Fan Cooling Unit

by Karl Shoemaker, AK2O



Most all communication equipment generates heat. Heat normally comes from current flowing through wires, and semiconductors. The latter is the most area of "IR losses" AKA, heat loss, due to current flowing through them and the fact they are semi conductors. That means they have resistance. Resistance to current flow generates heat. The higher the voltage drop worsens the heat issue. In the case of RF transmitters that run medium to high-power presents large heat issues. A linear transmitter is the worse, however class "C" transmitters have their issues as well. The latter is normally used for LMR-FM systems. Since SRG's system is such this discussion will focus on that.

Solution 1

Most of the link radios operate in the +4x dbm area, therefore, heat generation is moderate. The system output transmitters operate a few db higher, therefore, heat generation is more severe. Used are the Motorola Micor transmitters. For the micor is normally designed for +15 volts from the stock power supply. Running the transmitter at "rated" power out is just too hot for AK2O's specs. One (slick) method to keep IR losses down is to run the final (major heat source) transistor's supply lower than factory spec. A special power supply with custom transformer windings is used. Documentation for this supply is elsewhere on this site. They output + 10 volts nominal. "Nominal" means that vary +- 10%.

Even with lower voltage approach during long transmissions the transmitter still gets too hot for "comfort". Therefore, a second measure is taken with a fan. Fans are very much used and depended on as a primary source of cooling for all kinds of equipment such as transmitters, computers, servers, etc. Most of industry designs these fans to run continuously. This can lead to shorten fan life, dust accumulation and annoying-distracting noisy environments.

Solution 2

AK2O's spec calls for a fan cooling, if necessary, but only when cooling is needed. Therefore a FCU (Fan Cooling Unit) was designed. Several prototypes were built, along with a couple production units and put in service. Two or more fans are mounted in an assembly (in parallel) and only come on when the transmitter starts to get hot. This is done by a thermostat mounted on the head sink of the transmitter. Its normally open contacts close at 100 degrees F. Power to the fan "assy" is switched by the thermostat. The minus (-) side is switched making it very easy to "test" the fans by touching this control line to ground. Also, insulation is not important on the minus side, being the equipment is negative ground. The fan assy only occupies one rack unit (the silver panel). Power on the front; + runs to the fans, while the - runs to a remote switch, in the form of a thermostat in the appropriate spot. Of course, this circuit should be fuse protected. For this type the fuse is on a power supply's output buss-fuse array, located elsewhere. Other versions have a fuse holder right on the panel. Either use a panel mounted 3AGC holder Hosfelt Electronics part number 342014A or a surface mount single gang holder Mouser Electronics part number 534-3536.

Three version were built:

  • Commercial panel, thin, framing mounts; no welding needed.

  • Commercial panel, thicker stock steel for the mounts; some welding needed.

  • Stock steel for the panel and mounts; everything welded.



    We'll cover each version in order, starting with:




    Rear view with convenient "reminder" markings for flow direction. Yes, that's silver ink, from a felt pen. Very handy for marking (black) coax as well.




    Another rear view of the underside. Notice simple store-bought brackets can be used if you don't have access to welding equipment. The brackets are made up of Simpson framing braces with 8-32 screws and nuts to hold them together. The panel is from Bud Radio; already painted.




    For the Mitrek, showing the location of the thermostat (switch). Heat sink compound is used to ensure good switch sensing.




    Here's another view of the same type built, but a different unit as you can see by the black commercially made #1 panel. This also has a "patch" cord for power with a quick disconnect 2-pole molex connector for the power. This unit is designed to cool a mitrek radio with the two fans in parallel.

    The rear view better shows the layout for this type of fan cooling unit.




    The main advantage is easy, cheap assembly. The disadvantage is the thin metal tends to make the unit a little flimsy. This normally is not a problem since the fans are very light weight.





    This version is a combination of a commercially made #1 panel and the rest home-brew. This array has three fans for a site needing more and even cooling for a larger heat sink.





    Overall front view.





    Rear view.





    Bottom view.





    Side view.





    The type version is considerably heaver duty. Such that it could be used to mount some heaver equipment, such as a link radio or PC keyboard. It's made completely made from scratch. It consists mainly of two sections; the panel and the (horizontal) flat mounting area for fans or other equipment. Using flat steel 1 1/2" wide and 3/16" thick is cut into the 19" lengths which one standard for rack panels. Because of a #1 is 1 3/4" wide the difference is made up by mounting an addition "tab" extender on one edge of the "panel" for the proper rack spacing. The lower hole mounts were chosen for the extenders. That way give a little more clearance for the bottom of the flat steel, when fans are mounted, for screw head clearance. In this case a production of four units were produced in a few hours.







    Just more views of the units on the bench......








    Some close-up details; The "L" bracket part is welded directly on the back of the new "panel". In addition to having the "tabs" on the bottom side for clearance, you can also position the flat mounts up a little from the bottom edge of the panel so that screw heads clear the allocated space for this panel. When mounting the next rack slot above equipment makes tight clearances. The other option is to leave one rack space between the equipment, if you have enough room in the cabinet/rack at a remote site. as shown in this right picture close up. A 1/4 nut was used for the "tabs".








    The left picture is using a commercially made #1 panel as a "reference" check for proper dimensions. the right picture shows them being ground down to be even with the rest of the flat metal, as shown in the middle panel being worked on.








    Next, the panels are checked on a real rack for proper spacing and compatibility with other equipment.








    With a trip to the painting room and the result for a couple of them. The right picture shows the lower one just about ready for fans to be mounted toward the rear; that's the reason for the second cross-flat mount. If you were wondering if the picture aspect is distorted-it's not. Due to a mistake in mount alignment the two flat mounts "buck" each other a little causing some frame distortion. This is not a problem for just mounting fans, however if for equipment or otherwise you are anal about appearance pay close attention when welding the second mount to avoid this. This actual unit will be placed in service, rather than to discard the unit or repair this minor flaw. A good time to check for issues such as this is before painting, of course.








    The construction follows: Install a flat piece in front of the rear frame, depending on the fan size, of course. To mount the fan(s) with a 6-32 bolt and nut, drill 9/64" holes. Normally will be three mounting points per fan. For the terminal block drill 9/64" holes for the end holes for the #6 bolts. For the wire holes can be smaller. For the LED drill a 13/64" hole. In the event you wish #8 bolts drill 11/64" for those. For the AGC fuse holder drill a 1/2" hole. Ream all the holes a little to clear out any burrs and clean off the surfaces of any cutting oil. Glue the LED in place; it's a good idea to put some glue on the threads of the fuse holder as well. The three terminals are: 1-power, 2-a junction for the ground source and the thermostat low side, 3-for the control wire from the thermostats's high side. Here's some views of the complete unit, less labling.





    Rear side views:









    After careful planning and coming up with these protypes a production version of the FCU was done in the spring of 2012. For information on this version, click here for that page.

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