Schaudt LAS1216F Charger Module Schematic Availability and Alternatives

User question

De LAS1216F geeft een te hoge laadspanning; wat kan de oorzaak zijn?

Artificial Intelligence Response

Direct answer to the question

• A LAS1216F that delivers an abnormally high charge voltage is almost always the result of

1. An OPEN or defective battery-temperature sensor circuit (NTC) – the module “thinks” the battery is far below 0 °C and raises the voltage accordingly; or
2. A FAULT in the internal secondary feedback loop (precision resistor divider → TL431 reference → optocoupler).
   • Less common causes are a wrong battery-type selector position, poor wiring/remote-sense connections, or a completely failed battery drawing the regulator out of range.

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Detailed problem analysis

1. General operating principle
   • LAS1216F is a switch-mode IUoU charger embedded in Schaudt Electroblock installations.
   • Output voltage is regulated on the secondary side; a precision divider feeds a TL431 (2.495 V ref) that drives an optocoupler back to the primary PWM controller.
   • A 10 kΩ NTC on the battery provides ‑3 … -4 mV / cell / °C temperature compensation via the same divider.

2. Why the voltage can rise too far

A. Temperature-sensor problems (≈ 50-60 % of field cases)
• Open line / broken plug → charger assumes very low temperature (–20 … –30 °C default) → +0.8 … 1.5 V on a 12 V battery.
• Shorted sensor → charger may drop to 12.5 V instead.
Diagnostic: disconnect sensor and measure resistance; 8–12 kΩ at 20–30 °C is normal. ∞ Ω or 0 Ω = defective.

B. Feedback-loop faults (≈ 35-40 %)
TL431 drifts, optocoupler CTR collapses, or one of the divider resistors rises in value.
• Effect: charger “sees” too little voltage → increases duty-cycle → output climbs (often > 15–16 V).
• Quick check: measure TL431 cathode–anode voltage – should clamp near 2.5 V. If higher, ref or divider is off; if lower, opto LED open.
• Replace TL431 (precision grade), optocoupler (PC817/TLP-series), and check 0.1 % resistors.

C. Wiring / sense errors
• High resistance in battery negative or positive lead shifts the voltage seen at the module, causing controller over-compensation.
Diagnostic: compare voltage directly on LAS1216F output and on battery posts; difference > 0.3 V indicates cabling issue.

D. Wrong battery-type selector
• AGM/gel setting adds ≈ 0.2–0.4 V. Alone it seldom reaches damaging levels but can add to other faults.

E. Failed battery or extreme sulfation
• Very high internal resistance can allow the charger to spike; however, LAS1216F normally current-limits, so this is rare.

3. Circuit illustration (simplified)

12 V_OUT ── Rupper ─┬─┐
│ │ TL431 Ref
└─ Rlower ── GND
TL431 Cathode ── RLED ─► Opto LED ── GND
Opto Transistor (primary side) ──► PWM IC FB

Defect in any highlighted element upsets the 2.5 V reference point and lets the PWM run wide open.

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Current information and trends

• Users increasingly retrofit LiFePO₄ batteries; LAS1216F was not designed for those higher accuracy requirements. Modern DC-DC chargers therefore add their own closed loop and ignore the LAS1216F output.
• Newer Schaudt “E-series” chargers integrate microcontrollers and self-diagnostics; an open NTC now raises a fault code instead of silently boosting the voltage.

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Supporting explanations and details

Example:
NTC open → controller assumes –20 °C. Compensation ≈ –4 mV / cell / °C × (–45 °C) ≈ +1.08 V. Bulk voltage rises from 14.4 V to ~15.5 V – precisely the value veel campers meten tijdens “te hoge laadspanning”.

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Ethical and legal aspects

• Over-voltage can boil wet cells, dry out AGM, or ignite hydrogen/oxygen mix.
• Directive 2006/95/EC (LVD) and DIN EN 60335-2-29 require chargers to remain within specified limits; repair should restore compliance.
• Always disconnect mains and allow HV capacitors to discharge before servicing – lethal 325 V DC is present.

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Practical guidelines

Step-by-step check:
1 Disconnect 230 V and battery.
2 Verify NTC: measure resistance, inspect cable and plug. Replace if doubtful.
3 Plug a known-good NTC (10 kΩ) into Electroblock, power up, re-measure charge voltage. If normal → problem solved.
4 If still high, open LAS1216F:
 • Visual inspection for burnt parts / cracked solder joints.
 • Measure divider resistors out-of-circuit; replace any > 1 % off-spec.
 • Replace TL431 and optocoupler as a set (both < €2).
5 Re-test with an electronic load before reconnecting the battery.

Typical pitfalls & fixes
• Dry solder joints around the optocoupler are common; simply re-flowing can restore regulation.
• Electrolytic capacitors on the secondary side rarely cause sustained over-voltage but should be checked for ESR > 2 Ω.

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Possible disclaimers or additional notes

• LAS1216F is fully analogue; no firmware update is possible.
• If primary PWM IC (e.g. UC384x derivative) is shorted, the unit may be beyond economical repair.

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Suggestions for further research

• Download Schaudt service bulletin “Temperaturfühler prüfen” (available via Schaudt.de).
• Review TI application note SLVAE85 on TL431 stability for deeper insight.
• Consider upgrading to a charger with CAN-based battery communication for future lithium systems.

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Brief summary

A LAS1216F that produces too high a charging voltage is usually reacting to bad information: either no temperature data (open NTC) or a corrupted feedback signal (failed TL431/optocoupler/resistor). Verify the external sensor first; if intact, open the unit and service the feedback components. Always observe mains-safety procedures, because the charger contains hazardous voltages. Once the offending element is replaced, the output should return to 14.1 – 14.4 V (bulk) and 13.6 – 13.8 V (float), preserving battery life and meeting safety standards.

User question

de laadspanning is bijna 40V. De NTC werkt goe en ook de TL431 en optocoupler. Wat kan nu nog het probleem zijn?

Artificial Intelligence Response

Direct answer to the question

– A charge voltage that rises to ≈ 40 V means the regulation loop is wide open.
– If the NTC, TL431 and the optocoupler devices themselves test OK, the fault is almost always one of the following:

1. Open-circuit or out-of-tolerance resistor(s) in the secondary voltage-divider that feeds the TL431 reference pin.
2. Broken solder joint / PCB track anywhere between the divider, the TL431, the optocoupler LED or, on the primary side, between the optocoupler transistor and the PWM-controller feedback pin.
3. Failed primary-side parts of the feedback system: dried-out VCC electrolytic for the PWM IC, or the PWM-controller IC itself.
   In short: the PWM controller is no longer receiving valid feedback; locate the interruption in that path.

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Detailed problem analysis

1. Expected operation
   • The 12 V rail (≈ 14.4 V in charge mode) is scaled by two precision resistors, R₁ and R₂.
   • When Vout · R₂/(R₁+R₂) ≈ 2.50 V, the TL431 sinks current, driving the optocoupler LED.
   • The optocoupler transistor pulls the PWM-IC FB/COMP pin low, reducing duty-cycle and stabilising the output.

2. What 40 V tells us
   • 40 V ≈ three times the set-point: the converter is running at its start-up / maximum duty cycle.
   • Therefore the PWM IC “believes” the output is still too low → it receives no or too little feedback current.

3. Weak points once active parts are proven good
   a) Divider network
   – R₂ (the lower resistor) open → TL431 never sees 2.5 V.
   – R₁ drifted far lower → same effect.
   b) Series LED resistor open → TL431 saturates, but LED current is zero, so the optocoupler never talks to the primary side.
   c) PCB / solder defects
   – Hair-line crack between TL431 cathode and R₁/R₂ node is enough to break the loop.
   – Bad joint at the optocoupler or its return path.
   d) Primary feedback path
   – Track or series resistor between optocoupler transistor and PWM-IC FB pin open.
   – VCC decoupling electrolytic (often 22–47 µF) high ESR → PWM IC starts, drops out, starts again, effectively losing regulation.
   – PWM IC internal comparator damage.

4. Less common, but possible
   • Secondary rectifier short/open can distort feedback but rarely causes exactly 40 V unless loop is already broken.
   • Transformer or insulation breakdown would normally give wildly higher or mains-referenced output, not a clean 40 V DC.

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Current information and trends

• Modern SMPS chargers increasingly add a redundant “hard” over-voltage latch (e.g. an SCR crow-bar or dedicated OVP IC). In older LAS-series units that safeguard may not exist, so an open feedback loop still lets the output climb to several tens of volts.
• Lead-acid charger designs after ~2020 often replace the discrete TL431/opto with digital isolation (e.g. AMC1301 + MCU) which automatically logs a divider fault and shuts down. Your unit predates that; manual troubleshooting is required.

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Supporting explanations and details

Voltage divider check (example 12 V charger, set-point 14.4 V):
[ \frac{R_2}{R_1+R2} = \frac{V\text{REF}}{V_\text{OUT,nom}} = \frac{2.500}{14.400} \approx 0.174 ]
Typical values: R₁ = 47 kΩ, R₂ = 10 kΩ.
If R₂ goes open, K ≈ 0 → V_REF ≈ 0 → TL431 OFF → no LED current → full duty cycle → ≈ 40 V observed.

Opto LED path: measure with the supply powered up briefly into a dummy load (12 V/21 W bulb). If Vout is already > 16 V and the LED drop is < 0.2 V or 0 V, the series resistor or LED itself is open.

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Ethical and legal aspects

• 40 V on a “12 V” output violates Low Voltage Directive limits and can destroy downstream equipment or cause battery out-gassing.
• Work on the primary side exposes you to ≥ 325 V DC; follow EN 50191 safe-work rules (isolation transformer, discharge, one-hand technique).
• Restoring a charger used in caravans/motor-homes may fall under EN 60335; any repair must keep double-insulation and creepage clearances intact.

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Practical guidelines

Step-by-step isolation, mains unplugged:

1. Visually inspect divider resistors; re-solder any dull or cracked joints.
2. Desolder one lead of R₁ and R₂, measure with DMM; replace if > 1 % off spec or open.
3. Check continuity TL431 cathode ↔ opto LED anode, opto LED cathode ↔ secondary GND.
4. Power up via an isolation transformer with electronic load set to 1 A; monitor:
   – Vout (should drop rapidly when loop closes)
   – TL431 cathode to anode (target 2.5 V)
   – Opto LED VF (~1.2–1.8 V)
   – PWM FB pin (typically 0.1–1 V regulated, > 2 V indicates loss of pull-down).
5. If all secondary parts behave but FB pin stays high, recap VCC and, if needed, replace the PWM IC.

Potential challenges: conformal coating, multi-layer board, safety isolation slots; use fine wire bridges where tracks are carbonised.

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Possible disclaimers or additional notes

• Component ageing can mask intermittent faults; always test under heat (hair-dryer) and cold-spray conditions after repair.
• If the transformer shows any evidence of inter-winding breakdown, replacement is mandatory; re-varnishing is not sufficient for safety approval.

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Suggestions for further research

• Study ON-Semiconductor application note AND8454: “SMPS feedback resistor reliability”.
• Examine Keysight note 5965-7582E on sense-lead open-protection techniques.
• Consider upgrading to a charger with dual-loop OVP to avoid recurrence.

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Brief summary

A 40 V output means the feedback loop is open. Even with a good NTC, TL431 and optocoupler device, the feedback signal is lost—most commonly due to an open voltage-divider resistor or a broken connection in that small feedback network. Less frequently the primary-side feedback pin or its VCC supply is bad. Systematically verify every passive part and every PCB trace in the feedback chain before suspecting the PWM IC itself, and perform all tests with proper high-voltage safety precautions.