110-120 V AC Etched Foil Silicone Rubber Heaters

Etched Foil Silicone Rubber Heaters wound for 110-120 V AC. The supply decides the element: 500 W at this voltage is 24.2 Ω, and the same 500 W at a tenth of the voltage would be a hundredth of the resistance in a conductor ten times heavier. This page covers the circuit, the isolation, the switching and what the construction can be built to at this supply.

The Element at 110-120 V AC

Two equations do all the work: P = V²/R and I = P/V. At 110-120 V AC, 500 W is 24.2 Ω and 4.55 A.

The span matters more than the example. At 110-120 V AC a 20 W element is 605 Ω and a 500 W element is 24.2 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 mains voltage the difficulty is the opposite one. Current is modest — 4.55 A for the worked example — but the element is a long, fine conductor, and a low-wattage heater at 110-120 V AC is the hardest thing in this catalogue to build: 20 W means 605 Ω, which is a very long track to fit inside a small outline without the pitch becoming so tight that adjacent runs interact.

Where the arithmetic will not fit the outline, the usual answers are a lower supply voltage with a small power supply behind it, several circuits in series, or a different construction entirely. What does not work is thinning the conductor further: the track becomes the fuse.

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

110-120 V AC is a mains supply, and the construction is built to match: mains-rated insulation, a protective earth connection or a double-insulated build, and creepage and clearance distances held between the element and any accessible surface.

Leakage current is the figure that surprises people. A large heated area behaves as a distributed capacitance to earth, so a big mains heater has a measurable leakage even when it is perfectly sound, and an RCD protecting several of them can trip on the sum. Where several elements share a circuit, allow for the total.

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 Silicone Rubber Heaters Is

A photo-etched Inconel or copper foil circuit vulcanised into silicone: a wider, flatter conductor than wire, so heat is spread more evenly and higher watt density is safe.

Nichrome resistance wire, or an etched Inconel foil circuit, vulcanised between two sheets of fibreglass-reinforced silicone rubber. The construction is about 1.2 to 2 mm thick, conforms to a curved surface, and holds its dielectric strength from -50 °C up to 200 °C continuously.

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 fibreglass-reinforced silicone rubber. The element alloy would run far hotter; it is never the element that decides.

Switching an AC Element

AC switching has a zero crossing to hide behind. A zero-cross solid state relay makes and breaks at the crossing, which all but removes the switching transient and the conducted emissions that go with it — worth having on any installation that also has to pass an EMC test.

Control by burst firing — whole cycles on, whole cycles off, over a period of a second or more — rather than by phase angle. Burst firing generates almost no harmonics; phase angle control chops the waveform and is the reason a heater ends up in the EMC report at all. A heater has enough thermal inertia that a one-second period is invisible at the surface.

Size the relay for the inrush, not the running current, and heatsink it: a triac or SSR drops around one volt per amp of load, and that is real power to be got rid of inside the panel.

Watt Density: the Two Figures

Watt density, not wattage, is what the construction is limited by. 1.2 W/cm² unbonded and 8 W/cm² bonded to metal are the limits for this build — so if the wattage is fixed by the duty, the area is what has to move.

Where the required power will not fit into the available area at the free-air figure, the answer is a better thermal path rather than a thinner margin: bond the whole face down, or add a spreader plate. On 150 cm² the difference is roughly 18 W against 12 W, which is usually the whole argument.

Attachment and Thermal Contact

Attachment options are a factory-applied pressure-sensitive adhesive, vulcanising or bonding to the customer's part, or mechanical clamping with a plate or a strap.

A PSA bond wants a clean, dry, degreased surface, continuous pressure while it wets out, and no trapped air: a bubble under the element is a spot with no thermal path, and it will be the first place the heater fails. Acrylic adhesives reach full strength over 24 to 72 hours, so the assembly is not at its rated watt density the moment it is pressed down.

From the capability table: Free, 3M 468MP or 9077 PSA backing, vulcanised to the part, or mechanically clamped.

The bond line is part of the thermal circuit and should be specified like one. A 0.05 mm acrylic PSA layer conducts roughly 0.2 W/m·K; a filled silicone adhesive is two to five times better, and a bolted joint with an interface pad better again. Whatever is chosen, it has to be continuous — the heater cannot spread heat sideways to cover a gap, and the temperature under a void rises until something in the stack gives way.

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 8 W/cm² only where the heater is genuinely bonded to metal over its whole area; unbonded, the figure is 1.2 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.
• Mechanical damage at the edge. Silicone is tough but it tears from a cut, and a heater trimmed to fit on assembly has had its edge seal opened; moisture then tracks along the glass carrier to the element.

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 Silicone Rubber 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 the construction described. Watt density limits follow the published maxima for fibreglass-reinforced silicone rubber heaters (to about 12 W/cm² bonded to a heat sink, far less in free air); continuous temperature limits are those of the silicone insulation system, not of the element alloy.