Teardown of a big HEPA air filter unit (with schematic)

I got a big floor-standing air filter unit so we could take it apart and see what's inside. These were being sold at a very jacked up price during the pandemic, and then the price crashed when the residual stock had to be cleared. There's a good chance that you'll find these stuffed into cupboards of public spaces, or just thrown out after they have served their purpose of ticking the pandemic air quality box. In some cases the units will have been used without even taking the plastic wrapping off the filter first. (I came across two like that.)

The curse of these units is that they are sold in the same way as inkjet printers, where you are tempted by a cheap unit - but then pay the price later with the cost of replacement filters. Or worse still, the seller just stops selling the new filters and you are left with a useless unit.

Pink background is a silicone bench mat courtesy of Scott (Defpom's Electronic Repair channel).

The function is very simple. Air is pulled in through a multi-layer filter and then blown out the top past the obligatory ioniser. They don't need a circuit board or touch buttons. You could literally have an on/off switch and an optional high/low speed selector switch.
Because of the multi-tap motor winding, only one of the speed connections should be powered at any given time. Powering more than one would effectively add a shorted winding section.
If hot-wiring the motor the unused taps must be insulated.

Motor wiring:-
Black - common (I'd expect there to be a thermal fuse on this connection.)
Red - high power
White - medium power
Yellow - low power

The buck regulator number is LN8K05
The processor is an SC92F83A2
The triacs are JKT1D8W

I'd guess these units are mass produced in China with labelling and pricing to suit the company selling them.
The design is very cost optimised, and much more interesting as a result. They've literally squeezed every feature they could out of a cheap processor.

The filter construction is unusual. The honeycomb frame with loose carbon granules is quite neat.

Note that I flipped the 5V capacitors in the power supply schematic due to the odd topology. The negative side should be at 0V.

The bit that goes "PING" took me a moment to get my head around. It's genius. Basically a hardware envelope generator for a bell noise, where the processor just needs to provide a square wave at the desired frequency.


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