It's a Sunpentown (Taiwanese home appliance manufacturer) 14" stand fan. It has 3 speeds, infrared remote control, sleep timer, natural rhythmic wind function, etc. Height is adjustable on a telescopic column.
Like many high-end fans on the market today, this fan utilizes an additional small motor, instead of a reduction gear box driven by the fan motor, for the oscillation. In other words, the oscillation is triggered electronically (to power that small motor) instead of mechanically.
The fan worked great until last year, when a strange symptom began to appear. Whenever the fan is running, the fan will abruptly turn itself off if the oscillation is engaged. Selection of the rhythmic wind function will trigger this "instant shut-off" symptom too. Strangely, the fan can start right up again if one presses the on/off button. Other functions like the timer are fine. The on-board 1.5 A fuse has not been blown.
With summer here and fan usage increases, I believe now is a good time to fix this.
Before I continue, here is a friendly WARNING to viewers who wishes to try repairing their home appliances. Please read the disclaimer on this website. Most home appliances run on line voltage that can serious hurt or kill you. ALWAYS disconnect power before working on the appliance. My fan was unplugged during the whole repair process. If the appliance you need to work on is permanently attached to a connection unit on the wall (e.g. bathroom extractor fan), you have to switch off the power at the circuit breaker first. You should then confirm that the power is indeed off at the connection unit, as it's always possible to switch off the wrong breaker. As always, do NOT attempt the repair unless you absolutely 100% understand what you are doing and what the risks are.
Like most repair scenarios, full dis-assembly was the first thing to do. Due to the location, the fan motor housing was the first section I inspected.
The fan works fine at all 3 speeds, so I believe the fan motor and the starting capacitor are fine. Originally, I thought the small oscillator motor is dead. That assumption is wrong, as a simple hookup to 220 VAC proved that it works. The problem was definitely somewhere else - the base of the fan.
After removing the base (secured with a screw ring) and 6 screws, I found a circuit board that all wires connect to:
On first sight, the fact that the red AC capacitor is connected on the copper side of the circuit board really surprised me. This capacitor is a critical component at the beginning section of the non-isolated capacitive power supply. The power supply is used to generate low voltage for the electronics (LEDs, TRIACs, PIC, etc. - more on these below). It is hard to imagine how this important component was "left out" during circuit board layout (and thus have to be tagged on at the back). Well, enough about that. Let's look at the component side:
First off, a little tour around the circuit board.
The large IC chip is a microcontroller - a PIC16C57C in 28-pin DIP made by Microchip Technology Inc. There are eight LEDs to show fan status like current speed and timer settings. The black component next to the LEDs is an infrared sensor to receive the signals sent by the remote control. Surrounding the 470 uF electrolytic capacitor are five buttons to control the fan. A piezoelectric buzzer (circular black component next to the on/off button) provides audio feedback. On the bottom of the picture, numerous wires connects to the circuit board. From left to right, they are:
Grey - Connects to oscillator motor.
Brown - Connects to fan motor low-speed winding.
Pink - Connects to fan motor medium-speed winding.
Yellow - Connects to fan motor high-speed winding.
Blue - Input from the neutral wire of AC plug.
White - Connects to the motor starting capacitor, then to both motors. Basically a shared live wire after the fuse.
Brown - Input from the live wire of AC plug. Connects to the fuse.
An interesting point I noted was that the first four wires in the list above are all connected similarly. Upstream from each wire is a transistor-looking (TO-92 package) component named "97A8" and a corresponding resistor. A quick search on the Internet showed that it's actually a logic-level TRIAC with a maximum voltage of 600 V. The exact model for this component is MAC97A8. Basically, the TRIAC is like a on/off switch. When one of its legs, named Gate, receives a signal (sent from the microcontroller in this case), AC power is allowed to flow through the two other legs of the TRIAC.
The four TRIACs are in excellent shape so I believe the problem is before the TRIACs. That brings me to the microcontroller. The microcontroller has to be fine. Otherwise, nothing would work. The software (code) is out of consideration because it cannot be modified by this circuit. Looking back at the symptom, and reading this article from Microchip, I believe the malfunction has to do with the microcontroller shutting itself off (reset) due to brown out. It only exists when the rhythmic wind function or oscillation is engaged. I wondered why only these two functions, not other functions like speed change or timer, would cause this.
By using a flow-chart to analyze how the microcontroller would control the fan if I were to program it from scratch (but with this existing circuit), I discovered an interesting point. When the fan motor is running, only one out of the three TRIACs connected to the fan motor is triggered by the microcontroller at any time. That makes sense because the fan motor cannot be in two different speeds at the same time. Moreover, the oscillator motor can only be turned on when the fan motor is running. To turn on that motor, the microcontroller triggers the corresponding TRIAC. When this happens, the microcontroller is now triggering two TRIACs at the same time. This is also when the "instant shut-off" symptom appears immediately.
This finding, combines with the fact that the malfunction is likely to be related to age of the fan, turned me to the power supply section of the circuit. It is a non-isolated capacitive power supply with half-wave rectification. Capacitor is almost always the culprit when it comes to unusually-large voltage drop on high load or other weird power issues, and almost always the first type of component to die as time passes, so it's time for a change.
The capacitors in question were two metalized polyester film (MEF) non-polarized capacitors, both made by UTX and rated for 400 V operation. The large one is "105K" (or 1 uF / 1000 nF) and the small one is "104K" (or 0.1 uF / 100 nF).
That 470 uF electrolytic capacitor should be replaced as well, but its role isn't as critical as those two, so I didn't touch it.
Replacements are of the same construction (MEF type) and value, but with a higher voltage rating (630 V) and from a different (more reputable, in my opinion) manufacturer - Epcos, which is now TDK-EPC. My point of purchase for the new capacitors was RS Components. After acquiring the components, it's just a simple process of replacement using a soldering iron.
Since the new capacitors are larger than the old ones due to the increased voltage rating, I had to juggle and be creative with placement. The smaller one fitted in without much difficulty. It's installed at an angle in order to utilize an existing hole on the circuit board.
The big one was not as easy though. Due to the extra-large size, limited board real estate, and re-use of existing holes, I had to use jumper wires. Heat-shrink tubing and electrical tape were added to keep exposed conductors away from the neighbors.
Instead of hanging on the back side of the circuit board, the big capacitor now safely and neatly reside in a space next to the telescopic stand section.
After observing the fan operating for a few days, I am pleased to see that the repair is a success.
