One coil, a second compliance universe
A warming plate is a resistive appliance: a film, a thermostat, a controller, a cable. Add a wireless charging coil so the same surface can top up a phone and the product also becomes a radio device. That single addition pulls in a set of obligations that has nothing to do with heat: emission limits for the coil driver, immunity of the heater controller to its own radio stage, exposure assessment, and market-specific authorisation for the transmitter.
Buyers often approve the sample on the strength of the module supplier's paperwork and discover the gap at the marketplace onboarding stage. The module's approval covers the module. The product that reaches the customer is a heater with a coil inside it, in a housing, on a desk, with a mug standing next to a phone.
What changes technically
The coil is driven by a switching circuit, and that circuit is a noise source. It sits centimetres from a thermostatic controller that measures small temperature changes, and from a heating film carrying a chopped current. The two systems share a housing and a power input, so interference can travel in both directions: coil noise into the temperature loop, heater switching into the radio stage.
Then there is the thermal interaction. A phone charging on the plate adds its own heat, and the plate adds heat from below. The combination can push the phone into thermal throttling or into a protective shutdown, and it can push the plate's surface above what the thermostat expected, because the phone is a heat source rather than a heat sink. And there is the foreign-object question: a stainless cup base standing over the coil is exactly the kind of metal object a charging system is expected to detect and refuse.
Incremental scope, area by area
| Area | What the coil changes | Document to request | Question for the laboratory |
|---|---|---|---|
| Radio authorisation | The product now contains a transmitter | Module approval plus host integration evidence | Does the existing approval cover this host and antenna arrangement? |
| Emissions | A new switching source inside the housing | Test report covering charging and heating together | Which operating combinations were measured? |
| Immunity | Heater control must survive its own radio stage | Test report including the coil active | Was the thermostat observed during charging? |
| Exposure | A field is generated where hands and phones sit | Assessment for the intended use position | Does the assessment cover a mug beside a phone? |
| Foreign objects | Metal cup bases near the coil | Functional description of detection | How is a steel base treated during heating? |
The document burden
Start with the module. Ask for its authorisation document, the test report behind it, the antenna or coil description, and the conditions under which the approval holds — a module approved only with a specified ground plane and separation may not be approved inside your housing. Then ask the integration questions: who performed the host testing, on which sample, with which firmware.
Next, the labelling. A product containing a transmitter usually carries statements and symbols that a heater alone does not. Those must reach the artwork, the manual and the listing, and they must match the market. This is where a rushed dual-function launch most often fails: the coil was added late, the artwork was already approved, and the statement never made it onto the box.
Finally, the market-specific layer. Some markets require a notification or registration before a transmitter can be sold; some require the importer to hold specific evidence; some impose additional conditions on the combination of a charger and a heated surface. None of this can be settled by reading a supplier's brochure, and none of it should be guessed in a purchase order. Put the question to an accredited laboratory and to your compliance adviser in writing, with the product description and the intended markets, and keep the reply.
Test the combination, not the parts
A test plan that runs the heater with the coil off, and the coil with the heater off, will pass and tell you nothing. The customer runs both at once, with a phone on one side and a mug on the other. Ask for evidence of that state: heating at each level while charging, the thermostat behaviour recorded, the surface temperature under the mug measured, and any observation about the phone's own thermal response.
Also ask what happens when a metal cup base is placed over the coil area, and what happens when a phone is placed on the heated area. These are foreseeable misuse cases, they are cheap to test, and a supplier who has tested them can describe the outcome in a sentence.
A module approval is not a product approval
The boundary: an approved module inside an unapproved host is not an approved product. The approval follows the module's own configuration, and the host changes the antenna, the ground, the thermal environment and the use position. Equally, an emission report on the heater alone says nothing about the coil's behaviour.
A second boundary: the presence of a coil does not make the compliance file a single document. The heater still needs its electrical and electromagnetic evidence, the materials file still applies to the surface and the housing, and the two must describe the same configuration. The American authorisation question, including which route a combined unit takes, is set out in FCC: declaration or certification.
Questions to put to a supplier
- Which module is used, and can you send its authorisation document with the conditions of use?
- Who tested the combination of heating and charging, on which sample, and what was recorded?
- How does the system treat a stainless cup base placed over the coil area?
- Which labels and statements does the transmitter require, and are they on the current artwork?
- Has a laboratory confirmed the applicable route and any registration duty for each target market, in writing?
Next step
Treat the coil as a scope change, not an accessory: re-quote the compliance work before you approve the sample. We can outline the document set we hold for a combined unit through contact.
Prepared for desk-warming sourcing briefs. Numbers shown are placeholders; request measured data per model from the supplier.