Why Magnetic Transformers Behave Differently Under Dimming — Voltage Drop and Flicker Explained

Magnetic low-voltage transformers continue to play an important role in landscape, architectural, and marine lighting systems. When paired with LED loads and dimming controls, however, their behavior differs significantly from traditional halogen lighting. Two common field symptoms are:

• noticeable voltage drop under dimming, and

• visible flicker or instability at low dimming levels.

These behaviors are not merely a function of “LED compatibility” but are rooted in transformer magnetic design, core losses, magnetizing current, and the way AC waveforms are shaped by dimmers.

1. Magnetizing Current and Core Operating Point

A magnetic transformer must first energize its core before delivering useful power to the load. The magnetizing current required to establish flux is relatively constant and does not scale proportionally as dimmer output decreases. When dimming is reduced, the available conduction window becomes shorter, yet magnetizing current still consumes a fixed portion of the delivered energy.

As McLyman notes, the transformer cannot deliver power until the core is sufficiently magnetized. Under phase-cut dimming (leading-edge/triac or trailing-edge/MOSFET), the conduction period is truncated. For LED loads, where total power draw is low, the magnetizing requirement may consume a disproportionately large share of the waveform — resulting in apparent voltage drop at the secondary.

This is why a magnetic transformer that performs well driving halogen loads can show degraded performance when driving LEDs at low load power.

2. Core Loss, Hysteresis, and Phase-Cut Waveforms

Magnetic transformers are designed assuming sinusoidal excitation. Phase-cut dimming deforms the waveform into discontinuous segments with steep rise times. The core must repeatedly transition through hysteresis loops with incomplete flux excursions. This increases:

• core loss,

• heating,

• magnetization inefficiency, and

• conduction threshold.

As the conduction window narrows at deeper dimming, the effective RMS voltage can collapse faster than the dimmer’s nominal dimming percentage suggests. In field terms, the lights “dim quickly, then shut off abruptly.”

3. LED Load Compatibility and Minimum Load Requirements

Halogen lamps present a pure resistive load with predictable current proportional to voltage. LED systems present an electronic, rectified, and often capacitive load, introducing several complications:

1. LED drivers require minimum input voltage to regulate.

2. LED drivers require minimum power to remain in regulation.

3. LED drivers may not draw current continuously over the AC cycle.

Under shallow conduction angles, the LED driver may drop out of regulation, recovering only once voltage crosses a threshold, producing visible flicker.

This flicker is not caused by the transformer alone, but by the interaction between waveform shape + magnetics + LED driver electronic timing.

4. Voltage Drop Under Dimming — Not Just a Copper Issue

Installers often attribute dimmed voltage drop to:

“The transformer doesn’t have enough power,” or
“The wires are too long.”

While resistive IR drop in copper does contribute, the dominant factor is magnetic excitation inefficiency at reduced duty cycle. The transformer secondary voltage may measure correctly at no-load, yet collapse once a dimmer is introduced.

From McLyman’s viewpoint, the problem is that transformer utilization improves at higher flux swings. Reduced conduction denies the core the flux excursion needed to operate in its optimal region, lowering output capability.

5. Minimum Load and Stability

Therefore, many magnetic transformer manufacturers specify:

• minimum wattage loading, and

• compatible dimmer types.

The minimum load ensures:

• the magnetizing current is not the dominant load,

• the dimmer has a continuous current path for triac latching,

• the secondary voltage stays within regulation.

LED loads alone often fail to meet these conditions, particularly below ~20–30% dimming, resulting in stepwise dimming or dropout.

6. Why Halogen Worked Fine and LED Does Not

For designers and lighting specifiers, the key difference is:

LED loads simply reveal behaviors that magnetic transformers historically masked.

7. Practical Recommendations for System Designers

To reduce flicker and voltage collapse:

✓ Match transformer VA to actual load (avoid sizing x3–x5 for “future capacity”)
✓ Use compatible trailing-edge (ELV) dimmers when possible
✓ Avoid driving magnetic transformers far below rated VA
✓ Add dummy loads where minimum VA cannot be achieved
✓ Prefer toroidal transformers for better waveform and thermal behavior
✓ Verify LED driver compatibility with AC magnetic sources (not all are)

For marine and outdoor installations, stable operation is especially important due to sealed housings and long service lives.

Conclusion

Magnetic transformer performance during dimming is not incidental — it is governed by magnetic excitation, hysteresis behavior, waveform conduction, and the interaction with LED electronic loads. As Col. McLyman emphasized throughout his work, magnetic components must be evaluated under realistic operating waveforms, not just ideal sinusoidal excitation.

LED lighting has simply exposed dynamics that were always present but previously unnoticeable.