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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