-----Original Message----- From: Barry King To: lilel@toastmaster.com Date: Friday, May 07, 1999 11:33 AM Subject: Re: PIC-based Oven Controller >> Uh-oh! Bang-Bang control scheme - run away fast! >It is interesting that the duty cycling in the electric oven is also >balancing the radiation versus convection which make a huge >difference in browning / crisping effects as opposed to strictly >heating. So, Lawrence, what is the PWM frequency of the bimetal >thermostat in an electric range's oven? I think hear it "click" and >cycle the pilot lamp on the order of 5 minutes apart. In other >words, in practice it is heating in bursts, so there are bursts of >radiated, while heating, then a lagging rise in temperature, then it >cycles off. There must be a large hysteresis band, no? Let's see. My comments will apply most to small ovens, and your mileage may vary using large ovens. The thermostat controlled ovens I work on cycle at 10-20 second intervals at high temperatures (425F) and 30 sec to 1 minute intervals at low temperatures (350F). During these periods the unit will cycle "ON" for 2 seconds (hot end) to 10 seconds (cool end) or so. Hysteresis band is about 5 degrees C (9 degrees F) meaning the unit will vary in temperature by this much while trying to maintain a setpoint. A more massive oven might operate much differently. I'm thinking that the radiation effects will be smaller in a large oven. radiation is a square law phenomenon, and being 6 or 8 inches from a heating element is a lot different from being 2 inches from it. Besides, gas ovens have no radiation at all, just heat. I'm beginning to think the radiation effects are much more pronounced in a tabletop oven. THermostats work in a wierd fashion. They turn ON in response to oven temperature. But they contain a small internal heating element, caled an anticipator. They turn OFF when the anticipator runs the temperature of the stat up too high. This is how they prevent the stat from doing bang-bang control. Selection of the anticipator is a black art. >