Vivarium Cooling

One drawback of using high output lighting is the increased output of heat that comes along with it.  Ventilation and cooling can be tricky because too much ventilation can reduce humidity inside of vivariums.  I've experimented with several combinations of passive ventilation and 12v computer fans for cooling and have settled on a system that allows almost unlimited cooling without adversely affecting humidity.  The image below (Fig. 1) shows an example of this system.  The key to this system is a thin layer of acrylic beneath the lights that isolates the air surrounding the lights from the vivarium while still allowing penetration of visible and UV light.

Figure 1. A 1/16" thick sheet of acrylic covers the lights.  It is secured by a bead of 
weatherstrip stapled above and 4 sheet metal screws below.  A 12v fan draws cool 
air through side vents and exhausts hot air through a louvered vent in the top.  The fan 
speed is adjustable  and does not affect humidity inside the vivarium.  The reflector is 
household aluminum foil.

Homemade Speed Control for 12v Fans

(Thanks to Lars Österdahl, Mick Bajcar, Dave Calkins, Todd Newton and the FrogNet gang for information on building this gadget)

A simple method for controlling fan speed is to purchase an adjustable AC/DC converter.  The voltage can be adjusted to raise or lower the speed of all fans on the power supply simultaneously.  For more precise speed control and the ability to control fans independently, a simple circuit can be made using a rheostat and capacitor.  Following is a description of the controllers I've made.  I'm told that the resistance of 12v fans varies with make and model so different fans may require different rheostats or capacitors.  The rheostat (or potentiometer) is a variable resistor used to control the speed of the fan by turning a knob.  An appropriately sized rheostat will run the fan at full speed when the knob is turned to full on and run the fan at the slowest speed possible when the knob is turned fully in the other direction.  I found that a 500 Ohm rheostat works well for several models of 12v fans that I have.  Warning: a friend found that 500 Ohm was too much for his fans which are the same model as mine.  I can't explain this but when we figure it out, I'll update this page.  Just be aware that the 500 Ohm is not set in stone.  Check the links below for more information on sizing rheostats. Because it takes more voltage to start a fan spinning than it does to run it, you need to include a capacitor in the circuit to allow the fan to start spinning at the slowest speed.  The capacitor briefly supplies a full 12v to the fan at start up regardless of the control setting.  With my fans a 1000 µF capcitor reliably starts the fans at the slowest setting.  A 470 µF works also but occasionally a fan will fail to start because the 12v boost wasn't quite long enough.  Figure 2 shows the basic wiring diagram for a single fan.

Figure 2. Simple wiring for a single fan.  An AC/DC adapter can typically power 6 or more fans depending
 on the output of the converter and the amp rating of the fans.

To wire multiple fans to a single power supply you simply wire all of the black (negative) leads together and run a separate red (positive) lead to each rheostat fan pair.  The images below (Fig. 3-5) show what it actually looks like.

Figure 3. Wiring of a 2-fan control box.  The controls are wired 
to barrel connector jacks so fans with different cord lengths can 
be plugged in as needed.

Figure 4. Back view of the control box with jacks and  rheostats installed.  Ignore the lopsided jacks but use a drill press and a brad-point bit for drilling holes if you have them to avoid this.

Figure 4. The finished product with standard 1/4" knobs installed.

Vivarium Air Circulation

In addition to cooling, 12v fans are handy for creating air circulation inside a vivarium.  Circulation can be used to reduce fogging of glass or to promote plant growth by disrupting boundary layers.  The latter may be particularly useful for growing some orchids and this will be the subject of future experimentation.  I'm still looking for clever ideas for mounting and disguising fans used for interior air circulation.  Todd Newton sent the photo below (Fig. 5) of a clever way to get air into a vivarium.

Figure 5.  A method for providing air circulation inside a vivarium.  The 
fans mounts outside the vivarium and PVC pipe is used to direct air over 
the rim to the inside of a vivarium.  Nice features are that the air direction 
is adjustable and the fan stays outside the vivarium and away from delicate 
frogs and damaging humidity.  Photo and idea provided by Todd Newton.

Sources and Links