Norfolk Astronomical Society

Telescope Dew Heating Calculator #1

2002 Glendon L. Howell

Input Loop Parameters

Source Voltage  volts
Resistor Power  watts
Number of Resistors  

Computed Results (See Notes Below)

Calculated Resistance  ohms
Volts Per Resistor  volts
Loop Amperage  amps
Total Loop Power  watts


Source Voltage - chosen to be 12 VDC due to safety and remote availability.

Resistor Power - some standard power sizes are 2, 1, 3/4, 1/2, 1/4, and 1/8 watt. The 1/2 and possibly the 1/4 watt are best suited for use in this application.

Number of Resistors - 4 to 6 is a good number to work with around 1.25-inch barrel eyepieces. The necessary number will vary with your application.

Calculated Resistance - this is the exact resistance calculated that would be driven to the maximum Resistor Power at Source Voltage specified. The next higher stardard size should be used. Standard sizes readily available are 10, 15, 22, 33, 47, 68, 82, 100, 150, 180, 220, 270, 330,390, 470 ohms, etc. Precision resistors will get you closer to the calculated value but are also more costly.

Volts Per Resistor - in a series circuit, with each resistance the same, equal voltage is dropped across each resistance.

Loop Amperage - in a series circuit, the same current must flow through all component resistances in the circuit.

Total Loop Power - this is the total heating power of the loop. For best performance this value should fall at 2-3 watts, which should be sufficient to keep optics clear.