Source: https://guthlabs.ai/docs/instruments/linear-regulator-power-dissipation-heatsink-need/mechanism/

instrument mechanism / verified-clean

# Linear Regulator Power Dissipation & Heatsink Need mechanism

The inspectable method record for Linear Regulator Power Dissipation & Heatsink Need, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The power and temperature-rise calculations are acceptable as first-order steady-state estimates under explicit unit and thermal-path assumptions. The heatsink recommendation output is not computable from this specification, and the input/output constraints and quiescent-current treatment must be defined before catalog release.

- Pd=(Vin−Vout)×Iout is the standard first-order pass-element dissipation formula for an ideal linear regulator, but it omits regulator ground/quiescent current and any other input-current contribution; a more complete expression is Pd=Vin×Iin−Vout×Iout, or approximately (Vin−Vout)×Iout+Vin×Iq when Iq is known.

- ΔT=Pd×θJA is correct for junction-to-ambient thermal resistance under steady-state conditions, with θJA expressed in °C/W.

- Using ΔT=Pd×(θJC+θsink) is valid only if θsink is explicitly the complete sink-to-ambient path and all case-to-sink/interface resistances are included; θJC alone is not sufficient.

- The specification does not define a formula or threshold for the heatsink recommendation. A recommendation requires ambient temperature, maximum permissible junction temperature, regulator package/datasheet limits, and the applicable thermal path.

- The term 'dropout' is potentially misleading: the formula computes regulator power dissipation, not dropout voltage. Operation also requires Vin to exceed Vout by the regulator's specified dropout voltage.

- The formula should constrain Vin≥Vout plus the required dropout margin; Vin<Vout can produce a negative calculated Pd even though the regulator is not operating as claimed.

## Units and conventions

- Vin and Vout are DC volts (V), and Iout is amperes (A); if current is entered in mA it must be divided by 1000.

- Pd is reported in watts (W).

- θJA, θJC, and any sink-to-ambient or interface thermal resistance are in °C/W.

- ΔT is junction temperature rise above the stated ambient temperature, in °C.

- θJA is assumed to be the applicable steady-state, board- and airflow-dependent junction-to-ambient value.

- If θJC+θsink is used, θsink is assumed to include every remaining thermal resistance from case to ambient, including interface resistance.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "Vin_V": 12,
    "Vout_V": 5,
    "Iout_A": 0.5,
    "thetaJA_C_per_W": 50
  },
  "expected_outputs": {
    "Pd_W": 3.5,
    "junction_temp_rise_C": 175,
    "heatsink_recommendation": "Undetermined from the supplied formulas; requires ambient temperature and maximum junction temperature"
  },
  "working": "Voltage drop = 12−5 = 7 V. Pd = 7×0.5 = 3.5 W. Junction rise = 3.5×50 = 175 °C. A heatsink recommendation cannot be computed without T_A, T_J(max), and a defined decision threshold."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

Extreme finite inputs overflow to Infinity, and the specification lacks ambient-temperature and maximum-junction-temperature limits needed for a definitive heatsink recommendation. Reverse-polarity/sign handling is also unspecified.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/linear-regulator-power-dissipation-heatsink-need/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/linear-regulator-power-dissipation-heatsink-need/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/linear-regulator-power-dissipation-heatsink-need/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`