AUTOMATIC CONTROLS The basic function of an automatic control is turning current on and off as required to maintain a desired temperature. 

Bimetallic Blade Control Switch The bimetallic blade control switch, as shown in figure 7-12, is operated by the expanding and contracting effect caused by heating two pieces of dissimilar metals that are welded together. Repair of this control is limited to an adjustment in its temperature setting.

Figure 7-12.- Bimetallic blade control switch.

Figure 7-13.- A. Helix control; B. Helix components.

Helix Control A helix control (fig. 7-13, view A) is used in some domestic, semi-commercial, and commercial installations. It is operated by thermostatic metal that coils and uncoils when heat is applied, regulating a switch to make and break the electric circuit. Repairing this unit consists of replacing miscellaneous parts, such as the dials, switch assembly, helix, helix guard, and so on (fig. 7-13, view B).

Hydraulic Control A hydraulic control is probably the most frequently used control for automatically regulating the temperature of cooking appliances. In this control, a capillary tube filled with fluid is connected to hydraulic bellows (fig. 7-14). A bulb located on one end of the capillary tube is filled with fluid and then is put in the heat zone. It transfers heat-created pressure through the capillary to the diaphragm. The expanding diaphragm presses a system of levers that snap the electrical contacts open, thus cutting off electrical current to the controls. Automatic recycling takes place with slight temperature drops, maintaining a constant set temperature.

the two small, slotted lock screws (fig. 7-14) that are behind the dial. Now, with one hand, hold the main center hub of the control and move the slotted adjusting plate to the right to raise the temperature or to the left to lower the temperature. That is a delicate adjustment, so move the dial a little at a time.

Certain types of hydraulic controls have a safety device, normally set at 450 F, that trips upon reaching the set temperature and requires manual resetting. It may take several hours before the appliance has cooled down enough to allow the operator to reset the unit.

If you find that the circular slots in the adjusting plate prevent you from turning the plate for the desired temperature change, remove the two lock screws carefully so you can move the adjusting plate a full 180 degrees. Replace the two adjusting screws and continue the calibration process as before.

Repairing a hydraulic control is usually limited to adjusting the temperature setting to correspond with the temperature recorded in the appliance. You can adjust the temperature control by loosening, but not removing, A timer is used to control a circuit to an appliance automatically or give a warning to the operator that a predetermined time has lapsed. Figure 7-15 shows a typical wiring diagram of an electric range single-pole oven timer.

Figure 7-14.- Hydraulic control.

Figure 7-15.- Single-pole oven timer wiring schematic. APPLIANCE AND EQUIPMENT MAINTENANCE, REPAIRS, AND TROUBLESHOOTING

One of the primary responsibilities of a CE is to ensure proper operation of switches and elements used in appliances and equipment. That can be done by developing and following a good inspection and maintenance program. Periodically, you should check all equipment for loose connections, burned or pitted contacts, and the improper mounting of switches. You should also check for bad connections, improper mounting, and loose or broken insulators on coils or ribbon elements.

Identifying and locating problems within circuits, appliances, and equipment is a challenging job. This phase of your rating as a Construction Electrician (CE) working within a maintenance shop is where much of your career will be spent.

The inspection and maintenance of switches and elements on appliances and equipment go hand in hand, and, in most cases, a problem discovered during inspection is corrected on the spot and requires no further work until the next inspection.