) and it might get nasty. (I've seen this as a wiring error on the secondary of a small mains transformer and it did some quite odd things, but no damage occurred. On something with more VA I would not be so sure.)

Your third example - the note about tying the terminals implies that the manufacturer expects it to be used with paralleled secondaries (as that is making connections as described would do).

The voltage/current ratings are probably the maximum rated for each one, which I expect that that would be somewhat short of saturation, as any coil becomes a lot less efficient when saturated.

  • 2
    \$\begingroup\$ You explained how transformers work, which I kinda knew already. The question was - why the drawings of seemingly identical parts are different. Is there any physical meaning behind this and whether it explains the requirement to tie coils together. I apologize if I did not phrase the question properly. \$\endgroup\$ – Maple May 28 '18 at 23:08
  • \$\begingroup\$ OK. I'd say - unless there is some obvious difference between the devices that seems relevant - probably not. \$\endgroup\$ – dmb May 29 '18 at 4:40

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