Fan Controller

A case of really bad design

 

 This repair story is about a ceiling fan that was being used for the very first time after being installed.

It had been more convenient to mount the power supply for the so-called 24Volt fan motor in the loft above the bedroom ceiling. The reason for installing the fan was the series of UK heat waves in Summer 2026. Of course ceiling fans must predominantly be used when its hot weather and hot weather in a modern loft means really high temperatures in the loft space. Bearing all this in mind a fan controller design engineer must understand all the parameters involved especially temperature characteristics.

A picture from a sales brochure.

 

 

 The fan stopped working during the night and failed to respond to its remote control.

 

As I was visiting the next morning I suggested I fix the problem and was presented with this plastic-cased controller in a very sorry state.

 

You can see here that the fan is Model FYD-3517 and refers to its Instruction manual. I haven't found one yet.

The manufacturer appears to be

Foshan Shunde Fengyun Electric Appliance Co.,Ltd.

I've requested a copy of the instruction manual.

 

Extracting the circuit board wasn't difficult and I found a dead short across the three paralled smoothing capacitors for providing 24 volts DC.

The worrying point is that the mains fuse is still intact.

 

 I counted the components wired to the short and found two DPAK chips, three capacitors and three motor driver chips. The DPAK devices are marked MOT10200D and are dual Schottky diodes making one or more shorts in ten possible devices.

 

 

Below is the circuit showing the 24 volt PSU driven from the HF transformer (which I found was running at about 320KHz). Rather than removing and testing each component in turn I used my thermal camera. Connecting a current limited low voltage with the same polarity as the capacitors I cranked up the current and voltage.

 

One of the Schottky dual diode chips won easily by getting much hotter than the remaineder of the parts.

 

I removed the chip and found one of its diodes was short-circuit and the other quite leaky with the rest of the circuit normal (in fact I applied mains power and measured 24 volts across the capacitors).

 

 If you examine the circuit above you'll see there are four paralleled diodes. This is generally a really bad idea because, although each diode is rated at up to 5A, its actual forward current is influenced by the resistance of the diode when conducting meaning that each diode will rectify not a quarter of the load current, but a bit more or a bit less than this depending on its forward resistance relative to the others. At room temperature 20A capability seems OK to handle a mere 2A, but raise the ambient temperature and look at the diode junction temperature and the 10A capability of the MOT10200D starts to drop off. A worrying feature now becomes apparent in that any upward change in forward resistance due to rising temperature may begin a current hogging process. Once one of the four diodes sinking more current than its share its junction temperature will keep on rising until you get thermal runaway. One diode then sinks most of the current when its now too hot to handle this amount and its semiconductor junction fails short circuit.

This is what happened because the ambient temperature in the heat wave, in the loft, and under lagging (meaning very little ventilation) placed the MOT10200D outside its safe operating area. Basically at a junction temperature of 175C the "safe operating area" dictates zero current can be handled.

Below is the repaired controller with its RF handset.
 

 What was the solution? First of all I wanted to use a heatsink (none was originally fitted). I considered using a TO220 device but then decided a stud-mounted single diode was best. In my spares collection I found a BYW30-300. This is a stud mounted ultra fast rectifier (30nS) which can handle 20A at up to 300VRMS. Pretty special... and dead easy to mount on a heatsink. The repaired controller is not too easy to test as I don't have the 3-phase DC fan but I connected three 330 ohm resistors across the three output wires and using the remote control saw the voltages crank up.

In the picture above the mains wires are not UK standard, having USA colours of black-live and white-neutral. Red and blue feed a lamp via a small relay and white/pink/yellow feed the fan. The black wire is the antenna for picking up the remote control signal in the 70cm band.

 

Finally.. rounding up. The controller is potentially dangerous if operated in an environment having little or no ventilation. Although the controller was inoperative and with a badly melted and deformed casing the mains fuse was intact. I've contacted the manufacturer to see if I can get a user manual so I can see details of the installation.

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