AGL Isolating Transformer Calculator
Work out secondary lamp current, lamp voltage, input power and transformer loading for an airfield lighting series isolating transformer.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to see what an isolating transformer on an airfield ground lighting series circuit does to a lamp: the secondary current follows the circuit current in the ratio of the rated currents, and the transformer takes its own losses from the series circuit.
Limits of this calculator
The lamp is not a fixed resistance, so look up its power at the secondary current shown. An open secondary, such as a failed lamp, is not modelled and needs the manufacturer data on open-circuit voltage and core saturation.
Cross-check: 25 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 61820-1-2; FAA AC 150/5345-47B, IEC 61820-1-2:2024; FAA AC 150/5345-47B (30 June 2005) |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | AGL series-circuit requirements and the FAA series-to-series isolation-transformer specification are relevant. Verify transformer type, VA rating, leakage, insulation, secondary loading and current FAA qualification status against the actual procurement specification. |
| Cross-check test cases | 25 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Circuit (primary) current | 6.6 A |
| Rated primary current | 6.6 A |
| Rated secondary current | 6.6 A |
| Lamp power at that secondary current | 45 W |
| Transformer rating | 100 W |
| Transformer efficiency | 0.85 |
- Secondary current = 6.6 × 6.6 / 6.6 = 6.6 A
- Lamp voltage = 45 W / 6.6 A = 6.82 V
- Input power = 45 / 0.85 = 52.9 W, primary drop = 52.9 / 6.6 = 8.02 V
- Loading = 45 / 100 = 45 %
Assumptions
- Ideal current ratio: the secondary current follows the primary current in the ratio of the rated currents
- Lamp power is the figure at the secondary current, taken as a fixed power
- Resistive load, power factor near unity
The lamp is not a fixed resistance, so look up its power at the secondary current shown. An open secondary (failed lamp) is not modelled and needs the manufacturer data on open-circuit voltage and core saturation.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Circuit (primary) current | A | greater than 0 A | The CCR output current at the brightness step you are checking |
| Rated primary current | A | greater than 0 A | From the transformer nameplate, for example 6.6 A |
| Rated secondary current | A | greater than 0 A | From the transformer nameplate, for example 6.6 A or 20 A |
| Lamp power at that secondary current | W | greater than 0 W | From the fitting data sheet at the secondary current shown in the result |
| Transformer rating | W | greater than 0 W | Rated output of the isolating transformer, from its nameplate |
| Transformer efficiency | (number) | greater than 0.5, up to 1 | From the transformer data sheet at this current |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Secondary lamp current | A | Shown to 3 decimal places | |
| Lamp voltage | V | Shown to 2 decimal places | |
| Input power from the series circuit | W | Shown to 1 decimal place | |
| Primary voltage drop | V | Shown to 2 decimal places | |
| Transformer loading (%) | (number) | Shown to 1 decimal place |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Lamp power at that secondary current (W) | Secondary lamp current (A) | Lamp voltage (V) | Input power from the series circuit (W) | Transformer loading |
|---|---|---|---|---|
| 30 | 6.6 | 4.55 | 35.3 | 30 |
| 45 | 6.6 | 6.82 | 52.9 | 45 |
| 65 | 6.6 | 9.85 | 76.5 | 65 |
| 100 | 6.6 | 15.15 | 117.6 | 100 |
| 200 | 6.6 | 30.3 | 235.3 | 200 |
Other inputs are the same as in the worked example above.
Standards and further reading
- ICAO Annex 14, Volume I (aerodrome design and operations)
- ICAO Doc 9157, Part 5 (aerodrome design manual, electrical systems)
Photometric and circuit requirements come from the edition of Annex 14 and the regulator's rules that apply at your aerodrome, and from the fitting and regulator manufacturers' data. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter circuit (primary) current, rated primary current, rated secondary current, lamp power at that secondary current, transformer rating and transformer efficiency. Change the unit of a value with its selector and the result stays the same.
Why is the secondary current set by the primary current?
The circuit is a constant current series circuit, so the regulator holds the primary current and the transformer ratio sets the secondary current at each brightness step.
What is the transformer loading figure?
It is the lamp power as a share of the transformer rating. Check the highest brightness step, where the lamp power is largest.
What happens when a lamp fails?
The secondary opens and the voltage across it rises. This calculator does not model that, so use the manufacturer open-circuit data.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 25 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- AGL Isolating Transformer Formulas: AGL isolating transformer formulas: the secondary lamp current, the lamp voltage
- Transformer Formulas: Current and Fault Level: Transformer formulas for turns ratio, full-load current, impedance and fault current
- Transformer Efficiency and Regulation Formulas: Transformer efficiency formulas: efficiency from iron and copper loss, load at maximum efficiency, voltage regulation
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