Resistor Mats Heater & PowerBank Discharger
I wanted to learn about common resistors used as HEATING ELEMENTS in electrical circuit in order to achieve optimal working temperature in anti-mosquito tablets (mats) heating device. Additionally, this simple device can be used as a PowerBank USB discharger.
Yes, I’m aware of the fact that you can purchase factory-built units for like $1-$2, but where’s the fun in that? :)
- Soldering Iron (30 Watts)
- Soldering Wire (common 0.6 ~ 1.0 mm)
- Soldering Paste (optional)
- Wire Cutter Pliers
BILL OF MATERIAL
- 2x (or 3x) 5 Watts Cement/Ceramic Resistors*
- 1x PCB Board approx. size: 22×45 mm (0.9×1.8 inch)
- 1x USB Type “A” connector (obtained from a broken cable)
- 2x 5 cm (approx. 2 inch) wires
* I used two 8.2 Ohms, but ideally we should use three 12 Ohms resistors both for larger mats/pads cover surface and 85~90° C target temperature
ABOUT 5W RESISTORS
Resistors are basic passive electronic components which main purpose is to convert electrical work (energy) into heat. Also, they are used to limit current, reduce voltage, discharge batteries and capacitors etc.
Human skin tolerates temperatures below 43 °C (109 °F) (source: Wikipedia). Anything above that is perceived as burning hot and dangerous!
Five watts of power maybe doesn’t sound much, but when generated/released in a small resistor body like this, its surface can get very hot – near or above 150 °C (302 °F).
DO NOT TOUCH resistors while circuit is POWERED ON !!!
Common 5 watts cement/sealed wired wound type resistors have approx. 32 °K/W thermal resistance at 60% load (see detailed thermal design calculation in the video above), which may be used to recalculate target temperature at lower or higher power levels. Note that this value is only valid at that particular temperature, because power load vs temperature curve is not linear, as you can see from the example chart.
Alternatively, you can use above chart to approximately determine target temperature at rated load, but factors like ambient temperature, leads length, close placement near the PCB board surface, horizontal or vertical resistor mount, other nearby resistors/components and available cooling (air flow) will affect the exact value by few degrees.