Thursday, August 4, 2016

Why Canon Document Scanner AC Power Adapter Is A Consumable Item

A friend brought me a AC adapter use to power a Cannon Document Scanner, asking if it is possible to repair it. It has been in used for only six months and is not dead.  A replacement is not available from Cannon as Cannon classify the ac power adapter as a consumable.

So electronics has advance to such a stage that a ac power adapter can be consumed by providing clean DC power to a document scanner?

I decided to take a look inside the adapater to see where it has worn out due to fair wear and tear.

There is not much to see except for 3 burned resistors and a burnt diode.

Interestingly the heat sink for the power transistor or FET seems to be on the lighter side.

A multimeter check both in circuit and after removal of the transistor/FET show a short inside the device measuring at about 5 ohms.

Which brings me to the idea that this ac adapter fails because of insufficient heat sinking due to the thin copper plate.

Moreover on closer inspection, the transistor/FET is not affixed to the heat sink via a layer of silicone heat sink paste as was practice in most quality electronics.

So I think this ac adapter fails because of overheating of the transistor/FET due to:

1. Too small a heat sink
2.  No silicone heat sink compound use to ensure close thermal conduction between the tranistor/FET and the thin copper plate.

Dead Cannon Document Scanner AC Power Adapter

Immediately obvious is the thin copper plate use as heat sinks in this Cannon AC Power Adapter 

Burnt resistors feeding the switching circuit, give clue to a shorted switching FET.

No silicone grease between the FET and copper heat sink to ensure good thermal conductivity.

FET did not have a close contact to the copper surface of the heat sink, note the tiny air gap

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