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'[TLUG]: [OT]: Toaster oven thermostats (SMD cooki'
2002\11\30@173612 by Peter L. Peres

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Hi all,

I have done some testing and have found out that most (I tried three)
electric ovens and toasters have a max temperature of 150-170C after which
an internal safety thermostat turns off the heater no matter what.

Can anyone confirm this ? Esp. Lawrence L. who should be an expert on
this. I seem to remember that even fondue is limited under 200C, because
over 200C something or other is produced that may be toxic to people
(something carbonized or such).

Along the same lines, would it be safe (from the point of view of
insulation, melting materials etc) to tamper with this thermostat just a
little bit ? Say, 50 degrees up ?

Peter

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2002\11\30@175212 by Sid Weaver

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In a message dated 11/30/2002 17:36:32 Eastern Standard Time,
spam_OUTplpTakeThisOuTspamACTCOM.CO.IL writes:


{Quote hidden}

Stick your toaster in a regular oven, set it for 425 deg. and see what
happens.  That's about 200C.

Sid

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2002\11\30@181703 by Dave King

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At 05:51 PM 30/11/02 -0500, you wrote:
{Quote hidden}

200°C = 392°F

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2002\11\30@193014 by Wagner Lipnharski

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Peter L. Peres wrote:
> Hi all,
>
> I have done some testing and have found out that most (I tried three)
> electric ovens and toasters have a max temperature of 150-170C after
> which an internal safety thermostat turns off the heater no matter
> what.
>
> Can anyone confirm this ? Esp. Lawrence L. who should be an expert on
> this. I seem to remember that even fondue is limited under 200C,
> because over 200C something or other is produced that may be toxic to
> people (something carbonized or such).
>
> Along the same lines, would it be safe (from the point of view of
> insulation, melting materials etc) to tamper with this thermostat
> just a little bit ? Say, 50 degrees up ?
>
> Peter


Hi Peter.
I just ordered an electronic thermometer with a type K thermocouple to
build the temperature control for my oven.

The initial tests show that the thermostat is faaaar from being helpful.

The time the temperature takes to get to the thermostat, it bounced very
dangerously for the SMD components, I mean, the peak temperature inside the
oven goes way up until the thermostat reacts, it has a large time delay.

Using 3 x 1N4148 diodes in series, I got the following reading of VDROP
when exposing them into a professional oven;

80F...  1.82V
350F..  1.40V
400F..  1.28V
450F..  1.19V
500F..  1.09V
550F..  1.04V

Setting the thermostat of my cheap kitchen oven to 350F, the same 3 diodes
hanging in the middle of the toaster, generated the following VDROP;

Warm
1.41V            1.41V            1.41V
  \             .' \             .' \
   \          .'    \          .'    \
 60s\   300s.'    60s\   300s.'    60s\
     \    .'          \    .'          \
      \ .'             \ .'             \
     1.088V           1.088V           1.088V
       Hot

Both elements turn on for 60 seconds, the diodes takes down VDROP to
1.088V, elements off for 300 seconds (5 minutes), diodes takes VDROP up to
1.41V, turn on elements for 60 seconds, etc.

From the relative VDROP from the prof oven (darn until I don't have the
digital thermometer), it seems that the elements heat up the temp to more
than 500F for the thermostat turn them off, then the oven cools down to
around 220F to turn the elements on again.

For cooking food this temperature ping pong should be ok, since a beef can
take the temperature average, but for a tinny poor SMD transistor or
capacitor, this will be a killing.

The worse part is when you take the oven from cool to 200F at the first
stage of SMD soldering.  Setting the thermostat to 200F and turning on the
toaster, it takes 70 seconds with both elements full power, then the
thermostat turn then off. There is an enourmous temperature delta from the
oven chamber to the thermostat chamber.  From what I noticed, 70 seconds of
full power can take the oven chamber to more than 500F during the last 15
seconds.  This can probably force a premature damage in several SMDs in
near future.

Setting the thermostat for 200F, it seems that the temperature goes this
way into the oven;

500F             .-'-.OFF                      OFF
              .' |   '-.                     -'-.
             /   |      '-.                .' |  '-.
200F---------/----|---------'-.------------/---|-----'-.
           /     |            '-.        /    |
150F      .'      |               '-.    /     |
130F    .'        |                  '-.'      |
    .-'          |                    ON      |
80F-'ON----70s--->|<--------9min------->|-30s--|


---------

The 3 diodes were connected using small metal clips, since solder would not
hold under 450°F.  Used high temperature wire for 500°F to connect the
diodes to outside of the oven.  The same metal clips tied the diodes to the
wires.

One could build a simple microcontroller to drive a solid-state-relay (or
TRIAC with a zero-crossing opto-coupler), based on the diodes VDROP. It
would react much faster than the horrible thermostat.  To get better close
to the real PCB temperature, the diodes could be assembled on a real small
PCB (no soldering) and it could be sit side by side with the actual boards
being soldered.

As the microcontroller job would be to control the oven temperature for
200, 320 and 450F (SMD soldering profile), it needs to keep track of the
diodes VDROP for those 3 temps, something around  1.55V for 200F (4
minutes), 1.42V for 320F (2 minutes) and 1.19V for 450F (30 to 60 seconds).

This voltage is perfectly traceable by any uC with an internal ADC or at
least a comparator (as the AVR AT90S2313).

For the AVR 2313, I thought to use something simple like that;

+5V--o-----o-----------> AVR
    |     |
   R10k  R10k
    |     |      .-----------.
    |     |      |    2313   |
    o-----|------|+          |
    |     |      |Comparator |----> Relay
   _V_    o------|-          |
    |     |      |           |
   _V_    |      '-----------'
    |     |            |  |
   _V_    o--R1--->|---'  |
    |     |               |
    |     o--R2--->|------'
    |     |
    |    R2k
    |     |
   _|_   _|_
   GND   GND

Port pins driving R1 and R2 down change the voltage divider at the
Comparator (-) input. The possible 3 states should be very closer to the
diodes VDROP at the 3 tracked temperatures.

Another easy solution is using a timer to set a PWM value over a port pin,
this feed a capacitor acting as a filter to be compared to the Diodes
VDROP;

+5V--o----------------> AVR
    |
   R10k
    |           .-----------.
    |           |    2313   |
    o-----------|+          |
    |           |Comparator |----> Relay
   _V_    .-----|-          |
    |     |     |           |
   _V_    |     '-----------'
    |     |            |
   _V_    o-----R1-----' _     _
    |     |            _| |___| |___
    |    _|_           PWM Output
    |    --- Cap
    |     |
    |     |
   _|_   _|_
   GND   GND

The PWM output sets a voltage over the cap, to be compared to the diodes
VDROP.
Once the Comparator output is "1" (+ input is more positive than the -
input) it means the oven is cooler than expected and the uC should turn on
the relay (heater elements).  Remember, the diodes reduce their VDROP with
higher temperatures.

Of course this is the "poor's man temperature controller", and I wonder if
diodes from the same lot could have different VDROP profile.

If not enough port pins, you could use this simple design;

+5V--o----------------> AVR
    |
   R10k
    |           .-----------.
    |           |    2313   |
    o-----------|+          |
    |           |Comparator |----> Relay
   _V_    .-----|-          |
    |     |     |           |---.
   _V_    R1    '-----------'   |
    |     |                    |_|< Buzzer
   _V_    |       _     _
    |     |     _| |___| |___
    |    _|_      PWM Output
    |    --- Cap
    |     |
    |     |
   _|_   _|_
   GND   GND

The (-) comparator input can be switched to output and feed PWM for at
least 10 complete waveforms, then switched to input and used as the
comparator input, then switched back to output and so on.

The AVR can drive or sink 20mA at any port pin, enough to drive directly
any Solid-State Relay 3-32Vdc control, 240V @ 25A unit.

All this temp control will cost less than $15 ($10 for the solid-state
relay). The only problem is to physically install everything, since the
damn oven heats all covers to almost the internal temperature.  Probably it
should be an external box that the power cord goes in middle, along with
the diodes wires to the oven.

The idea is to turn the thermostat to 500F (full power) and feed power to
the whole set, the AVR resets and starts the SMD soldering profile,
powering off and alarming at the end.

This oven has a special thermostat position named BROIL that turns full
power to the elements via an extra switch tied to the thermostat shaft.  If
things went not so hot into the thermostat chamber, everything could be
installed there, and this switch feeds power to the electronics. It means
that BROIL position would start the SMD profile soldering, but... it is too
hot inside there.

VV46NER

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'[TLUG]: [OT]: Toaster oven thermostats (SMD cooki'
2002\12\01@064137 by Roman Black
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Peter L. Peres wrote:

> I have done some testing and have found out that most (I tried three)
> electric ovens and toasters have a max temperature of 150-170C after which
> an internal safety thermostat turns off the heater no matter what.
>
> Can anyone confirm this ?


I have repaired many appliances that had 150'C thermal
safety cutouts. BUT these are normally located so as
to measure cabinet temp, not the main heating area
temp. They are for fault conditions, ie if the thermostat
contacts weld closed, a common fault.

A small electric oven will (inside the oven cavity)
get to MUCH more than 200'C. Most of the thermostats
go to 220'C or 250'C and yes the oven will get to
that temp and regulate at that temp, give or take 10%.
:o)
-Roman

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2002\12\01@065839 by Roman Black

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Wagner Lipnharski wrote:

> I just ordered an electronic thermometer with a type K thermocouple to
> build the temperature control for my oven.
>
> Setting the thermostat for 200F, it seems that the temperature goes this
> way into the oven;
>
> 500F             .-'-.OFF                      OFF
>                .' |   '-.                     -'-.
>               /   |      '-.                .' |  '-.
> 200F---------/----|---------'-.------------/---|-----'-.
>             /     |            '-.        /    |
> 150F      .'      |               '-.    /     |
> 130F    .'        |                  '-.'      |
>      .-'          |                    ON      |
> 80F-'ON----70s--->|<--------9min------->|-30s--|



Hi Wagner, that is a great post, especially the text based
charts! :o)

Interesting the use of 1N4148 diodes as temp sensors.
Quite cheap and seems to work well. But are you sure
this will be a reliable long term solution? Normally
SMD soldering charts etc show that silicon deteriorates
badly if kept at an excess of 200'C. The diodes might
not last very long...

One more point, your oven has a HUGE thermal lag!!
This is why you are getting the long cycling times.
As a comparison, my little toaster oven which is about
40cm wide and 25cm high, cycles for 2 seconds ON and
3 seconds OFF when regulating at around 150'C.
The total thermal ripple is only a few percent.

When choosing a toaster oven, find one with large
power (1500W minimum) in a TINY box if possible, with
elements above and below the cavity, pref exposed.
The right oven will regulate at temp with no mods
needed. If you need to go to production quantities
then a proper controller will be a great help, but
if your oven is thermally unsuitable then maybe you
should be looking for a better toaster oven??

Also, you can buy good quality mechanical thermostats
with external copper sense bladder (about $25), if
you couple the sense bladder to the element they
will regulate very closely, within a couple of degrees.
Most of the problem with mechanical thermostats is that
they are cheap garbage or just located too far away
from the heat source.
-Roman

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2002\12\01@102852 by Wagner Lipnharski

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Roman Black wrote:
{Quote hidden}

That's correct. The 1N4148 is not suitable to work above 150°C, but it
works great (for my surprise) up to 230°C even after many cycles to that
temp.  Of course a better sensor is required, perhaps a thermocouple or
even a high-temp thermistor.  Even so, replace the 3 diodes after a while
is so cheap that it may worth the pain.


>
> One more point, your oven has a HUGE thermal lag!!
> This is why you are getting the long cycling times.
> As a comparison, my little toaster oven which is about
> 40cm wide and 25cm high, cycles for 2 seconds ON and
> 3 seconds OFF when regulating at around 150'C.
> The total thermal ripple is only a few percent.


Yes, as I wrote in the later post, an old toaster I had here has much
better temp regulation, thermostat is internal.


> When choosing a toaster oven, find one with large
> power (1500W minimum) in a TINY box if possible, with
> elements above and below the cavity, pref exposed.
> The right oven will regulate at temp with no mods
> needed. If you need to go to production quantities
> then a proper controller will be a great help, but
> if your oven is thermally unsuitable then maybe you
> should be looking for a better toaster oven??


Of course, I would not mind to pay more for a better toaster, it worth's
the final PCB soldering results.
I bout this cheap $19 one (two crystal heaters - top and bottom - 500W
each) and trying to solve its problem with an external temp regulator. Once
the regulator is working nicely in this cheap unit, it can be incorporated
to any other better oven. The idea of the regulator is the automated SMD
soldering profile.  Press [start] and wait for the beep to remove the
boards.  The microcontroller also consider 3 minutes delay after complete
power shutdown, to reduce boards temperature naturally without stressing
components by a possible fast temp break down (fast door open).  The total
operation takes 10 minutes; 4min=preheat, 2min=heat, 1min=melt,
3min=cooldown.


> Also, you can buy good quality mechanical thermostats
> with external copper sense bladder (about $25), if
> you couple the sense bladder to the element they
> will regulate very closely, within a couple of degrees.
> Most of the problem with mechanical thermostats is that
> they are cheap garbage or just located too far away
> from the heat source.
> -Roman

That's quite correct.

I will not bother to build a stainless steel oven, double wall, with a vent
duct window system to control the cool down rate.  I believe that a well
insulated oven can hold and distribute the temperature pretty well without
much re-heat, mostly for a max of 4 minutes time.  This home-build oven
doesn't need to be bigger than 15x22x8cm (6x8x3") inside (useful area).
Heater elements could be anything well distributed, even plain
Nickel-Chromium wire trying to cover most of the walls for a better heat
distribution.  I am considering even the regular spiral heating element of
a regular range, one on top another on bottom. Other than easy to hookup,
the spiral format can helps to apply better distributed heat on top all
around the board under soldering.

I also wonder (will check it next week), what is the average accuracy of
the temp control system for a regular spiral range heater, since the
thermostat is not existent, it rather use current control (power
thermostat) at the knob mechanics.  In a well made pre-warm toaster (lets
say 130°F), the external temperature will not influence much "power vs
temperature" adjustment, then probably just controlling power PWM can set
internal temperature pretty closer to the desired one, without effects from
the external ambient temp.

I believe this oven can be built much cheaper than professional ones for
such purposes.

Any information about the oven insulation material will be welcome (should
I disassemble an old oven?)
How a force ventilation works inside an oven?  I was never lucky to see one
yet. It is a small metal fan blades with a long shaft and external motor?

VV46NER

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2002\12\02@094835 by Roman Black

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Wagner Lipnharski wrote:
> The idea of the regulator is the automated SMD
> soldering profile.  Press [start] and wait for the beep to remove the
> boards.  The microcontroller also consider 3 minutes delay after complete
> power shutdown, to reduce boards temperature naturally without stressing
> components by a possible fast temp break down (fast door open).  The total
> operation takes 10 minutes; 4min=preheat, 2min=heat, 1min=melt,
> 3min=cooldown.

When I worked in the instrument fitters we maintained
a lot of heating ovens for baking steel product.

There is more to consider than just the electronic
control, the most important part is the thermal
characteristics of your cabinet and load (target PCB).

For instance, if you put your accurate sensor closely
coupled to the heating element, it will keep the element
at the right temp, BUT it may take considerable time
for the load temp to rise to match, and worse on
cool-down. It your sensor is too close to the load,
there is a large thermal lag at the element and thermal
ripple will be very high and you get overshoot problems
similar to what you show in your charts. A fan helps a lot,
as does 4 elements around the cavity (like my small oven)
and smaller cavity vs element power helps too.

With correctly considered thermal properties of the oven
you can get great temp regulation even with a crappy
mechanical thermostat. But with bad thermal properties
even a perfect controller might leave you with overshoot,
slow response, or hot/cold spots on your PCB.


> I will not bother to build a stainless steel oven, double wall, with a vent
> duct window system to control the cool down rate.  I believe that a well
> insulated oven can hold and distribute the temperature pretty well without
> much re-heat, mostly for a max of 4 minutes time.  This home-build oven
> doesn't need to be bigger than 15x22x8cm (6x8x3") inside (useful area).
> Heater elements could be anything well distributed, even plain
> Nickel-Chromium wire trying to cover most of the walls

Why not just buy a decent toaster oven? These are the
right size and a good one has the elements correctly
spaced and enough elements to give goo thermals.

The cool down can be done from the controller, I think
the oven will cool quick enough (thats why you don't want
extra insulation) that your controller can "ride" the
down curve adding a tiny bit of power and keep it exact. :o)


> How a force ventilation works inside an oven?  I was never lucky to see one
> yet. It is a small metal fan blades with a long shaft and external motor?

I have, mainly in microwave convection ovens. The fan
is a blower (tube) type, motor is on a shaft a few
inches away. Most appliance repairers will have a
heap of these junked around the place, a polite phone
call should get you one to play with. :o)
-Roman

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2002\12\03@124159 by llile

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Yes, most toaster ovens do have a limit thermostat.  Now, I can't really
recommend tinkering with safety controls ;-)  If you do you are on your
own.  This limit stat will be a bimetal type stat, and there won't be any
way to adjust it really.  Shorting it out would allow the oven to get
hotter, and as they say on Star Trek, "holodeck safeties are offline". If
you do this, make sure you run the oven on a heat-proof surface like a
metal bench or on top of four bricks or a concrete paver, and make sure
you DON"T EVER leave it unattended.

The main thermostat is another animal.  It can be hacked to run hotter,
because the elements can be tweaked with a pair of needle nose pliers.
Examine it closely and you'll figure it out pretty quick.

Make sure you are using an oven with a metal top.  The plastic top ovens
in this line are run just below the safe point, and running them hotter
will probably deteriorate or melt the plastics.  Some of them brown before
melting, some melt before browning, all make an awful stink.  Plastic
sides or front will probably be OK.

You are right about 200C being a good limit for foods, cooking hotter than
this creates some chemicals that are not good to eat, carcinogens and
such.  Since youa re also working with lead, put a sign on this thing that
says DO NOT COOK FOOD IN THIS OVEN, especially if you are working at home
or around idiots.

Most of the wiring in an oven will be rated for 200 or 250 C.  I wouldn't
recommending using such an oven anywhere over 250 C continuously because
of that.  Most ovens will reach 260 - 300 C as they overshoot when you
first turn them on, but the areas with wiring will generally be cooler at
that time.

And remember, I can't recommend you do any of this stuff.

-- Lawrence Lile





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11/30/02 04:32 PM
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       Subject:        [TLUG]: [OT]: Toaster oven thermostats (SMD cooking)


Hi all,

I have done some testing and have found out that most (I tried three)
electric ovens and toasters have a max temperature of 150-170C after which
an internal safety thermostat turns off the heater no matter what.

Can anyone confirm this ? Esp. Lawrence L. who should be an expert on
this. I seem to remember that even fondue is limited under 200C, because
over 200C something or other is produced that may be toxic to people
(something carbonized or such).

Along the same lines, would it be safe (from the point of view of
insulation, melting materials etc) to tamper with this thermostat just a
little bit ? Say, 50 degrees up ?

Peter

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2002\12\03@175640 by Mike Singer

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Roman Black wrote:
>                    ...It your sensor is too close to the load,
> there is a large thermal lag at the element and thermal
> ripple will be very high and you get overshoot problems
> similar to what you show in your charts...

  Automatic Control Theory at its very basics should help
even with slow sensors. It's not a rocket science (Dale
used this term recently).

  Best Regards,
  Mike.

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2002\12\04@135145 by Peter L. Peres

picon face
Lawrence, thank you for your excellent response.

My oven is very old and all metal with bakelite handles. The wiring is
probably in very poor shape. Good point about wiring temperature limit. I
believe that I can tweak the safety thermostat as it is of the open type
(ceramic body + bimetal in hole thereof).

I have nothing to lose so I will try. The oven should have been thrown out
a long time ago. There is no way anyone is going to cook anything in it.

Peter

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