DSLR cooling, cold finger with air cooled TEC - working version

[Gina]

Drying of the air in the box was better now but still taking a long time to get the humidity down. Again after discussion it was decided that some air circulation should help so I got a little 30mm fan. This was run at reduced voltage using a Zener diode to drop the volts from the 12v supply. The fan was placed between the camera and the side of the box with some polystyrene foam to form a duct and force air through the silica gel beads in the cartridge.

These photos show the fan surrounded by foam. (Note - the piece of foam on the left was later removed.) The little circuit board with the DHT22 can also be seen - in the place where the viewfinder used to be. The cooler attachment can be seen beside it.

[Gina]

I should have mentioned earlier that the box was cut down in height to make the lid level with the TEC so the the hot side could be outside the box.

Here's a photo of the completed camera unit. The rubber gasket was sealed to the box with hot melt glue - in the event of needing to get inside again this seal is quite easily broken. The removable drying cartridge was not glued in and is easy to remove and replace. I have now done this several times and the humidity reduced sufficiently to get the dew point several degrees below freezing. It would be nice to get this down further but it would seem impossible to get a perfect airtight seal and I guess with changes in temperature and maybe barometric pressure, moisture gets sucked in. Of course we have had months of very damp/wet weather and the atmospheric humidity has been very high.

[Gina]

Here is the camera unit on the scope before some of the cables and control box were added.

[Gina]

The Control Unit - Parts and Construction Details

Before giving details of the cooling control and power distribution box, here's a photo of the test setup as I started developing the Arduino control. This shows an Arduino Uno, a breadboard and jumper wires plus LCD display with adjusting pot and piece of strip board with IDC PCB header for the data cable to the camera box. Also connected to the breadboard is a DS18B20 digital thermometer in a little copper clip - this read the ambient temperature.

[Gina]

Parts List (with some suggestions for sources but I suggest anyone wanting to build this to check yourself as offers vary)

• ABS Box - http://www.ebay.co.uk/itm/197-4-X-113-X-63-Black-ABS-Project-Case-Box-Enclosure-/140819949889?pt=UK_Sound_Vision_Other&hash=item20c985e941
  
• Arduino Nano - http://www.ebay.co.u...984.m1439.l2649
  
• LCD Display - http://www.ebay.co.u...=item1e729dc7c7
  
• Push Button Switches - http://www.ebay.co.u...984.m1439.l2649
  
• Potentiometers & Knobs - http://www.ebay.co.u...984.m1439.l2649
  
• Preset pot - http://www.ebay.co.u...984.m1439.l2649
  
• Connectors - I used XLR connectors for reliability - lots on ebay
  
• Switch Mode PSU x 2 - I got these off ebay from Hong Kong for a few pounds each but can't find them now.
  
• Digital Ammeter - http://www.ebay.co.u...=item4167973c11
  
• Strip board and various components.
  

[Gina]

Construction of Power Distribution and Cooling Control Box.

Here are some photos showing progress of the construction. I'll post details shortly.

[Gina]

Cooling Control Modes :-

• 0 - All Off
  
• 1 - TEC Off, Fan Full
  
• 2 - Gradual Warm Up
  
• 3 - Set Point Temperature
  
• 4 - Dew Point + 2C
  
• 5 - Dew Point + 1C
  
• 6 - Maximum Cooling
  

LCD Display shows data as follows :-

1. Mode or set point temperature - mode in letters, temperature in digits.
   
2. Box internal temperature.
   
3. Relative humidity.
   
4. Dew point.
   
5. Cold finger temperature.
   
6. Heat sink temperature.
   
7. Ambient temperature.
   

Data connections are :-

1. Gnd
   
2. DHT22 data
   
3. Gnd
   
4. 1-wire data
   
5. Gnd
   
6. Camera power
   
7. Gnd
   
8. Fan PWM
   
9. Gnd
   
10. TEC PWM
    

[Gina]

Camera cooling control circuit diagram :-

[drhemlock2]

cool write up as i plan on doing this soon to one of my cameras the information is great and thanks for the hard work.

doc

[Gina]

Thanks doc

[Gina]

Circuit diagram of power distribution including provision of +5v for Arduino and USB hub and +7.5v for camera power supply plus over-voltage crowbar to protect all 3 power supplies. A digital ammeter reads the total current load.

[Gina]

Added values to diagram :-

[Gina]

Maybe an explanation of how the over-voltage crowbar works wouldn't go amiss...

The supply is protected by a fuse of appropriate value. This both protects the supply and wiring against an overload or short citcuit and, in this case, is part of the over-voltage crowbar circuit. A Zener diode is connected between the positive rail and the gate of a thyristor, which is across the supply. When the PSU fails and the voltage rises above the Zener voltage plus trigger voltage of the thyristor, the latter fires and blows the fuse, thereby removing power from the load.

Where there are several supply voltages, one thyristor can serve for all by using a Zener diode for each to the gate of the thyristor, as shown above.

For many applications an over-voltage crowbar may be considered "overkill" but in this case the supply feeds an expensive mount where a replacement circuit board may cost a hundred pounds or more plus one or two cameras again representing several hundred pound in value. The cost of components for this valuable protection is pennies. Many commercial supply units have this built in but without either the full circuit diagram or opening up the case one can't be sure.

[cfaranda]

Thanks for sharing your design. Can you give detail about the TEC controller programing, and the sensitivity of the temperature regulation? Thanks!!