36 V DC Etched Foil Polyimide Heaters

A 36 V DC element is not a 230 V AC element with a different label on it — the resistance differs by the square of the ratio, and with it the conductor gauge and the whole circuit layout. This page sets out what 36 V DC does to the design of etched foil polyimide heaters.

The Element at 36 V DC

Two equations do all the work: P = V²/R and I = P/V. At 36 V DC, 200 W is 6.5 Ω and 5.56 A.

The span matters more than the example. At 36 V DC a 20 W element is 65 Ω and a 500 W element is 2.6 Ω — a factor of twenty-five in resistance, met by changing the conductor cross-section and the length of the path, both of which are constrained by the outline.

At low voltage the difficulty is current, not insulation. 5.6 A wants 16 AWG rather than 22 AWG lead wire: 16 AWG is about 13 mΩ per metre, so a 2 m pair adds roughly 0.053 Ω to the circuit and dissipates 1.6 W in the leads instead of in the heater. Contact resistance at a crimp or a connector sits in series with the element in exactly the same way, and on a 1 Ω element a 20 mΩ joint is 2% of the power lost where nobody is looking for it.

The design consequence is that lead length and termination are part of the specification at low voltage. On a 5 V element the leads and the connector can account for a tenth of the total resistance, which shows up directly as a heater that never reaches temperature.

Resistance tolerance is u00b110% standard, u00b15% on request, and power follows resistance inversely: a u00b110% standard, u00b15% on request tolerance band is the same band on the delivered wattage. Where the wattage matters — a warm-up to a deadline, a pack of heaters that must match — say so on the enquiry and the tighter band is selected and tested.

Isolation, Earthing and Safety Class

36 V DC is inside the safety extra-low voltage band, so the supply itself is not a shock hazard and the heater does not need a protective earth or double insulation on that account. The low voltage directive does not apply below 75 V DC.

What still applies is everything else: the element is at the far end of a circuit that can deliver a large current, so short-circuit protection, conductor sizing and the thermal cutoff are unchanged. Low voltage removes the shock risk, not the fire risk.

Every heater is tested to these before it ships, element to any exposed surface. A dielectric test is a pass or a fail, not a measurement — a part that passes at 2500 V is not one that may be run at 2500 V.

What an Etched Foil Polyimide Heaters Is

Precision-etched circuits with varied trace width across the outline, for temperature uniformity better than ±1 °C on instrument and optical assemblies.

A photo-etched copper or Inconel foil circuit laminated between polyimide films, 0.2 to 0.4 mm thick in total. It is the thinnest and fastest flexible heater made: almost no thermal mass of its own, so what it heats is the part rather than itself. It will wrap a cylinder; it will not take a compound curve, and a nick in a 0.05 mm film propagates from the edge, so outlines are drawn with radiused corners.

The heat leaves through the bonded face. Everything that is not in contact with the load radiates and convects from the back of the heater instead, which at these thicknesses is a poor path — so an air gap under part of the outline is not a small loss of efficiency, it is a hot spot at that spot.

The continuous limit of 200 °C is set by polyimide film. The element alloy would run far hotter; it is never the element that decides.

Switching a DC Element

DC has no zero crossing, so a mechanical contact has to break the arc itself. A relay rated 10 A at 250 V AC may be rated 1 A or less at 30 V DC for exactly that reason, and using the AC number is how contacts weld closed — with a resistive heater on the other side, a welded contact is a heater that cannot be turned off. At 36 V DC and 5.6 A, use a MOSFET, a DC-rated solid state relay or a contactor with a DC rating.

PWM is the natural control method on DC, and a low frequency is right: a few hertz to a few hundred hertz is plenty for something with the thermal time constant of a heater, and it keeps the switching losses and the emissions down. There is nothing to gain from switching a thermal load at 20 kHz.

One DC-specific failure mode is worth naming: a continuous DC bias across a damp printed circuit drives electrochemical migration between adjacent tracks. It is why a silver-ink element in a humid installation is specified with a conformal barrier or reversed periodically, and why carbon ink is still used where the geometry allows it.

Watt Density: the Two Figures

Two watt densities are quoted because there are two thermal situations. With nothing conducting heat away, 0.8 W/cm² is what the insulation survives. Bonded to a heat sink over the full outline, 7.5 W/cm² is. Anything in between depends on the quality of the joint, and the joint is the customer's to make.

The failure is silent and quick: the element does not know how it is mounted, so at 7.5 W/cm² in free air the heat simply accumulates in the insulation until it degrades. This is the single most common way these heaters are destroyed, and it happens on the first run rather than after a year.

Leads and Terminations

Lead wire as supplied: PTFE or silicone insulated 24 AWG to 20 AWG, or an FFC tail etched as part of the circuit. Length, gauge, exit position and termination are drawing items — the exit position in particular, because it decides where the heater can be routed and it cannot be changed after the tool is cut.

The lead exit is a thermal discontinuity as well as an electrical one. The joint between a fine element and a stranded lead is the hottest point of many heaters, so it is kept out of the highest watt density zone, strain relieved, and — where the exit sits in a hot area — insulated in PTFE rather than in silicone.

On a 0.2 mm film the lead joint is thicker than the heater. Where the assembly has to stay flat, the leads are brought out to a tail or a flat flexible tab rather than soldered onto the heated area.

Control and Protection

This heater has no way of limiting its own temperature. Resistance heating draws roughly the same power whatever the temperature, so with the load removed, the fluid gone, the fan stopped or the sensor open circuit, the element keeps producing watts until something stops it.

The design must therefore contain a control device and, separately, a protective one: a controller with a sensor to hold the set point, and a bimetal thermostat or a one-shot thermal cutoff wired in series with the element to open the circuit if the loop ever fails. The two are not interchangeable — a controller that has lost its sensor cannot protect anything, which is exactly the fault the cutoff exists for.

Sensors can be built into the part: NTC thermistor, PT100 or PT1000 RTD, or a K-type thermocouple, fitted under the same insulation and tested with the heater as one assembly. A sensor laminated into the heater reads the heater; a sensor on the load reads the load. Both are legitimate, and the drawing has to say which is wanted.

How This Construction Fails

• Run at the bonded watt density in free air. The insulation reaches its limit within minutes and the element burns through where the local density is highest. Specify at 7.5 W/cm² only where the heater is genuinely bonded to metal over its whole area; unbonded, the figure is 0.8 W/cm².
• Voids under the bond. Air trapped during application, a contaminated surface or an adhesive that never reached full cure all produce the same result: a patch with no thermal path, running far above the rest of the heater, which is where the failure starts.
• Loss of the control loop. An open-circuit sensor, a welded relay contact or a controller in manual leaves the element at full power with nothing watching it — the case the series thermal cutoff exists for, and the reason it must not be omitted to save a part.
• A nick in the film. Polyimide tears from a defect at the edge, so sharp internal corners and hand-trimmed outlines both propagate. It also hydrolyses in continuous steam, which rules out sterilisation duty for the bare film.

How to Specify One

This is a made-to-drawing part, so there is no catalogue number to quote. A quotation is built from the following, and the more of it that arrives with the enquiry the fewer rounds it takes:

• An outline drawing, or a sketch with dimensions: overall size, holes, cut-outs, radii and the lead exit position.
• The supply voltage, and whether it is AC or DC. The element is wound for one voltage; it is not a switchable range.
• The power required, or — better — the temperature to be reached and the mass or flow it has to be reached in. Given the second, the wattage can be calculated instead of guessed.
• What the heater is attached to, and how. This is what decides which watt density figure applies, and it is the question a quotation cannot proceed without.
• The ambient, and the lowest temperature the assembly must start from. A heater sized for a 20 °C start is undersized for a -30 °C one.
• Duty cycle: continuous, intermittent, or a warm-up to a deadline. Time to temperature is a specification, not a by-product.
• Sensor and protection: which sensor, where it goes, and which limit device is fitted in series with the element.
• The certification the end product is built to, named on the enquiry rather than after approval.
• Quantity and target price, because the construction that is right for fifty pieces is often not the one that is right for fifty thousand.
• Whether adhesive backing is wanted, and what surface it goes onto — a low-energy plastic needs a different adhesive from bare aluminium.

Lead time: Samples 7-10 days from approved drawing; production 15-20 days.

Etched Foil Polyimide Heaters — Capability Table

These are process limits for the construction, not the specification of one stocked item. Every line is what the factory can hold; what your part holds is fixed on your drawing.

Capability figures are this factory's process limits for etched-foil polyimide construction. The 200 °C continuous limit and the watt density ceiling (to about 7.75 W/cm² with heat sinking) are properties of the polyimide film and the adhesive system, and are published consistently across the industry.