Heat removal is an electrical problem in disguise. Temperature difference plays voltage, power plays current, and each interface plays a resistor. Solve the chain from junction to ambient and heatsink selection becomes arithmetic.
At a glance: 7 minute guide · part 10 of 10 in the amplifiers complete guide track · includes a worked example and a quick-reference table.
The resistance chain
Tj = Ta + P × (RθJC + RθCS + RθSA). Junction-to-case is fixed by the package; case-to-sink by paste or insulator (~0.2-1 °C/W); sink-to-ambient by the heatsink itself. You buy the RθSA that lands Tj safely below its limit (125 °C silicon, design to ~85-100 °C).
| Interface | Typical Rθ (°C/W) | Note |
|---|---|---|
| Junction-case (TO-220) | 0.5-5 | Fixed by package |
| Paste joint | 0.1-0.3 | Thin smear |
| Mica + paste | 0.3-0.6 | Electrical isolation |
| Sil-pad | 0.5-1.0 | Clean, insulating |
| Heatsink (small extrusion) | 2-8 | Size and airflow |
Real-world derating
Datasheet sink ratings assume natural convection in open air. Enclosures, dust and adjacent heat sources degrade them 30-50 % margin is standard practice. Forced air roughly doubles capacity but adds failure modes and noise.
Assembly details that dominate
Paste quantity (a thin smear), torque. Insulator choice shift case-to-sink resistance more than many sink upgrades. A rough, dry or over-torqued joint wastes the finest extrusion ever machined.
How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
- Sum the dissipated power in the device
- Chain the thermal resistances to ambient
- Size the heatsink for Tj ≤ ~100 °C worst case
- Assemble with correct paste and torque; verify by measurement
Worked example
A regulator dissipating 5 W with RθJC 4 °C/W and a 3 °C/W sink. Tj = 25 + 5 × (4 + 0.3 + 3) = 86 °C safe. The identical setup with a dried-out joint (1.5 °C/W interface) reaches 100 °C before the room warms. Run the numbers yourself with the PCB Trace Width and the result should agree to within rounding.
Practical note from the bench. Our build sheets list every hot part with its chain maths no heatsink gets chosen by anecdote.
Common mistakes to avoid
- Designing to the 125 °C absolute instead of ~100 °C practical
- Enclosing a sink rated for free convection
- Insulating tabs thermally but not electrically (or vice versa)
Key takeaways
- The resistance chain the foundation of this guide; revisit it if any measurement here surprises you.
- Real-world derating the foundation of this guide; revisit it if any measurement here surprises you.
- Assembly details that dominate the foundation of this guide; revisit it if any measurement here surprises you.
Prerequisites and preparation
Before starting: sum the dissipated power in the device and chain the thermal resistances to ambient. Keep the PCB Trace Width and Electrical Power Calculator open every number in the worked example is reproducible. Total time including the bench steps: about 6-7 minutes.
Who benefits most
Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.
Quick reference card
| Aspect | Where to find it in this guide |
|---|---|
| Core theory | The resistance chain |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |
How this fits the amplifiers complete guide track
This guide is one stop in the structured learning path. Start from the amplifiers complete guide complete guide for the full map, or continue with AB output stage heat and regulator heat budgets. For the arithmetic, open the PCB Trace Width or Electrical Power Calculator.
Frequently asked questions
Is bigger always better? Thermally yes, until mechanical and cost realities object; the maths finds the minimum, taste finds the margin.
Paste or pad? Paste for lowest resistance, pads for clean repeatability and isolation both beat dry joints.
Is there a calculator for this? Yes the PCB Trace Width and Electrical Power Calculator tools run the formulas from this guide instantly, client-side, with no signup.
Keep going with this track
- The complete amplifiers & audio guide: Amplifiers & Audio complete guide
- Read next: inverting op-amp: gain, virtual ground and design
- Also in this track: non-inverting op-amp: high-impedance gain stage
- Continue with: the op-amp voltage follower: small circuit, big leverage
- Calculate as you go: op-amp gain calculator · power dissipation tool · gain to dB converter
- Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.
Working method notes
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
Formulas and checks from this guide
Verification checklist for this track: measure bias at idle before signal testing, check heatsink temperature under load. Confirm gain across frequency rather than at 1 kHz alone. Amplifiers forgive nothing at their edges.
Bookmark this page against your next build in the track. The checklist above is the same one used across 19 guides in this series.
What the bench taught us
Across the emitter resistors at idle, no signal. Compare each output device.
Thermal drift in bias. Check the bias tracker is bolted to the heatsink and re-set per the service values.
A parting thought for builders
A note on thermal reality, which ends most arguments in this track: dissipation is set at idle, verified under load, and never forgiven by a bigger heatsink alone.
Working through The resistance chainand Real-world derating with that habit in mind takes minutes, and it is the difference between reading about this topic and owning it.
Extended Application Notes
This section expands the practical application of heatsink thermal design: junction maths made simple beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. Designing to the 125 °C absolute instead of ~100 °C practical Enclosing a sink rated for free convection Insulating tabs thermally but not electrically (or vice versa). Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of heatsink thermal design: junction maths made simple deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
Last updated 23 August 2026