Linear regulator heat in a sealed enclosure

Reduce the regulator’s voltage drop or provide a verified heat path to the enclosure exterior. A larger internal heat sink can lower the regulator temperature, but it does not remove heat from the box. Calculate the electrical losses, measure temperatures with the lid closed, and check junction temperature against the exact device data sheet. Thermal shutdown is a protective feature, not an acceptable operating strategy.

What the symptoms point to

A hot regulator and a hot enclosure are different problems. Record which temperatures rise, when they rise, and whether the output stays regulated.

• The regulator heats quickly while the enclosure stays relatively cool: suspect excessive regulator dissipation or poor heat transfer from its package.
• The regulator, circuit board, and internal air warm together: suspect insufficient heat rejection through the enclosure walls.
• The output cycles off and back on: investigate thermal protection, current limiting, input dropout, or an overloaded supply.
• The output dips at maximum lighting load: check the minimum input voltage between rectifier charging pulses, not just its average.
• The problem appears only outdoors or with the lid closed: include ambient temperature, sunlight, and the completed enclosure in the assessment.
• Nearby capacitors or connectors become hot: check local heating, component losses, current loading, and proximity to the regulator.

Do not estimate temperature by touching the heat sink. Surface temperature does not establish junction temperature, and a hot accessible surface can present a burn hazard.

Several hours of operation also do not prove thermal equilibrium. The explanation of enclosure time to overheat covers why thermal mass delays the final temperature without necessarily reducing it.

The likely causes in order

Too much voltage across the regulator

A linear regulator converts most of the voltage difference between input and output into heat. For steady operation, estimate its dissipation as:

P_reg ≈ (V_in − V_out) × I_out + V_in × I_q

Here, I_q is the regulator’s ground or quiescent current. Use the definition and operating conditions in its data sheet.

For an illustrative measured input of 8 V DC, a 5 V output, and a 0.6 A load, the main loss is 1.8 W. If ground current is 5 mA, another 0.04 W brings the total to approximately 1.84 W.

A transformer’s AC rating is not the regulator’s DC input voltage. A nominal 6 V AC secondary has an ideal sinusoidal peak of approximately 8.5 V before rectifier losses. Actual voltage depends on line voltage, transformer regulation, load, rectifier drops, and capacitor ripple.

Too little heat transfer from the package

A small heat sink may leave the junction hot even when total enclosure dissipation is modest. Mounting pressure, interface material, package orientation, and surrounding clearance affect the result.

A heat-sink rating measured in open air does not automatically apply inside a confined enclosure. Use heat sink thermal resistance in sealed enclosures to distinguish package cooling from whole-box cooling.

More internal heat than the regulator alone

Include rectifier losses, transformer losses, resistors, control electronics, and any lighting located inside the enclosure. A 5 V load drawing 0.6 A receives 3 W, but its location matters.

If that load is outside the box, do not count all 3 W as internal enclosure heat. If it is inside, include its contribution and account for any optical energy escaping through a window.

A warmer installation than the bench

An outdoor enclosure can face warm ambient air, direct sunlight, and restricted clearance around its walls. A transparent cover can also admit sunlight onto internal parts. A comfortable workshop test does not represent those conditions.

Check the electrical and thermal limits

Start with the exact regulator part number and package. Confirm input limits, dropout requirements, output current conditions, required capacitors, thermal data, and protection behavior.

Keep mains parts enclosed. Take measurements only on an appropriately isolated low-voltage circuit using suitable instruments and safe access arrangements. Mains-side investigation belongs with qualified electrical personnel.

Measure both ends of the input range

Check regulator input voltage under maximum continuous load and at lighter loads. Capture ripple where necessary; an average-reading meter can hide the minimum voltage.

For a full-wave rectifier with a reservoir capacitor, a first estimate is:

ΔV ≈ I / (f_ripple × C)

At 0.6 A, with 4,700 µF and 100 Hz ripple from a 50 Hz supply, the estimated ripple is about 1.3 V peak-to-peak. This approximation excludes transformer impedance and detailed charging behavior.

The lowest ripple trough must remain above the required regulator input voltage. The highest input voltage must remain within component limits and be included in the heating calculation.

Separate junction temperature from enclosure temperature

Measure outside ambient, internal air near the regulator, heat-sink temperature, and temperatures at vulnerable nearby components. Use insulated, securely positioned sensors that cannot short electrical connections.

A data-sheet maximum of 125 °C may refer to junction temperature, a recommended operating limit, or another specified condition. Read its definition. It is not permission to operate the heat sink or enclosure at that temperature.

For illustration, assume a design junction target of 110 °C, local air at 65 °C, and regulator loss of 1.84 W. The available junction-to-local-air thermal resistance is:

R_allowed = (110 − 65) / 1.84 ≈ 24 °C/W

That is a complete thermal-path budget, not a heat-sink specification by itself. The example target is not a recommendation for every regulator. Select the actual target from device limits, tolerances, reliability needs, and operating conditions.

Do not apply a published junction-to-ambient resistance unless its board and test conditions represent your installation. Where appropriate, use the manufacturer’s case-temperature method and thermal model instead.

Check the whole-box balance

At equilibrium, heat entering the enclosure must equal heat leaving it. A useful simplified relationship is:

Internal temperature rise ≈ internal heat × effective enclosure thermal resistance

The effective resistance must represent the enclosure, mounting, surroundings, and heat distribution. It is not a universal value for a particular material.

For example, 5 W inside an enclosure with an established effective resistance of 8 °C/W implies approximately 40 °C rise. At 35 °C ambient, that suggests 75 °C internal air before considering additional local hot spots.

Use the broader power supply enclosure cooling checks when several power-conversion stages contribute to the total.

Reduce heat without opening the enclosure

Reduce losses at their source

Lowering the regulator input voltage reduces heat directly, provided the lowest input still meets dropout requirements. Check that condition at maximum load and minimum supply voltage.

A low-dropout regulator helps only when its lower dropout permits a lower input supply. At the same input voltage, output voltage, and load current, its main dissipation remains approximately unchanged.

A suitable switching converter can reduce the conversion loss substantially. For a 3 W output and an illustrative efficiency of 90%, converter loss is approximately:

P_loss = 3 × (1 / 0.90 − 1) ≈ 0.33 W

Actual efficiency depends on voltage, current, temperature, and the selected converter. Check electrical noise, protection, output ripple, startup behavior, and required layout before substitution. A replacement module is not automatically compatible because its pins appear to fit.

Moving conversion outside the enclosure can also help. An appropriately approved external isolated supply can remove transformer and conversion losses from the sealed volume.

Improve the thermal path

A larger internal heat sink can reduce the junction-to-air temperature difference. It cannot eliminate the regulator’s watts or guarantee a cooler enclosure interior.

A designed conductive path to a metal enclosure wall may transfer heat outside more effectively. The article on sealed enclosure conductive cooling explains the wall and interface considerations.

Check whether the regulator tab is electrically connected to a circuit node. Where necessary, use a suitable electrically insulating thermal interface and mounting arrangement. Preserve required electrical clearances and enclosure bonding.

Do not improvise longer regulator leads without reviewing stability and wiring requirements. Place capacitors and any protection components exactly as the device manufacturer specifies.

Avoid solving a heat problem by drilling ventilation holes into an enclosure that must remain sealed. New heat-sink penetrations also need sealing and mechanical review; they do not inherit the original ingress rating automatically.

Stop the problem returning

Validate the completed assembly with its final lid, seals, cable entries, mounting arrangement, and operating load. Use the procedure in thermal soak testing for a sealed enclosure to establish a repeatable test rather than relying on elapsed time alone.

Record these acceptance checks:

• Temperatures stabilize under the worst credible continuous operating mode.
• The estimated junction temperature remains below the selected design limit with an appropriate margin.
• Output regulation remains acceptable through input ripple and load changes.
• Capacitors, wiring, terminals, plastics, and seals remain within their applicable temperature limits.
• Accessible surfaces meet the application’s touch-temperature requirements.
• Protection does not cycle during normal operation.

Thermal limiting does not prove the surrounding assembly is safe. Its threshold and behavior vary, and it does not protect every nearby component.

Likewise, solder melting is not the correct acceptance threshold. Capacitor life, plastic limits, wire insulation, and semiconductor reliability can become concerns well before solder melts. Copper tracks and leads also conduct heat into nearby components.

When to re-specify the enclosure

Change the enclosure when the circuit losses are reasonable but the final assembly still exceeds its thermal budget. A larger enclosure may provide more external area and better component separation. Verify the benefit rather than assuming volume alone solves the problem.

For an existing IP67 enclosure, temporary immersion protection does not establish thermal suitability or continuous underwater suitability. Any modification needs review against the required final ingress performance.

We build custom enclosures with the material, width, height, depth, and cutouts defined for the application. Our enclosure ratings are manufacturer ratings for the specified design; our enclosures are not UL listed.

Send the following requirements with the enclosure specification:

• Maximum ambient temperature, sunlight exposure, and mounting orientation.
• Internal losses at maximum continuous load, separated by component.
• Maximum permitted internal air and component temperatures.
• Required ingress protection and any independent listing requirement.
• Board dimensions, heat-sink envelope, cable entries, and electrical clearances.
• Any proposed conductive cooling interface and its electrical isolation requirements.

Use the enclosure selector once those requirements are defined. A dimensional match is only the starting point; the selected enclosure still needs its thermal performance checked.

Enclosures for this job

Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

Questions