Temperature control devices continue…

Variable thermostats

Aschematic diagram of the internal workings of a pod-type thermostat. This type is used in appliances that have a variable temperature control. It consists of an oil-filled pod connected to the switch by a capillary tube. When the oil in the pod is heated or cooled, it expands or contracts in the tube and operates a diaphragm. The diaphragm acts on the switchgear thus breaking the circuit and in this instance, ‘making’ the other. The cooling oil contracts, pulling the switch the opposite way — back to its original position; the process repeats if temperatures change.

When removing a variable thermostat you should ease the pod gently from its position at the base of the capillary tube, taking care not to kink or pull unduly on the capillary tube itself. When fitting or refitting this type of thermostat, make sure that the capillary tube does not touch any electrical contacts, such as the heater terminals, or any moving parts or sharp edges. After fitting re-check the entire length of the tube for contact with these items. Any coiled sections in the original should be duplicated in the replacement unit.

The iron stat shown is a variable thermostat using the bi-metal strip principle. The large bi-metal strip has one contact point on it. The opposing contact point is mounted on a movable threaded shaft which is linked to the temperature control knob. Heat causes the bi-metal strip to distort which causes the contact to make or break, depending on the degree of distortion required relevant to the movable contact point’s position — a simple yet effective means of temperature control.

Unfortunately, the constant operation and flexing of the bi-metal during normal use can quickly tire it, resulting in ineffective temperature regulation. In some cases, a degree of adjustment to the stat is possible and recalibration will often be necessary.

A pyrometer is required if an iron needs to be recalibrated. With many appliances, it is advisable to obtain a new thermostat which has been factory calibrated. Take care when handling such stats as the bimetal strips can be very sharp and can also easily be knocked out of calibration.

Thermal overload cut-out

The thermal overload cut-out (TOC) is a safety device that is generally connected in-line with the item it is protecting. For example, power supplied to a heating element will first pass through the TOC and on to the heater. If the safe working temperature of the heater is exceeded, the TOC will operate, go open circuit and cut power to the element.

There are two ways in which the TOC can reset when normal working temperature has been reached.

The simplest is a self-setting version which works on the bi-metal strip principle. This type can give rise to cycling of the fault, that is, constant heating and cooling of the element via the TOC’s tripping on overheat and resetting when cool. If this is not spotted and the cause for the overheat rectified, the TOC will eventually fail. This may simply be a failure to reset, or worse, the contacts of the TOC may short or weld together and render the device completely useless.

The second type of TOC requires manual resetting once it has tripped, usually simply by pressing a reset button. Remember to unplug the appliance before doing this and always check for the cause of the tripping and rectify as required. Because this type of TOC cannot reset itself the danger of cycling is eliminated.

The shapes, sizes and styles of bi-metal TOCs vary. Some look identical to fixed thermostats and are housed in an insulated container. In others the bi-metal disc or blade is not covered. Such exposed types can also trip if overload current occurs. For example, if a motor fails to run due to seizing up, current loading would be higher than normal causing the TOC in circuit with the motor to heat up and trip out..

Other thermal safety devices

Many modern appliances requiring protection from overheating are fitted with a different overheat fail-safe device that avoids the use of moving parts and contact points that may in themselves fail. This device also has the benefit of being very small. It has several names: thermal fuse, micro-temp and safety diode. Its function is the same as a TOC, but once it has tripped, it cannot reset itself or be reset manually and renewal is the only answer. This type of system provides an extremely high level of safety to many items that, in the past, could give rise to very dangerous situations through overheating. Although small, it is easily recognizable but it may be housed in a protective sheath in the appliance. The outer shell will give details of the normal operational temperature.

Testing is as before: if safe, closed circuit, if tripped, open circuit. Make sure that an identical replacement is obtained and properly fitted. Crimp-fit securely, ensuring that the new fuse is in the same direction and fitted in the same position as the original. Do not by-pass this item.

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