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New Wasmachine and Tumble Dryer Repair
These days a single bad component can stop a complete machine from working. It can be a simple lambda probe in your car that needs fixing before the motor starts again. Or even 5 to 6 different hardware built-in safety protection procedures in a modern Beamer why a Projector won’t power up.
This repair is about at least 20 identical used AEG t55640 2500 Watt tumble dryers (and also washing machines) that I fixed in these last weeks that all suddenly stopped working.
There is a controller board in these machines that powers down when a LNK304 chip breaks down. And together with that LNK304 chip a single 47 ohm burns through on the controller boards. After both are replaced the machine will work again just fine.
These special LNK 302/304-306 chips can be found in all sorts of non-isolated power supplies like in rice cookers, dishwashers, and other white goods. What makes this repair interesting is that these LNK304 chips are excellent PWM modulated Buck controllers capable to withstand a HV 700 input
voltage. And only the LNK304 to 306 type off-line switcher controller has a built-in Auto-Restart function.
Next example circuit shows the way how the AC Line voltage is directly connected to the LNK304 chip. Only the shown resistor RF1 that also functions as a safety fuse resistor isn’t a 8.2 ohm 2Watt in our AEG tumbledryer repair but the already mentioned 47 ohm 2Watt component.
The non-isolated input circuit components are the fusible resistor RF1, D3 and D4, C4 and C5, and coil L2. (See Red circle).
The active power processing stage circuit components are the LNK304, D1, L1 and output capacitor C2.
C3, R1 and R2 are the Feedback components and are selected in such a way that the voltage at pin FB is a stable 1.65V at the chosen output voltage.
The D pin is the Drain Mosfet connection that provides internal operating current to both start-up and steady-state operation. The BP is the Bypass Mosfet connection for the internally generated 5.8V supply.
The normal LNK304 operation will terminate when the current that goes into the Feedback FB pin is greater than 49 uA. The internal LNK (LinkSwitch-TN) chip runs of the energy stored in the bypass capacitor thanks to the extremely low energy consumption needed. A capacitor of 0.1 uF (C1) is sufficient for providing enough energy for the chip to operate.
The circuit also provides undervoltage protection when the BP voltage drops below 4.85V. This will disable the Power Mosfet in our LNK chip until it rises back to 5.8V. (because only when the Mosfet is on , the LNK304 runs of the energy from the bypass capacitor). And the LNK is also over-temperature protected by a treshold set at 142 degrees Celsius which will disable the Power Mosfet. And there is an internal over-current protection too that also will disable the Power Mosfet. Resistor R4 is a small pre-load that prevents tracking errors between the output voltage across C3, at light load or no-load conditions. The forward voltage drop over both special UltraFast diode D1 and normal silicium diode D3 are identical. So the voltage developed over C3 tracks the output voltage. And resistors R1 and R3 are chosen so that at the desired voltage output, the feedback voltage at the FB pin exactly is 1.65V.
Regulation (at about 66 kHz) is maintained by skipping Mosfet switching cycles. When the output voltage rises, the current through pin FB will rise too, which will disable the Power Mosfet. This means that when less amperage is drawn at the output the output voltage will rise which will inhibit switching the Power Mosfet as in the previous regulation method described.
And in case no cycles are skipped during a 50 ms period (only the LNK304-306 types) the average output power will be limited to provide overload protection by entering an internal auto-restart mode. Next photos show the position of the LNK304 on the AEG controller board.
And following photo shows the mentioned fusible 47 ohm 3W resistor (R76) that is placed at the component side. Here a 3W type, but sometimes they are probably also just 1 or 2W. And if a 8.2 ohm 2W type in our example circuit is okay, a 47 ohm 1W certainly will do just fine too. Besides, if we choose a too high Wattage it won’t work as a fusible protective resistor anyway!
Next circuits, extracted from the Power Integrations Datasheet, show how we also can use this switcher IC to make a simple non-isolated AC Line operated protected LED Light.
As it seems, this LinkSwitch IC is becoming almost as popular as the old and still used 555 IC.
Above the internal circuit of the Off-Line Switcher LNK304-306 IC. Next photo shows where the frontpanel with here still the controller board in it was extracted.
It is still not entirely clear what makes all these identical machines suddenly fail. But according to my good shopowner friend Anton this likely occures when customers do not clean out the dust in their dryer machines on time. Which makes the tumble motor running harder, causing it to use more current, and often also destroys the motor capacitor at the same time. Anyhow after all my recent repairs it looks like these problems are real designer flaws. Maybe made on purpose to sell new machines? And it looks like something that is only happening in our modern day world. Something that would have been bad advertising for a manufacturer in the old days.
Following photo shows the entire component side of the controller board of the AEG tumble dryer. With the green colored 47 ohm 3W resistor on the top right, just above the blue AC switch. The LNK304 Off-line switcher IC is at the solder side about under the blue AC switch.
Previous photo shows the tracks after removing the old bad LNK304 chip.
Looking at the bright side, it could mean more jobs for repair engineers. Nevertheless I was only helping a good friend on a pro deo base. Hoping to make the world a little better!
By-the-way: If you wonder how you can tell if a LNK304 chip is defect? That one is most likely shorted between pins Source and Drain! But in most cases your RF1 resistor needs replacing too.
Albert van Bemmelen, Weert, The Netherlands.
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