Practical case: Measure transformation ratio with a tester

Materials

• 1× T1: Small transformer (unknown ratio; isolated), e.g., 230 V:12 V or 120 V:12 V
• 1× AC1: Isolated low‑voltage AC source, ~6 VAC (function generator or 6 VAC wall adapter)
• 1× DMM: Digital multimeter with AC V and Ω ranges
• 1× R1: 100 kΩ, 0.25 W resistor (light load/reference for the secondary)
• 6× Test leads with alligator clips (insulated)
• 1× Breadboard or terminal strip (optional, for tidy connections)

Wiring guide

• Verify isolation: ensure AC1 is an isolated low‑voltage source (do not use mains directly).
• Identify windings (power off):
• Use the DMM in Ω mode to find the two separate windings. The pair with higher resistance is usually the high‑voltage winding (primary in a step‑down).
• Prepare the primary test:
• Connect AC1 across the winding you intend to energize (the “primary under test”).
• Tie one end of that winding and one terminal of AC1 together as the common reference (GND in the diagram).
• Prepare the secondary measurement:
• Connect the other winding (“secondary under test”) with its bottom end tied to the same common reference.
• Place R1 (100 kΩ) across the secondary (top node to common) as a light load and to define a reference.
• Multimeter setup:
• Set the DMM to AC volts.
• You will measure VP (across the energized winding) and VS (across the opposite winding), both relative to COM (common).

Schematic

           +V
            │
            ● Vp+
            │
            ├──────────────┌──────────┐
            │              │          │
            │              │          │
            │              │  AC1     │
            │              │  9 VAC   │
            │              │          │
            ├──────────────└──────────┘
            │
            ├──────────────────────┌──────────┐         ┌──────────┐
            │                      │          │─────────│          │
            │                      │  T1 (LV) │         │  T1 (HV) │
            │                      │          │─────────│          │
            ├──────────────────────└──────────┘         └──────────┘
            │                                               │    │
            ● Vp-                                           │    │
            │                                               │    │
           GND                                              │    │
                                                            │    │
                                                ┌───────────┘    └───────────┐
                                                │                            │
                                                │          R1 100 kΩ         │
                                                │        ┌──────────┐        │
                                                │        │          │        │
                                                └────────┤          ├────────┘
                                                         │          │
                                                         ┴          ┴
                                                         ● Vs+      ● Vs-

Measurements and tests

• • Safety prep:

  • Power OFF. Confirm no mains is connected anywhere.
  • Double‑check winding pairs with the Ω range; no cross‑connections between windings.

• • Baseline checks:

  • With power OFF:
    • • Measure DCR_P: Resistance of the energized winding. Typical: a few to hundreds of ohms.
    • • Measure DCR_S: Resistance of the opposite winding. Typical: lower for low‑voltage, higher current windings.

• • Apply low‑voltage AC:

  • Power ON AC1 (~6 VAC).
  • Ensure the DMM is on AC volts.

• • Measure primary voltage:

  • Place DMM leads between VP and COM.
  • Record VP (RMS). Target near the source value (e.g., ~6.0 VAC).

• • Measure secondary voltage:

  • Place DMM leads between VS and COM.
  • Record VS (RMS). This is the induced no‑load secondary voltage.

• • Compute ratio:

  • Turns ratio n = Np/Ns = VP/VS (when you energize the primary).
  • If you energized the lower‑voltage winding instead, invert accordingly:
    • • n = VP/VS still holds; interpret which side is Np or Ns based on which winding you excited.
  • Example: VP = 6.00 VAC, VS = 0.49 VAC → n ≈ 6.00/0.49 ≈ 12.2:1 (about a 230:19 V transformer).

• • Cross‑check (optional):

  • Swap which winding you energize (still using a low‑voltage AC source), repeat VP and VS.
  • The two ratios should be consistent (nforward ≈ 1/nreverse within measurement error).

• • Abbreviations used:

  • VP: AC voltage across the energized winding (measure between VP and COM).
  • VS: AC voltage across the opposite winding (measure between VS and COM).
  • COM: Common reference node (diagram ground). Use the black probe here.

Common mistakes

• Energizing with mains: Never apply mains to an unknown transformer for ratio tests. Use an isolated low‑voltage source.
• Floating secondary: Without any reference, the secondary reading can be noisy. Keep one end tied to COM and/or add R1 (light load).
• Loading the secondary too much: Heavy loads reduce VS and skew the ratio. Use no load or a very light load (R1 ≥ 100 kΩ).
• Wrong range on the DMM: Use AC volts; auto‑range can be slow—manual range near expected values improves stability.

Safety notes

• Use only isolated, low‑voltage AC for excitation (≤ 12 VAC).
• Keep primary and secondary leads well separated to avoid accidental shorts.
• Power down before moving clips or reconfiguring windings.
• If the transformer has any connection to mains in its original product, do not test it while connected to that product.

Improvements and extensions

• Frequency sweep: If you have a function generator, test at 50/60 Hz and a few hundred Hz to see if VS is stable; core losses rise at higher frequencies.
• Polarity marking: Briefly excite with low AC and check phase on a scope (if available) to mark dot‑convention on leads.
• Uncertainty reduction: Average multiple readings; note DMM AC bandwidth limits at low voltage.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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