I was nicely asked if I could take a look at a disabled toaster unit that stopped doing its job.
Post-cleanup it looks like this:
The quick repair job turned out to be quite entertaining in figuring out post-mortem how the unit failed and the questionable design decisions. It involves no complicated electronics and it might help other people figure out what to look for before buying an appliance. With another layer of failures stacked on top, this unit could have set the house on fire or shocked someone.
Fortunately no damages were recorded, but the deep-dive might prove interesting to some people.
The stainless (?) top comes off by loosening the screws that hold the bottom and short sides together. It can then be gently pried apart, yielding its crummy treasures.
I guess the way the toaster collects crumbs is already a preview to the attention that went into the functional design of it.
The right side houses the temperature indicator daughter board (top) and the high power electronics (bottom). For details on how a toaster works there's a great video by Technology Connections.
The left side contains just a slider that mirrors the movement of the right one. It also mirrors the spring arrangement that drives the bagel bars up. This seems unimportant for now but will become relevant until the end of the post. Notice that the spring is connected at the bottom with the "bagel bar" but not at the top side.
The main board has a classical setup where the electromagnet (yellow) pulls on the part above it when the slider is pushed down all the way. That is: as the toaster slider is pushed, it rotates the white roller and brings in the magnet (black) close to the electromagnet. The white roller has an eccentric which pushes down on the two contacts below it.
As the springy bars below the roller contact the bottom surface, the electromagnet becomes energized and keeps the black magnet drawn to it. The contacts also close the circuit to the heater bars, that's why they spark when the slider is first brought down - a lot of current is being drawn by the heaters.
After the microprocessor has determined that time has run out, it kills the current to the electromagnet and the white piece springs up, breaking the contact.
After removing and cleaning the circuit board, it's easy to see that a big chunk of copper was vaporized, just below and to the right of the BSHG silkscreen text.
This is just below a spring contact, the one which is apparently stuck down (right, on the picture before). At this point I was thinking that the contact was welded from the factory, which doesn't make much sense in hindsight. If that contact would be welded, it would mean that, depending which way the unit was plugged in, there could always be 220V on the heater bars, shocking anyone trying to reach inside for a bread.
Looking again on the right side of the unit, there is a thin black wire that's connected to the 220V line and is going inside the heater. It is also connected to the "bagel bar" and thus to the entire unit's case.
However, on the left side, the small bagel bar eyelet is connected to another bar, this way you can see how the bars are supposed to work. It seems that on the right side, the bar got disconnected from the other bar which is supposed to raise and lower it.
At this point I was sure that: the black wire should be touching the bar and that the spring contact was not supposed to be welded down. So I freed up both of those and moved the bar to its home position.
As you can see, it only takes about 1 mm of flexing for the top bar to get disconnected from the bottom bar. As soon as that happens, the bottom bar (with the hole/eyelet) is free to fall on the heating element below it, on the terminal, and make contact with it. If that happens while the unit is powered on, it will connect the entire case to the line voltage.
If the case is earthed (as it should be) it would just cause a big spark, a mild explosion, vaporize all the copper on the way, weld the contacts and (eventually) trip the house breaker.
If the case is earthed and the unit is unplugged, on the next plug-in there will be a big spark and the house breaker will trip (darkness).
If the socket in which the unit is plugged has a defective earth, the toaster will become live and might electrocute anyone touching it.
But how did the two bars become disengaged in the first place? Why would they flex if the path they take is almost carefully designed?
Enter initial failure stage:
The spring connected the two sides popped off from one of them. There are two springs (left and right) and each connects two sides each (front and back). If one get disconnected, the disconnected bars will be forced to move outside their range of motion by the other side. They will get stretched as the person tries to manually lower and raise the bagel bars.
Under normal circumstances, the middle of the spring is moved by the slider and the two side connections (front/back) to the bars will follow. If one gets disconnected, the bar assembly will be moved by the forces from the other side of the toaster. If this happens many times, there is a chance that the two connecting bars will spring from each other. This could have been prevented with a 0.1 cent retention ring or just a bend in the inner bar.
My actions:
Post-cleanup it looks like this:
The quick repair job turned out to be quite entertaining in figuring out post-mortem how the unit failed and the questionable design decisions. It involves no complicated electronics and it might help other people figure out what to look for before buying an appliance. With another layer of failures stacked on top, this unit could have set the house on fire or shocked someone.
Fortunately no damages were recorded, but the deep-dive might prove interesting to some people.
The stainless (?) top comes off by loosening the screws that hold the bottom and short sides together. It can then be gently pried apart, yielding its crummy treasures.
I guess the way the toaster collects crumbs is already a preview to the attention that went into the functional design of it.
The right side houses the temperature indicator daughter board (top) and the high power electronics (bottom). For details on how a toaster works there's a great video by Technology Connections.
The left side contains just a slider that mirrors the movement of the right one. It also mirrors the spring arrangement that drives the bagel bars up. This seems unimportant for now but will become relevant until the end of the post. Notice that the spring is connected at the bottom with the "bagel bar" but not at the top side.
The main board has a classical setup where the electromagnet (yellow) pulls on the part above it when the slider is pushed down all the way. That is: as the toaster slider is pushed, it rotates the white roller and brings in the magnet (black) close to the electromagnet. The white roller has an eccentric which pushes down on the two contacts below it.
As the springy bars below the roller contact the bottom surface, the electromagnet becomes energized and keeps the black magnet drawn to it. The contacts also close the circuit to the heater bars, that's why they spark when the slider is first brought down - a lot of current is being drawn by the heaters.
After the microprocessor has determined that time has run out, it kills the current to the electromagnet and the white piece springs up, breaking the contact.
After removing and cleaning the circuit board, it's easy to see that a big chunk of copper was vaporized, just below and to the right of the BSHG silkscreen text.
This is just below a spring contact, the one which is apparently stuck down (right, on the picture before). At this point I was thinking that the contact was welded from the factory, which doesn't make much sense in hindsight. If that contact would be welded, it would mean that, depending which way the unit was plugged in, there could always be 220V on the heater bars, shocking anyone trying to reach inside for a bread.
Looking again on the right side of the unit, there is a thin black wire that's connected to the 220V line and is going inside the heater. It is also connected to the "bagel bar" and thus to the entire unit's case.
However, on the left side, the small bagel bar eyelet is connected to another bar, this way you can see how the bars are supposed to work. It seems that on the right side, the bar got disconnected from the other bar which is supposed to raise and lower it.
At this point I was sure that: the black wire should be touching the bar and that the spring contact was not supposed to be welded down. So I freed up both of those and moved the bar to its home position.
As you can see, it only takes about 1 mm of flexing for the top bar to get disconnected from the bottom bar. As soon as that happens, the bottom bar (with the hole/eyelet) is free to fall on the heating element below it, on the terminal, and make contact with it. If that happens while the unit is powered on, it will connect the entire case to the line voltage.
If the case is earthed (as it should be) it would just cause a big spark, a mild explosion, vaporize all the copper on the way, weld the contacts and (eventually) trip the house breaker.
If the case is earthed and the unit is unplugged, on the next plug-in there will be a big spark and the house breaker will trip (darkness).
If the socket in which the unit is plugged has a defective earth, the toaster will become live and might electrocute anyone touching it.
But how did the two bars become disengaged in the first place? Why would they flex if the path they take is almost carefully designed?
Enter initial failure stage:
The spring connected the two sides popped off from one of them. There are two springs (left and right) and each connects two sides each (front and back). If one get disconnected, the disconnected bars will be forced to move outside their range of motion by the other side. They will get stretched as the person tries to manually lower and raise the bagel bars.
Under normal circumstances, the middle of the spring is moved by the slider and the two side connections (front/back) to the bars will follow. If one gets disconnected, the bar assembly will be moved by the forces from the other side of the toaster. If this happens many times, there is a chance that the two connecting bars will spring from each other. This could have been prevented with a 0.1 cent retention ring or just a bend in the inner bar.
My actions:
- repair the blown-out trace with a thick piece of wire
- disconnect the welded spring contact and clean all the mating surfaces
- bend the inner (upper/outside) bar so it is able to protrude more from the eyelet
- bend the outer (lower) bar so it is able to hold better to the other one
- install a blocking device that will prevent the inner bar from falling too far down
- install an insulating harness on the electrode sticking out of the heater element
- recommend the owner that they shop for another toaster
Summary of the failure
The spring that kept the bagel assembly in place failed on one side of the toaster. This meant they either have to be moved by hand into position or that the toaster slider moved much harder than usual. Either one of these actions would cause the bagel assembly to bend and stretch on one side and eventually to become mechanically disconnected. The now loose lower bar rotated (fell) down onto the live heating element, connecting the entire metal case to 220V. Because the case was earthed, this caused 220V to flow like this: socket -> spring contact -> heater -> case -> Earth welding and melting everything in its path. There was a big spark and then the lights went out.
Comments
Post a Comment
Due to spammers, comments sometimes will go into a moderation queue. Apologies to real users.