How conductive is a coffee bean or convection oven failure mode.

[GiskardReventlov]

A while back (before I even heard of eevblog) the kitchen counter-top convection oven started burning. Unplugging solved the problem. I kept it around so that I could look inside to see what happened later.  I am asking you all what do you think caused the failure?  My guess is that a coffee bean found it's way in through the top miraculously landed across these diodes causing a short. Yes the vents on the top left are unobstructed and allow things to fall right through. I found a scorched coffee bean inside.


The burnt part is 1N5408 KEL

According to digi-key it is:

DIODE GEN PURPOSE 1KV 3A DO201AD

I no longer own a toaster oven or convection oven, over the years there have been too many problems with these things.

[SeanB]

I doubt it was the bean, more the poor match between the diodes that meant the one ended up carrying most of the current. It then got hot, lowered it's forward voltage and went into thermal runaway. Not helped by being too low in any case to carry the typical oven current of 6-10A in any case, so it was really cooked to death.

[GiskardReventlov]

@SeanB,
How do components in parallel become out of sync (for lack of correct description)?  Would 3 or 4 in parallel offered more protection or just higher probability of failure?

[AG6QR]

@SeanB,
How do components in parallel become out of sync (for lack of correct description)?  Would 3 or 4 in parallel offered more protection or just higher probability of failure?

If you keep a constant voltage across a diode, then as it gets hotter, it will conduct very much more current.  This behavior is the opposite of most resistors, and it leads to a positive feedbag spiral of "current hogging".

Suppose you have to handle a current of one amp.  And let's say you can easily find diodes that are capable of handling 0.5A.  So you decide to put three of them in parallel, so that each one will handle 0.33A, giving you a nice margin of safety, right?

Problem is, the diodes won't be perfectly matched, or even if they're perfectly matched at the factory, they won't be exactly at the same temperature in your device.  Suppose one gets a tiny bit warm and starts conducting 0.35A, while the others are handling 0.325A each.  The one carrying more current will get relatively warmer (because of the energy dissipated by the higher current), and carry more current, while the ones carrying less current will get relatively cooler, and conduct less.  Soon one is carrying 0.4A, while the others are carrying 0.3A.  This is a positive feedback spiral, and it won't be long before the one that's conducting more warms up still further, conducts more, and eventually exceeds its 0.5A rating.

It will soon fail open circuit and cause the full 1.0A to be shared among the remaining two diodes.  One of those two will warm up more than the other, hog the current, and fail.

Google "current hogging" for more explanation and examples.

The only good solution is to insert some means distributing the current among the various diodes, so that if one starts conducting a bit better, it won't be allowed to hog all the current.  Or to use one big diode instead of lots of little ones in parallel.

[GiskardReventlov]

@AG6QR,
This makes sense to me, what would happen if you had 1amp current and you use two 1.1A diodes? Either one alone can handle the total and they couldn't cost that much more. 

FWIW the oven brand was Euro Pro, the website is no longer but was listed as euro-pro.com on the back label.

I wonder if you can tell me what the device is that is used in the temperature control, it's to the right of the burnt diode in the last two pics. The control, when turned, moves in/out over the range of a few millimeters. It's got white insulators and aluminum.

I was surprised when I tore down the oven as there is just not much there.

 

[SeanB]

Thermal regulator, basically a bimetal strip that heats up from the current flowing through it and bending out to open the contacts. As you turn the knob from zero the contacts start out at open, and as you go around they oscillate slowly till at full they are closed. Used to give a rough and ready power control by doing coarse pulse width modulation mechanically with the oven and heater doing the averaging.

[amyk]

What would an oven which doesn't have an electronic control, like this one appeared to be, even need a diode for?

[SeanB]

I would guess to lower the heater power, as likely they wanted a 600W or so heater, but in the size they got the cheapest was a 1.5kW unit, so the diodes were added to lower the power of the heater to a low level so as not to cremate the contents of the oven. Simple solution as heaters are made in specific power ratings related to length. If you want a lower power then it becomes a special manufacture and a lot more expensive to do instead of just using the cheap mass made winding and size.

[GiskardReventlov]

@SeanB,

Are you saying that it uses feedback (i.e. oven temp.) to maintain ~constant temperature?

That's really interesting.  An interesting approach. I will guess it's an old idea that is cheap. I don't know how accurate it was. Now the aluminum makes sense too.  I've kept it so will have to study it some more. It was not easy to extract. They epoxy-glued the knobs on.

[GiskardReventlov]

Simple solution as heaters are made in specific power ratings related to length.

The heating elements are off-the-shelf (or leftover from some other house-burner project) so they just slap these too-small diodes in to reduce the heating output of the elements because the heating elements are over-sized.  I will never shop the same way again!

I guess I can look for datasheets. It seems then I would look for length of heating element, diameter, composition, max desired temps.

[SeanB]

No, totally open loop. The knob adjusts duty cycle and is crudely calibrated ( if you can call it that) to keep a constant ( sort of) temperature inside the oven. The current through the switch is used as a heat source only, not as a temperature measurement. It does compensate slightly for mains voltage variations, but that is a side effect of the construction. It really is cheap and simple, calibration is by bending parts during manufacture.

[GiskardReventlov]

It really is cheap and simple, calibration is by bending parts during manufacture.

Oh my holy crap. No wonder so much stuff burnt in that heap-O-shit.

So the knob is probably made in a location separate from oven assembly and if that's true it's conceivable that while assembling the "calibration" could get mangled and voila!!  A 1500watt foodburner/firestarter.

[SeanB]

I buy them as spare parts, used in both cheap 2 plate stoves or cheap ovens. You kind of have to bend them to get it working during installation, but with a stove the cal points are off at off and full on near full on. No saying what it does in the middle other than switching somewhat. It is a truly Guess-o-matic control.

[GiskardReventlov]

What are some alternative solutions for this kind of control?  The cost on these must be very low, there's just not much there. I'm asking as a novice what other approach or design might be used to match the cost but provide a better solution.  I was thinking a rheostat. But just guessing.

[AG6QR]

What are some alternative solutions for this kind of control? ...  I was thinking a rheostat. But just guessing.

The problem with a rheostat is that it expends energy in the control.  Power is I^2 * R, or alternatively, V^2 / R.  If the resistance is zero or infinite, power disspiated is zero.  That thermostat does a pretty good approximation of staying at either zero resistance or infinite resistance, so it does a pretty good approximation of dissipating zero power in the thermostat itself. 

On the other hand, a rheostat would have to dissipate significant power in the rheostat itself. 

If you really wanted to control a large AC mains-powered heater with more finesse than a simple on/off switch, a variac might be a good solution.  But not cheap.  Some sort of triac-based solution of the sort commonly used for dimming incandescent lights could also work, but would probably produce RFI, and probably wouldn't be as cheap as what they used.

Furthermore, for temperature control, you usually have a target temperature to maintain, so you want to turn the dial to set the temperature, not to set a target power level.  You need a temperature sensor and a negative feedback mechanism to maintain the target temperature.  Without a feedback mechanism, a rheostat, variac, or triac-based dimmer is only part of the solution to temperature control.  The thermostat incorporates the temperature monitor, feedback mechanism, and the on/off switching control into one inexpensive device.

When the application is cooking food, a simple on/off control is normally plenty good enough to maintain the temperature within tolerance, and it's cheap, simple, and effective.  It's also what we often use for residential heating and air conditioning.