Automatic
Heater Control for Dew Prevention
designed
by Don Clement
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.
How
the Circuit Works
The schematic
shown (at the bottom of the page) is of 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.
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 & Tel 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
International
Rectifier Datasheet for IRF510
National
Semiconductor Datasheet for LM311
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.
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.
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