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 automat-ically maintains the temperature
of the optics, at a preset amount above the ambient air temper-ature, so dew will not form on the optics. Many
ATMers have tried this circuit and found it useful.
The schematic at the bottom of the page 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
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.
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.
The circuit is based on an idea from a Sky & Telescope August, 1978 p.161 entitled "An Automatic Electronic
" 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:
I have built this circuit and it works on the bench. Q1 can switch 12W
without a heatsink and 24W with an added heatsink.
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 avilable at this website
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.