Adequate Ventilation for Electronic Equipment

If an electronic assembly dissipates an appreciable amount of power, the designer, technician, or builder must provide adequate means of getting rid of the heat generated. Some designs use the case as a heat sink for power semiconductors. Examples of this include car alternators, car stereo amplifiers, and many power inverters currently on the market. Other equipment - quite likely including the computer you are reading this on - uses forced-air ventilation.

It is beyond the scope of this post to supply design formulae for calculating air flow. Electrical or mechanical engineers who read this will not find anything new here. For other home-brewers, hobbyists, and tinkerers, I offer the following pointers I have learned from practical experience:
Keeping It Cool
1) "Muffin", a.k.a. "biscuit" type fans - such as used in most PC power supplies and as case fans - are commonly available from many electronic supply houses. Usable ones can often be scavenged from old equipment. Fans as small as 1/2" to 6" diameter are made and can be purchased from some distributors. Units measuring 2-4" across are the most common I've seen and used.
2) RETURN air supply is needed - just as with an air conditioner. I typically allow from 1.5 to 2X the fan's area for total return air grille area. This is strictly an empirical value that has worked well for me - it is NOT based on any precise calculations. Your mileage may vary!
3) The fan should draw the air through the case and discharge it outside. Having the fan blow air into the case, by drawing it in from outside, will often cause "dead spots" to form around components. These stagnant air pockets may cause overheating - even if the air flow is otherwise adequate. This is why most equipment fans discharge OUTWARD.
4) Try to position the fan as close as possible to the hottest components. This quickly gets the heat out of the box - preventing it from cooking the rest of the circuit assembly.
5) When operating at "normal" room temperature, a semiconductor should be at most warm, but NOT hot to the touch. If you cannot comfortably leave your finger in continuous contact with the device at "normal" room temperature, it is too hot and WILL suffer thermal damage and failure. See manufacturer's data sheets on your diode, transistor, or IC for more precise information on safe operating temperatures. Please bear in mind too, that even with fans and heat sinks, devices cannot dissipate nearly as much power when operated in high ambient temperatures as they do at "normal" room temperature. Study carefully the device dissipation v.s. ambient temperature curves for the device(s) in question to be sure it will do what you intend to do under worst-case conditions.
6) Beware that a circuit that stays nice and cool in your lab at "room temperature" may overheat and fail if you are operating in a vehicle or outdoors on a hot, sunny day! Verify that your equipment is staying cool enough under the hottest conditions you ever intend to run it.
This is why engineering companies test their prototypes in special environmental chambers. I have on occasion used my kitchen oven (set to 170 F) and refrigerator freezer compartment to test home projects built for use in the car! Granted, most home freezers do NOT go near low enough to test for "nighttime in the dead-of-winter" conditions in places like Montana and the Dakotas.
Keeping It Clean
1) To keep dirt and dust to a minimum inside the equipment, you will want a return air filter placed directly behind the return air hole(s). A piece of 3/16" thick foam of the type sold as replacement filter material for window unit air conditioners works well and looks professional. If you electronic supplier has them, a frame to hold the filter foam looks classy and provides easy removal for cleaning. You will see an example of this later on in my "serial port relay controller" project writeup. Otherwise the filter may be glued to the inside of the case.
2) Lacking proper foam to make a filter - other expedient materials such as "cheese cloth" and even women's panty hose material also work reasonably well in a pinch.
3) Removable filters may be washed, blown out with "canned air", or vacuumed. Filters glued in place will need to be blown clean or vacuumed.

First Post

Greetings!! I created this blog to showcase some of my skills and to share information that others might find interesting or useful. As this blog develops I plan to post on a wide range of topics including, but not limited to, the following: Anything computer related, electronics, amateur "ham" radio, shortwave, digital photography, and information on electronic engine controls in cars/trucks/motorcycles.

Hope readers of this blog will find the content enjoyable and will check back regularly.