ew

designed by Don Clement

Introduction
This is a description of a circuit for the automatic control of a heater element for
telescope optics. The optics maybe a corrector, secondary mirror, eyepieces, or
telrad finder. The circuit automatically maintains the temperature of the optics, at
a preset amount above the ambient air temperature, so dew will not form on the
optics. Many ATMers have tried this circuit and found it useful.

How the Circuit Works
The schematic at the top of page 3 is a circuit that I designed, built, and tested on
the bench. The temperature is measured using LM335 precision temperature
sensors that output 10mV/degree K. D1 measures the ambient temperature. D2
is mounted close to the heated optics for measuring the optics temperature.
The output of U1 pin7 either pulls the gate of Q1 to close to the ground rail or
allows R4 to pull the gate of Q1 up to the upper rail when there is a very small
voltage difference on pins 2 and 3 of U1. When the gate of Q1 is pulled to the
upper rail, current flows through the heater.

Positive feedback through resistors R5 and R6 cause the comparator (U1) to
work as a schmitt trigger. This prevents U1 from oscillating with the slow
changing inputs.

How to Perform One Time Calibration
One time calibration is accomplished by allowing the circuit to thermally stabilize
at room temperature. The voltage from pin2 of D1 to pin2 of D2 is monitored with
a voltmeter. Adjust R3 for a predetermined offset of 10mV/degree C. So if one
wants the optics to be 2 degrees C above ambient, then adjust R3 so the
voltmeter reads 20mV. The LM335 voltage output is directly proportional to
absolute temperature in degrees K. This means that at room temperature the
output of the LM335 is approximately 3V. R3 adjusts the slope of the
voltage_out/deg K at one temperature. So if the offset voltage is set to 20mV (2
deg C) at room temperature, then when the system is in use at 0 deg C, the
offset will be about 1.83 deg C. This is close enough to 2.0 degrees C offset at
room temperature for the purpose of this circuit. Automatic Heater Control


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How to Determine the Resistance of the Heater

To determine the resistance of the heater the following equation is used: where
V=battery voltage, Rh=resistance of heater, Rds=drain to source resistance of
Q1, P= power dissipation of heater

Rh= ((V^2)/P - 2*Rds + ( (2*Rds - (V^2)/P)^2 - 4*Rds )^1/2 )/2

So lets assume one wants 12W heater and V=12V and Rds=0.5 ohm then Rh=
11 ohms The actual heater size will have to be determined by the size of the
optics. I haven't gone beyond a bench test yet and will report back later with
results of testing this circuit on my scope.

Where to Find Parts
The circuit is based on an idea from a Sky & Telescope August, 1978
p.161 entitled "An Automatic Electronic Dewcap" I made many improvements
and implemented some suggestions from ATM List members. All parts are
commercial and available from Digikey. Some parts, like the IRF510, are
available from Radio Shack. Total cost should be under $10. Datasheets can be
found at:

National Semiconductor Datasheet for LM335:
http://www.national.com/pf/LM/LM135.html
International Rectifier Datasheet for IRF510:
http://www.chipdocs.com/datasheets/datasheet-pdf/International-
Rectifier/IRF510.html
National Semiconductor Datasheet for LM311:
http://www.national.com/pf/LM/General%20Description

I have built this circuit and it works on the bench. Q1 can switch 12W
without a heat sink and 24W with an added heat sink. If you need to control
a heater with more that 24W consider paralleling the IRF510.

Ken Lowther has a circuit board layout for this circuit available at:
http://www.atmsite.org/contrib/Clement/dewheater/pcb/index.html.

If you have comments or suggestions, email me at:
clement.focuser@charter.net


See Temperature Control Schematic next page. Automatic Heater Control


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Temperature Control Schematic

All Rights to this design are reserved by Donald Winfield Clement, 1999 and is
copyrighted material. The right to download, use, and distribute this material is
granted for personal use only. No text, image, plan, software, or other material
may be incorporated into a web site, commercial product, or publication (except
for short extracts for review purposes) without prior written consent.