Heating pad
A heating pad or a heating blanket is an electrically operated heating device with a textile flexible cover. Heating pads are used by humans and animals as a heat source close to their body or to preheat the bed . Another application is heat therapy for chronic pain.
history
The first heating pads and electric blankets appeared at the end of the 19th and beginning of the 20th century when the population began to be supplied with electrical energy. They almost completely replaced the bed warming pans operated with glowing coal and the warming stones that had to be heated in the oven, but could not replace the hot water bottles made of rubber that appeared in the 1920s .
Historical heating pads were often made from asbestos and were not waterproof.
Function and nature
In contrast to heating cushions , which can only give off limited latency heat , heating cushions and electric blankets are constantly heated by a heating wire . The power required for heating is comparatively low and is only approx. 60 W for heating cushions and approx. 100–200 W for heating blankets. Even in older devices, step switches made it possible to operate a different number of parallel-connected heating wires and thus to control the heat output. Modern heating pads and blankets have a temperature switch and a thermal fuse to prevent overheating. Depending on the manufacturer's instructions, this may result in unsupervised operation or use while sleeping.
Most devices are operated directly on the power supply and at the same time close to the human body. Therefore, high demands are made on security. The sheathing of the heating wires must be insulated in a break-proof and watertight manner with the required flexibility at the same time. This is the only way to avoid electrical accidents in the event of contact with liquids, heavy sweating or urine .
Special heating pad switches
With two differently strong heating circuits, three different power levels can be configured. For example, if you use a heating circuit with 30 watts of power and a second heating circuit with 60 watts of power, a step switch can activate the following states:
- Heating circuit 1 in operation: 30 watts
- Heating circuit 2 in operation: 60 watts
- Both heating circuits in operation: 90 watts ( parallel connection )
As early as the 1930s, step switches for so-called series-parallel switching were known, which also achieve three different power levels with two equally powerful heating circuits, but with a greater spread of the levels. The heating circuits are operated either in series or in parallel . With two heating circuits with 50 watts each, the following output levels result:
- Both heating circuits connected in series : 25 watts
- One heating circuit in operation: 50 watts
- Both heating circuits connected in parallel: 100 watts
This results in a factor of 2 for both level transitions, which cannot be achieved without these special switches with only two heating circuits.
As with the simple step switch, the series-parallel connection can also be implemented with a three-wire cable between the switch and the cushion.
