What the heck are we referring to when we talk about a thermal switch or thermal fuse in an overhead projector?

Let's start with the thermal fuse. In many cases the manufacturer will place a thermal fuse in the electrical circuit of your overhead projector to protect the unit from overheating and damaging some of the internal components like the fresnel lens to due excessive heat.

How the thermal fuses works is rather simple in nature. The fuse is rated a a particular temperature rating pre determined by the manufacturer. If for some reason the cooling system in your overhead projector fails the thermal fuse will open once it has reached the pre-determined temperature, hence shutting power off to the projection lamp. Once the thermal fuse has done its job, it will need to be replaced, as it can not be re-set.

A small knowledge of Electricity and soldering will be necessary when replacing the thermal fuse in your overhead projector. I am often asked if this thermal fuse can be bi-passed when it has been used as described above. Keep in mind that this thermal fuse was placed in your overhead projector as protection against damage due to overheating. It is highly recommended that this part always be replaced once determined defective or open.

Now on to our next subject which is thermal switches. There are two types of thermal switches used in overhead projectors today. Normally open and normally closed. Both types of thermal switches are heat activated or heat sensitive.

The normally open thermal switch is generally used to control the cooling fan. Normally opens refers to an open circuit. No electricity will flow through this thermal switch until it reaches a certain operating temperature at which time the switch closes and makes and electrical circuit allowing the cooling fan to operate. When the overhead projector is turned off the fan will continue to run until the unit cools off enough to let the thermal switch open back up turning the fan off.

Now the normally closed thermal switch acts in complete opposite. It is a closed circuit until the overhead projector heats to a certain temperature. Once this temperature has been reached the thermal switch opens shutting off power to the unit. It acts in the same manner as the thermal fuse except for the fact that the thermal switch will reset itself several times before needing to be replaced. A more economical feature than using a thermal fuse that must be replaced once it has been used only one time.



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Mark Boehm