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The 10 volt secondary transformer will be used to ATMEGA328P while the 25 volt primary transformer will be used for logic gates.

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    \$\begingroup\$ Depending on transformer ratings, you may have some trouble finding a shared fuse that will adequately protect the 10 VAC transformer while allowing enough current for both of the transformers at full load. \$\endgroup\$
    – jpa
    Mar 24, 2021 at 15:00
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    \$\begingroup\$ As a terminology note, when talking about transformers the words "primary" and "secondary" will generally be understood to be referring to the "input" and "output" sides of the transformer. Your usage of "the 25 volt primary transformer" is understandable enough, but given that we're talking about transformers it could be confusing and would be better to phrase it another way (ie: the "main" transformer, the 25V transformer, etc). Otherwise, we end up saying things like "the primary transformer has a 25VAC secondary, but, like the secondary transformer, has a 220VAC primary" 0_o \$\endgroup\$
    – J...
    Mar 24, 2021 at 19:01

5 Answers 5

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Yes, you can connect transformer primaries in parallel - after all, EVERYTHING you connect to your house wiring is connected in parallel.

Incidently, the switch in your schematic is wired incorrectly - exchange terminals 2 and 3. As shown, the switch will have no effect, and the circuit will not get any power.

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  • \$\begingroup\$ Thank you for reminding me about the switch that was wired incorrectly. \$\endgroup\$ Mar 24, 2021 at 8:59
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No problem at all doing this; in fact the whole electrical distribution network is done in this way. With some care you can also parallel the output of the transformers (if they are in phase, same voltage and so on) to raise the available current.

If possible it could be better to use one transformer with two separate winding: it will cost less and primary losses will be somewhat reduced.

On the other hand I've seen pro audio equipment with two separate transformers to completely avoid interference between the logic and the analog section. A bit extreme but it works.

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Yes. Compare that to your house, all the loads and devices are separately plugged in so they are parallel.

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If a parallel combination of two transformer primaries is driven by a low-impedance source (one that supplies a voltage that is relatively unaffected by the load imposed by the transformers), and there exists only a single-point connection between the two secondaries, the behavior of the overall system may be modeled by evaluating each transformer separately as though it were an ideal transformer, adding some "uncertainty factors" to accommodate the difference between ideal transformers and those that are actually built, and then adding the current demands.

If a series combination of two transformers is driven by a high-impedance current source (one that supplies a current that is relatively unaffected by the voltage drop across the transformers), the behavior of the overall system may likewise be modeled by considering the transformers separately and combining their effects.

It's possible to wire transformers in parallel or series in other scenarios that don't meet those criteria, but modeling the behavior of such systems is much more complicated. Fortunately, your schematic appears as though it likely fits the low-impedance-source scenario which allows easy analysis of parallel-connected transformers.

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Yes, You can connect them in parallel or even series.

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    \$\begingroup\$ It won't hurt anything to put them in series, but the circuit won't work as expected. \$\endgroup\$
    – Mattman944
    Mar 24, 2021 at 7:05
  • \$\begingroup\$ It was general comment about transformers that they CAN be connected in series. \$\endgroup\$ Mar 24, 2021 at 13:05

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