OK - perhaps you're looking at this problem from the wrong end - you'll probably get thermal inertia problems with type-k thermocouples, so why not measure the resistance of the heating element? I have a temperature controlled soldering iron design that uses this principle - all it needs is a triac to turn the heating element on and off, a comparator IC, a transistor, and a few passive components. Temperature is set with a linear pot. Calibration requires the use of a type-k, but once it's set up, it will maintain the temperature you've set +/- 2 C, and only takes about 20 seconds to get there. A check of the tip temperature at various settings showed that once calibrated, it was pretty good.... If anyone wants a sketch of the circuit, you can e-mail me off-list. It might take a day or so to locate the original design (it's buried in a filing cabinet), but some of you may find it useful.... Chris ----- Original Message ----- From: "Roman Black" To: Sent: Sunday, December 01, 2002 11:49 AM Subject: Re: [TLUG]: [OT]: Toaster oven thermostats (SMD cooking) > 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 > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body