Okay, let's break down what could be happening with terminals 7 and 8 on your industrial control transformer, given the 15V you're seeing between 2 and 7 when energized.

Possible Explanations for Voltage on Terminals 7 & 8

Based on your description and the lack of information in the documentation, here are the most likely scenarios:1.

Auxiliary Winding (Most Likely)

This is the most common and your initial hunch is probably correct. Many control transformers do include a separate, smaller auxiliary winding specifically to provide a lower voltage, often around 12-24VAC. This lower voltage is used for powering things like: PLC inputs (e.g., 24VDC sensors, etc.) Small indicator lights Relay coils (especially smaller relays) Timers Other low-power control circuit componentsThe voltage you're seeing (15V between 2 and 7) could be the result of just being energized. The auxiliary winding, it's output is rarely rated for high currents and as you loaded it, it would sag down to 12V (depending on the load of course)

Why it's not documented

Sometimes, this auxiliary winding is considered a "bonus" feature and not always prominently featured in basic documentation. It might be mentioned in a more detailed datasheet or specifications, or just not covered at all since it's an optional/flexible feature.2.

Taps on Secondary Windings

Some transformers have taps on their secondary winding. These taps are used to increase or decrease the output voltage by a small amount. If terminal 7 is a tap, energizing the windings could indeed result in 15V between terminal 2 and 7.3.

Capacitive Coupling (Less Likely, but Possible)

It's possible that the voltage you're measuring is due to capacitive coupling between the primary and secondary windings. Transformers have inherent capacitance between windings. However, this is usually a very small voltage and very low current. While 15V is a bit high for capacitive coupling alone, it's not impossible, especially if the transformer is old or the insulation is degraded. The key here is that the voltage would collapse almost immediately under even a very light load.4.

Autotransformer Configuration (Less likely with Industrial Control Transformers)

While less common with industrial control transformers designed for isolation, sometimes transformers are wired as autotransformers to step voltage up or down. If this were the case, and terminal 7 was a tap, you might see a voltage difference. However, this configuration usually means that the primary and secondary are not isolated, which is a safety concern in many control applications. Industrial control transformers are designed for isolation.5.

Manufacturing Defect (Least Likely, but Possible)

It's always possible there's a manufacturing defect in the transformer, such as a shorted turn in one of the windings or unintentional winding crossover. This is unlikely but should be considered if nothing else seems to fit.

How to Investigate Further


Manufacturer's Website/Datasheet

The best approach is to find the manufacturer's website and look up the exact model number of your transformer. Download the detailed datasheet. The datasheet should provide a wiring diagram and specifications that will clearly identify the purpose of terminals 7 and 8.2.

Load Test

This is crucial! Connect a small resistor (e.g., 100 ohms, 1/2 watt) between terminals 2 and 7. Measure the voltage again. If the voltage completely collapses, it's likely capacitive coupling. If the voltage drops, but remains relatively stable, it's probably an auxiliary winding. Measure the voltage between terminals 7 and 8. You will probably see 12 - 24V. If this is the case, make sure that the voltage between 7 and 8 is isolated from the primary terminals. Use a multimeter to measure continuity between them when the transformer is de-energized.
3.

Continuity Test (Transformer De-energized!)

With the transformer completely de-energized, use a multimeter in continuity mode to check the resistance between: Terminals 7 and 8: You should see some resistance (not an open circuit). Terminals 7 (or 8) to any of the primary terminals (e.g., 1, 3, 4): You should see infinite resistance (open circuit). This confirms isolation. Terminals 7 (or 8) to the secondary terminals other than 2: You should see infinite resistance (open circuit). This confirms isolation.4.

Visual Inspection

Carefully inspect the transformer for any obvious signs of damage, such as burnt windings, loose connections, or cracked insulation.

Important Safety Considerations



Work De-energized

Always disconnect power to the transformer before performing any tests or making any connections.

Isolation

Industrial control transformers are supposed to provide isolation between the primary and secondary circuits. Never assume isolation is present. Test it with a multimeter as described above. If there's any continuity between the primary and secondary, do not use the transformer as it presents a serious shock hazard.

Fuses

Use appropriately sized fuses to protect the transformer and the circuits it powers.

In summary: I'm 90% sure that terminals 7 and 8 are for a small auxiliary winding. Finding the datasheet is key! Let me know the model number if you have it. Flag for review