A 12VAC to 12VDC converter must explicitly accept the actual low-voltage AC source and deliver regulated DC within the load’s limits. A bridge rectifier alone changes polarity; it does not hold the output at 12 V. Adding a capacitor raises the unloaded voltage toward the AC peak and introduces charging pulses. Specify RMS input, frequency, source variation, load, and the required DC tolerance before comparing converters.
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This guide is for equipment OEMs deciding whether to retain an existing 12 VAC transformer and qualify a downstream conversion assembly, or change the source architecture. It does not identify a standard WEHO model for 12 VAC input. A mains-input supply whose documented minimum exceeds the transformer’s output is not a candidate, even if its output label says 12 V DC. Any proposed assembly must cover the complete source envelope.
Define what 12 VAC means at the converter input
Start with the source, not the output connector. Record whether the 12 VAC rating belongs to a 50/60 Hz transformer secondary, an electronic lighting transformer, or another waveform. A converter qualified for sinusoidal low-frequency AC is not automatically suitable for a chopped or high-frequency source. Include any dimmer or controller upstream.
For a sinusoid, 12 V RMS corresponds to approximately 16.97 V peak before rectifier losses. A transformer label also needs a loading condition: secondary voltage can change with mains voltage, winding resistance, and load. Obtain the maximum no-load voltage and minimum loaded voltage, or measure them under an agreed test plan. Do not replace these limits with an arbitrary percentage and call the result a product rating.
| Field | Specify | Reason |
|---|---|---|
| Voltage and waveform | RMS range, waveform, measurement conditions | RMS and peak limits answer different questions |
| Frequency and control | Frequency range; any upstream switching or dimming | Rectifier and control behavior may change |
| Source capacity | Transformer VA, regulation data, other loads | DC watts do not define transformer RMS current |
| Load requirement | DC tolerance, continuous demand, startup profile | Defines the voltage and current acceptance limits |
| Reference and insulation | Secondary bonding and required separation | Rectification does not create galvanic isolation |
Separate rectification, filtering, and regulation
A full bridge produces a unidirectional waveform. A reservoir capacitor stores energy between charging peaks, but its voltage still depends on source amplitude and current demand. A regulator then attempts to maintain the specified output over a documented input range. A finished converter may integrate these functions; its input specification must say so.
The external rectifier design guide distinguishes rectifier/filter arrangements, treats AC voltages as RMS, and highlights transformer current and capacitor ripple-current effects. It supports checking the assembly rather than treating transformer amperes as interchangeable with DC output amperes.
A linear regulator also needs headroom above its output and dissipates the voltage difference as heat. For a documented example of a headroom requirement, see the external regulator datasheet, dropout-voltage section. Its limits describe that component under stated conditions; they do not establish a WEHO converter’s minimum input.
For an input-rating counterexample, the WEHO LRS-100 input specification (mains-input example) lists mains-level limits, not 12 VAC. Its 12 V output variant is not a candidate for the low-voltage AC conversion discussed here.
Screen the highest peak and lowest trough
Hypothetical screening example: assume a sinusoidal secondary of 10.8–13.2 V RMS at 50 Hz, two conducting diodes dropping 0.80 V each, a 4,700 µF reservoir with a −20% capacitance tolerance, and an approximately constant 1.00 A reservoir discharge current. These are chosen calculation inputs, not a WEHO specification or a complete design.
Minimum capacitance = 4,700 µF × 0.80 = 3,760 µF. For a full bridge, the charging-peak repetition rate is 2 × 50 Hz = 100 Hz. The first approximation is ΔV ≈ I/(2fC) = 1.00/(100 × 0.003760) = 2.66 V peak to peak. The external reference design explains the charge-balance approximation and doubled ripple frequency in TIDA-00896, section 3.1.
| Secondary condition | Peak after assumed bridge drop | Approximate trough at 1.00 A |
|---|---|---|
| 10.8 V RMS | 10.8 × √2 − 1.60 = 13.67 V | 13.67 − 2.66 = 11.01 V |
| 12.0 V RMS | 12.0 × √2 − 1.60 = 15.37 V | 15.37 − 2.66 = 12.71 V |
| 13.2 V RMS | 13.2 × √2 − 1.60 = 17.07 V | 14.41 V under the same assumed discharge |
Suppose the load allows 12.0 V ±5%, or 11.40–12.60 V. The approximate low-source trough is below its minimum, while even the nominal-source peak exceeds its maximum. A capacitor has therefore not turned this source into an acceptable regulated 12 V rail. An average-reading meter could conceal both problems.
The screening envelope is roughly 11.01–17.07 V under the stated assumptions. It is not a qualified operating range. Diode drop varies with temperature and pulse current; transformer impedance affects charging; capacitor ESR, ripple current, and tolerance matter. A downstream switching stage draws a changing input current, so its reservoir discharge cannot simply be equated to its output current. Measure or model that interaction before selecting the regulator. If the resulting DC envelope crosses the desired output, review regulation when input crosses the nominal output voltage before choosing the downstream conversion stage.
Check current and temperature at each boundary
Specify transformer RMS current, bridge current, capacitor ripple current, regulator input current, and DC output current separately. A capacitor-input rectifier can draw short charging pulses with an RMS value that exceeds the average DC current. Increasing capacitance changes those pulses and startup demand; it does not automatically permit a smaller transformer.
For a linear stage, use loss ≈ (Vinput − Voutput) × Ioutput as an initial heat estimate, then apply its actual operating limits. For a switching stage, use efficiency at the relevant input, output, load, and temperature. Include rectifier and connection losses outside the regulator boundary. Review the enclosure with all heat-producing parts operating.
Validate the retained source with the complete assembly
Test the production transformer, conversion assembly, harness, and load together at the defined source extremes. Retain the assembly identity and integration records described in OEM converter-module qualification. Capture the rectified bus and load voltage during startup, steady load, load changes, and restart. Record measurement bandwidth and probe location. The load-terminal voltage-budget method helps keep converter output and delivered load voltage separate. Check no-load behavior as well as rated load, and verify any protective device against the resulting charging demand.
Keep insulation review separate. An existing transformer may provide a required barrier only if its documentation and installation establish that function. A bridge or non-isolated regulator adds no new barrier. Retain the defined secondary bonding arrangement and evaluate the finished equipment against its applicable requirements.
Prepare a supplier inquiry that states the input correctly
For an OEM equipment review, contact WEHO about the source and regulated-output requirement. Include the source RMS range and waveform, transformer VA rating, required DC output and startup profile, installation temperature and space, and expected quantity where known. State whether the existing transformer must remain or whether the source architecture can change. Ask whether a documented model matches the requirement and obtain its exact input limits before treating it as a candidate; this guide does not establish availability of a 12 VAC-input product or a custom-conversion service.
