On 05/11/2020 18:46, Paul wrote:
Dave Plowman (News) wrote:
In article ,
Â*Â* Andy Burns wrote:
Dave Plowman (News) wrote:

The time display goes blank after use.

I think the time display on my neff m/w blanks between certain hours,
yours doesn't think it's 9pm when it's really 9am does it?

Mine doesn't. And since with this fault you have to reset the time after
using it, unlikely. ;-)


Sounds like the DC power supply for the controller board
is winking out for some reason. Like maybe when the
microwave oven door opens and the light comes on ?

You'd need to dig up a sample schematic for a microwave,
to see what "best practice/cheapest method" looks like
and from there, figure out the fault modes possible.

My previous microwave needed a pretty decent DC voltage,
since the display was vacuum fluorescent and the segment
drive for that is like 18-27VDC or so. Other technologies will
need different voltages. A LED based display could be
neatly done off 5VDC. The voltage doesn't particularly
matter, just find how they're making that voltage, and
trace what could be shorting it out and causing it
to current limit.

There's nothing adventurous about this one. A 5V design
based off a 7805 (1 amp) regulator. The unregulated
rail is used for relay drive, so that the relay current
doesn't overtax the 7805. 7805 has thermal cutout, if
it gets too hot, output is cut off. And naturally,
they'll put the 7805 in there, with no heatsink on it.
Because heatsinks cost money.

https://www.electronicsforu.com/wp-c...owave-oven.jpg


That schematic is "too pretty" to be real. Real schematics,
the controller board doesn't have the 7805 on it, and
the power components would be on another page of schematic,
poorly labeled and impossible to follow. That's how real
schematic drawing works :-)

The relays around the edge, could be feeding interfaces
on the microwave board, whatever that is. They'd likely
want some isolation between the two circuits, to reduce
the opportunities for HV leakage from the microwave
section, blowing up the controller.

You'd be looking for a center tapped transformer wire
set, three wires, feeding from some transformer to the
controller board. And right after that should be
some rectifiers to make pulsating DC. Which needs to be
filtered by a cap. Check the cap for brown leakage.
High voltage caps, the fabrication is pretty careful.
Filter caps for 5V, any Chinese junk will do.

Â*Â* Paul


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