Bit OTT - Telescope dew Heater controller

I have started to put the observatory together again and find i need to create a dew heater and controller for the LX90 8" SCT telescope at M1KTA. After some googling I came up with a couple of options and I like one from Don Clements, I had a look in the junk box (!!) and found I had some LM355 temperature sensors.

So decided to attempting to build a circuit that will provide automatic control of a heater element (made from simple resistors, soldered into a ring and then covered in shrink wrap) that will surround the telescope optics. These optics maybe any of the secondary mirror, primary, eyepieces, or the telrad finder. The concept is for the circuit to automatically maintain the temperature of the optics, at a few degrees (preset) above the ambient air temperature, so dew does not form on the optics.

The initial idea is the temperature is measured using LM335 precision temperature sensor that output 10mV/degree K. There are two one measures the ambient temperature, the other the temperature as close to the optics as possible by the heaters. I will call these tempsense1 and tempsense2 and these have a small bias applied to them, these voltages are fed into an opamp in a simple comparator configuration so that a small difference is amplified and this is output such that it can drive the gate of a switching MOSFET. If the two temperatures are close then there is no voltage difference so the MOSFET is turned off, if there is a difference the MOSFET is switched on, what is switched is the current that goes through the heater.

In the telescope configuration most likely to be used there are four potential heaters so the circuit is replicated three times.

One important calculation that needs to be done is detailed here. You need to determine the resistance of the heater used and the following equation is used: where V=battery voltage, Rdh=resistance of dew heater, Rmosfet=drain to source resistance of the MOSFET used, P= power dissipation of heater

Rdh= ((V^2)/P - 2*Rmosfet + ( (2*Rmosfet - (V^2)/P)^2 - 4*Rmosfet )^1/2 )/2

So lets assume one wants 12W heater and V=12V and Rmosfet=0.5 ohm then Rdh= 11 ohms The actual heater size will have to be determined by the size of the optics. In my case I have an approximate 9" tube for the secondary, same for the primary and two 3" heaters, one for the finder scope and the other for the eyepiece.

If all 4 are 12W (bit high) the current needs are simple to calculate using I=W/V or 12/12 or 1amp per heater.