Introduction:
There are a few methods of controlling the cooling temperature of peltier systems, including:
Manual Control:
1. On/Off 2. Variable Voltage
Circuit Controlled with temperature sensor feedback:
3. On/Off 4. Pulse-Width Modulated (PWM) Variable Voltage
Methods No. 3 and 4 above are not necessary with the Peltier Cooled DSLR. This is because the thermal load (DSLR camera) is reasonably constant and very precise temperature control is not needed for obtaining acceptable dark frames with DSLRs.
1. Manual On/Off method:
For the Peltier Cooled DSLR System that I built, the simple manual On/Off method works well in most cases. Repeated On/Off control is not recommended by one peltier device manufacturer (Marlow). According to some other peltier manufacturers, their peltiers are designed to handle thermal cycling. The concern is that thermal cycling of the peltier, because of switching from full On to full Off, may degrade the life of the peltier. Repeated thermal cycling does not occur for my DSLR cooling system. For the all-night imaging sessions I have done to date, I have left the peltier in full On power condition. A typical temperature drop curve for Version III of the DSLR cooler I built is shown below:
As depicted in the above temperature drop graph for my system operating from a 12 Volt DC (6 Amp) power supply, it is apparent that the cooling system quickly cools to an almost steady state temperature. This steady state temperature occurs after a temperature drop of about 40 degrees Fahrenheit. In most ambient temperatures where I image, this provides dark frames that are essentially free of noise. Operation of the peltier device in this manner is very similar to the operation of the peltier as it was originally configured for use as a beverage/food cooler, powered on and off when transported inside a vehicle.
The graph below shows actual noise reduction with cooling, using five-minute exposure dark frames at temperatures of 77, 50, 34 and 5 degrees.
For the Canon 450D (Xsi), the dark frames I have been obtaining with the peltier cooled system are almost completely free of noise when the measured camera body temperature is near 10 degrees Fahrenheit. There is no need to subject the DSLR to lower cooling temperatures. Since the peltier cooling system at 12 volts provides a 44 degrees temperature drop, when I am imaging in cold winter temperatures, I prefer to limit the cooled temperature to keep the camera body temperature above 0 degrees Fahrenheit. An example is when the ambient temperature is around freezing (32 degrees Fahrenheit). At that ambient temperature, I would like to limit the peltier cooling to a drop of only 25 degrees Fahrenheit. One method of doing this is by switching the 12V power supply off when the camera temperature reaches 5 degrees Fahrenheit and back on again when the camera temperature reaches say about 15 degrees Fahrenheit. This cycle of manual On/Off switching could be continued for the duration of the imaging session and may require an On or Off switch about every one-half hour . Such long period On/Off switching of the peltier device is not detrimental to its life.
2. Manual Variable Voltage Control:
Another method for controlling the temperature is by using a variable voltage power supply. This is from the ferrotec.com web site: "In applications where the thermal load is reasonably constant, a manually adjustable DC power supply often will provide temperature control on the order of +/- 1°C over a period of several hours or more."
I purchased a heavy duty regulated 5 Amp variable power supply, Model DF-1732T for this purpose. This variable power supply is inexpensive and available from a number of online suppliers. I purchased mine from Suburban Electronic Wholesalers for $30 at:
http://www.suburban-elect.com/display/DF1732T/DF-1732T
The DF1732T power supply has a rotary switch for eight DC voltage settings:
This is a good voltage range for most peltier devices, especially those that typically operate at 12 Volts DC. I do not know the specifications of the peltier used in the RubberMaid (Vector) cooler. Since it has a 5 Amp fuse, the whole cooler/heating system was designed for a maximum power draw of 60 Watts. Typical 40mm by 40m peltiers sold by peltier manufacturers for 12 Volt beverage cooler applications have specifications that range as follows:
It is important that the Maximum Volts or Maximum Amps specification values for a peltier device is not exceeded.
When you build a peltier cooling system, you will learn that there is an optimum voltage and current for dissipating heat. This optimum voltage depends on the peltier itself and the thermal and electrical characteristics of the complete cooling system including the load, which in our case is the DSLR camera. Too high of a setting and the cooling effect will diminish because of internal Joule heating of the peltier itself. Too low of a setting and the maximum cooling effect of the peltier device will not be realized. It is expected that the RubberMaid (Vector) beverage cooler was thermally designed to operate best at 12 volts, the standard vehicle DC power supply voltage. Remounting the peltier module in the DSLR camera cooler changes the characteristics of the cooling system.
An easy way to find the optimum power setting for the DSLR system you build is by using a variable power supply. For the DSLR peltier cooler I built, while the Canon 450D camera is in operation and taking long exposure images, I recorded camera body temperature inside the cooling chamber at various voltage settings of the variable power supply. The graph below shows a test imaging session over a 5.3 hour period:
After one and one-half hours of initial cooling with the peltier operating at 12 volts, the above graph shows a temperature drop of about 44 degrees Fahrenheit. I then switched the voltage from 12 volts to 9 volts. Over the next hour at 9 volts, the camera body temperature was in a nearly steady state mode. To increase the camera body temperature, I next changed the voltage to 7.5 volts. As the above graph shows, the camera body temperature increased very slowly over the next one hour period. The voltage was then changed to 6 volts and the temperature increased at a slightly greater rate. Changing the voltage back to 12 volts decreased the camera body temperature once again as shown. Understanding the effect of varying the voltage of your peltier cooling system allows one to manually control (fine tune) the desired camera body temperature for various ambient temperature conditions while monitoring the camera body temperature.
It is possible that the cooling effect could be further increased by using the variable power supply setting of 15 volts. Since the DSLR cooler has been working so well for me operating at 12 volts, I have not tried the 15 volt setting. Peltier devices usually do not perform at their optimum when near Maximum Volts.
More information on use of peltier devices can be found at these links:
TELLUREX - Introduction to Thermoelectrics MARLOW - Introduction to Thermoelectric Cooling MELCOR - Thermoelectric Handbook