Hottop Heater Control

It could be. There are two problems, though. One is a cost issue and the other is related to physics.

The first problem is that it is much cheaper to switch a fixed voltage on and off than to vary it up and down. Particularly an AC voltage. A simple solid-state (or even mechanical) relay that costs $5.00 or less can switch AC on and off. That's the way the Hottop works. (This technique is sometimes called "digital proportional control.") An analog controller would be much more expensive and the inefficiencies of analog power transfer would limit the amount of beans that could be roasted.

The Hottop divides up time into slices (wandering John mentioned two seconds.) The amount of time that the heater is enabled during that two seconds is called the duty cycle. For instance, if the heater is enabled half the time, then the duty cycle is 50%. If it's enabled one-fourth of the time the duty cycle is 25%.

Temperature from a sensor is fed back to the controller (in this case a microprocessor) that determines what percentage of the time to leave the heater on. This sounds very simple, but in order to avoid temperature overshoot or, in the worst case, oscillations, some sort of integrative control must be established. You've probably heard us talk here about "PIDs." The microprocessor implements a PID in software.

Because the line voltage is not regulated, but simply switched on and off, it should be obvious that the heating capability of the roaster is related to the line voltage. Because power is equal to the applied voltage squared divided by the resistance of the circuit, the effects of variations in line voltage are magnified.

As the line voltage *increases*, the heating element emits more heat, and the Hottop will reduce the duty cycle so that the average applied heat is the same. In that respect, the Hottop does compensate for line voltage variations. But that's not where the problem is.

The problem is that as line voltage *decreases*, the Hottop will increase the duty cycle until at some point the duty cycle is 100%. At that point, the Hottop must lose regulation because it has no way to apply more heat.

So the answer to the first part is that because the Hottop uses digital proportional control its range of control under low line voltages is limited. A roaster could be designed to use an analog heat controller, but it would be much more expensive and would run into the next problem: lack of energy at the wall outlet.

The second problem, related to physics, is that the amount of energy available for heating at a typical 117VAC 20A wall socket is limited. If you want to roast half-pound batches of coffee in less than 20 minutes, then it takes a lot of energy. Household appliances made to run off the aforementioned wall socket are limited to about 1500 watts. That isn't enough to give a comfortable margin of control. In other words, if the roast gets behind, there is not enough power available to catch it back up again. The duty cycle becomes 100% and that's all she wrote.

On the other hand, if you designed a roaster to run off a 240VAC 30A circuit, you could design it so that it never went out of control with a half-pound batch.

Or you could design a 117VAC roaster that was insensitive to line voltage changes but only roasted a quarter-pound at a time. But would you pay $600 for such a roaster?