E
Electronics Tech.
- Jan 1, 1970
- 0
This is probably a problem with your microwave oven door interlock switches.
Here's a quote from repairfaq website:
http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris
"Even slamming the door really hard has been known to knock an interlock
switch out of position, resulting in breaker tripping at the electrical
service panel whenever the microwave oven door was closed. (Another reason
to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
if there was some kind of "event" after which the microwave failed, check
the interlock mechanism first - a switch may just need to be popped back
into place."
Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves
unless the door is closed completely. At least one of these will be directly
in series with the transformer primary so that a short in the relay or triac
cannot accidentally turn on the microwaves with the door open. The
interlocks must be activated in the correct sequence when the door is closed
or opened.
Interestingly, another interlock is set up to directly short the power line
if it is activated in an incorrect sequence. The interlocks are designed so
that if the door is correctly aligned, they will sequence correctly.
Otherwise, a short will be put across the power line causing the fuse to
blow forcing the oven to be serviced. This makes it more difficult for an
ignorant consumer to just bypass the door interlocks should they fail or to
run the oven with an open door as a room heater - and protects the
manufacturer from lawsuits. (That interlock may be known as a "dummy switch"
for obvious reasons and is often not even mentioned in the schematic/parts
manifest.) Of course, should that switch ever actually be used, not only
will the fuse blow, but the switch contacts will likely be damaged by the
high initial current! This also means it probably wouldn't be a bad idea to
replace the interlock switch which might have been affected if your oven
fails with a blown fuse due to a door problem.
Failed door interlocks account for the majority of microwave oven problems -
perhaps as high as 75 percent. This is not surprising considering that two
of the three switches carry the full oven current - any deterioration of the
contacts results in increased resistance leading to their heating and
further deterioration. And, opening the door to interrupt a cook cycle
results in arcing at the contacts. Complete meltdowns are not unusual! If
any defective door switches are found, it is probably a good idea to replace
all of them as long as the oven is already apart.
The typical door switches and their function:
* Door Sensing: Input to the microcontroller to indicate the state of
the door.
* Interlock Monitor: Shorts out the AC line (and blows the main fuse)
should the Primary Interlock not open due to incorrect sequencing of the
door switches or a failed switch.
* Primary Interlock: In series with the high voltage (magnetron) power
supply so cuts power when the door is open.
Note that if the Door Sensing switch should malfunction, peculiar behavior
may occur (like the fan or turntable operating at the wrong time) but should
never result in microwaves being generated with the door open.
Testing and replacing of interlock switches
With the oven unplugged, put an ohmmeter across the AC input just before the
interlocks (but beyond the power relay or triac if it precedes these). Open
and close the door slowly several times - there should be no significant
change in resistance and it should be more than a few ohms. If it approaches
zero while opening or closing the door, the interlock switches and door
alignment should be checked. (You may need to disconnect one side of the
transformer primary since its resistance is a fraction of an ohm. Refer to
the schematic pasted inside the cover.)
Replace with switches having a precisely identical fit and equal or better
electrical specifications (terminal configuration, current rating). When
removing the old switch make a note as to where each wire goes. Check the
embossed marking on the old switch - don't depend on location as your
replacement might just have a different arrangement. Make sure the new
switch aligns correctly with the actuating mechanism and then check for
correct electrical operation with an ohmmeter before applying power.
Even slamming the door really hard has been known to knock an interlock
switch out of position, resulting in breaker tripping at the electrical
service panel whenever the microwave oven door was closed. (Another reason
to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
if there was some kind of "event" after which the microwave failed, check
the interlock mechanism first - a switch may just need to be popped back
into place.
Credit to author :
Notes on the Troubleshooting and Repair of Microwave Ovens
Version 3.44
Copyright © 1994-2004
Samuel M. Goldwasser
--- All Rights Reserved ---
http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris
From Jim
[email protected]
Remove 99 to reply.
----- Original Message -----
From: Pat
Newsgroups: sci.electronics.repair
Sent: Tuesday, November 16, 2004 9:29 AM
Subject: Sharp microwave oven problem
My Sharp convection microwave (model R-9H66) trips the circuit breaker
everytime it's turned on. It started doing this after its door was slammed
shut by someone. It's obviously shorting somewhere when turned on, and I
took the outer housing off to look at the wiring to see if there was
anything obvious,but didn't notice anything.
Does anyone have any idea what might be wrong or what to look for? It was a
fairly expensive unit when I bought it 6 years ago, and I'd hate to just
through it away, especially if it turns out to be something relatively easy
to repair.
Thanks for any help. I appreciate it.
Pat
Here's a quote from repairfaq website:
http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris
"Even slamming the door really hard has been known to knock an interlock
switch out of position, resulting in breaker tripping at the electrical
service panel whenever the microwave oven door was closed. (Another reason
to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
if there was some kind of "event" after which the microwave failed, check
the interlock mechanism first - a switch may just need to be popped back
into place."
Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves
unless the door is closed completely. At least one of these will be directly
in series with the transformer primary so that a short in the relay or triac
cannot accidentally turn on the microwaves with the door open. The
interlocks must be activated in the correct sequence when the door is closed
or opened.
Interestingly, another interlock is set up to directly short the power line
if it is activated in an incorrect sequence. The interlocks are designed so
that if the door is correctly aligned, they will sequence correctly.
Otherwise, a short will be put across the power line causing the fuse to
blow forcing the oven to be serviced. This makes it more difficult for an
ignorant consumer to just bypass the door interlocks should they fail or to
run the oven with an open door as a room heater - and protects the
manufacturer from lawsuits. (That interlock may be known as a "dummy switch"
for obvious reasons and is often not even mentioned in the schematic/parts
manifest.) Of course, should that switch ever actually be used, not only
will the fuse blow, but the switch contacts will likely be damaged by the
high initial current! This also means it probably wouldn't be a bad idea to
replace the interlock switch which might have been affected if your oven
fails with a blown fuse due to a door problem.
Failed door interlocks account for the majority of microwave oven problems -
perhaps as high as 75 percent. This is not surprising considering that two
of the three switches carry the full oven current - any deterioration of the
contacts results in increased resistance leading to their heating and
further deterioration. And, opening the door to interrupt a cook cycle
results in arcing at the contacts. Complete meltdowns are not unusual! If
any defective door switches are found, it is probably a good idea to replace
all of them as long as the oven is already apart.
The typical door switches and their function:
* Door Sensing: Input to the microcontroller to indicate the state of
the door.
* Interlock Monitor: Shorts out the AC line (and blows the main fuse)
should the Primary Interlock not open due to incorrect sequencing of the
door switches or a failed switch.
* Primary Interlock: In series with the high voltage (magnetron) power
supply so cuts power when the door is open.
Note that if the Door Sensing switch should malfunction, peculiar behavior
may occur (like the fan or turntable operating at the wrong time) but should
never result in microwaves being generated with the door open.
Testing and replacing of interlock switches
With the oven unplugged, put an ohmmeter across the AC input just before the
interlocks (but beyond the power relay or triac if it precedes these). Open
and close the door slowly several times - there should be no significant
change in resistance and it should be more than a few ohms. If it approaches
zero while opening or closing the door, the interlock switches and door
alignment should be checked. (You may need to disconnect one side of the
transformer primary since its resistance is a fraction of an ohm. Refer to
the schematic pasted inside the cover.)
Replace with switches having a precisely identical fit and equal or better
electrical specifications (terminal configuration, current rating). When
removing the old switch make a note as to where each wire goes. Check the
embossed marking on the old switch - don't depend on location as your
replacement might just have a different arrangement. Make sure the new
switch aligns correctly with the actuating mechanism and then check for
correct electrical operation with an ohmmeter before applying power.
Even slamming the door really hard has been known to knock an interlock
switch out of position, resulting in breaker tripping at the electrical
service panel whenever the microwave oven door was closed. (Another reason
to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
if there was some kind of "event" after which the microwave failed, check
the interlock mechanism first - a switch may just need to be popped back
into place.
Credit to author :
Notes on the Troubleshooting and Repair of Microwave Ovens
Version 3.44
Copyright © 1994-2004
Samuel M. Goldwasser
--- All Rights Reserved ---
http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris
From Jim
[email protected]
Remove 99 to reply.
----- Original Message -----
From: Pat
Newsgroups: sci.electronics.repair
Sent: Tuesday, November 16, 2004 9:29 AM
Subject: Sharp microwave oven problem
My Sharp convection microwave (model R-9H66) trips the circuit breaker
everytime it's turned on. It started doing this after its door was slammed
shut by someone. It's obviously shorting somewhere when turned on, and I
took the outer housing off to look at the wiring to see if there was
anything obvious,but didn't notice anything.
Does anyone have any idea what might be wrong or what to look for? It was a
fairly expensive unit when I bought it 6 years ago, and I'd hate to just
through it away, especially if it turns out to be something relatively easy
to repair.
Thanks for any help. I appreciate it.
Pat
