Transformer Help - 2 primary 3 secondary?

Thread Starter


Joined Aug 20, 2012

I am confused. Is there a standard for the image above that I am missing.
I normally don't work with transformers - I work primarily with embedded systems. Thanks for the help.
I am used to see 3 on primary - 3 on secondary.
Also - this is a custom part made in 2002 for the company I work for. I can't find specifications or data. The documentation is very poor. All I have is the image above, schematic (only view-able by CADSTAR viewer). The "datasheet" has barely anything else besides this image!

Sorry to be so vague - wish I could provide more info.


Joined Nov 30, 2010
Dual primarys are common enough. Center tapped or separately wound, equal, secondaries are too. The third winding brings us into the realm of custom made stuff like power transformers for vacuum tube guitar amplifiers. Without the whodunit, you are pretty much threaded in a tapering fashion.


Joined Jun 26, 2012
The primary is likely designed to work with different input voltages, depending on how its wired. For example, for 120VAC you would wire them in parallel for 240V in series.
#12 beat me to it but the secondaries may be for 3 different supply voltages/currents or designed to be combined. If its a custom, its likely the former. Depends on your circuit.

EDIT: original series/parallel mix up.
Thanks to subsequent posters.
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Joined Jul 18, 2013
What was the original function of the transformer?
For a normal 1ph power supply transformer the primary winding options would be a continuous winding with various taps.
This multi winding one you show smacks more of a special purpose type, other than Mains type.
If unaware, the dots are the start point of each winding to indicate phasing.


Joined Feb 19, 2010
For example, for 120VAC you would wire them in series, for 240V in parallel.
This part did not makes sense to me.

Assuming that we are talking about primary side (on the right), and each winding is 120VAC. In series they would give 240VAC because you are doing 120+120=240VAC. In parallel they would give 120 VAC.


Joined Oct 2, 2009
1. Take an Ohmmeter to the primaries...
2. Measure DC Resistance of the two primaries
3. Select the primary with the highest DCR.
4. Connect a 120V Variac to the primary with the highest DCR.
5. Slowly bring up the voltage to (say) 25Vac.
6. Measure the AC voltage across the other windings (both primaries and secondaries)
7. Put an AC ammeter between the Variac output and the primary winding. Crank up the voltage toward 120Vac while monitoring current. Stop if the input current exceeds ~1Aac. If you make it to 120V without excessive current, that is good indication you are connected to a 120V primary winding..., at which point you may repeat the Vac measurements at the other windings.

8. Post the results, and we can speculate what the tranny might be good for.
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Joined Nov 30, 2010
2. Measure DC Resistance of the three primaries
the way it is normally portrayed is the primary(s) are on the LH side.
C'mon guys. Have a little flexibility. The TWO primaries are labeled on the right side of the drawing. If I just climbed out from under a dashboard, after twisting half a dozen, 8 mm, hex head screws, upside down, and left handed, you can read a schematic backwards.:rolleyes: