As mentioned in a previous blog entry, I’ve been trying to arrange that the observatory PC can remain outdoors in our frigid winter temperatures and still start reliably when I need it. The problem has been the hard drive – at sub-zero temperatures it won’t start, and the PC fails to boot. I tried replacing the mechnical hard drive with a solid-state unit and, while this solved the temperature problem, it created other problems, and I returned to a mechanical drive.

Heating just the interior of the observatory with a heater designed to keep things just above freezing wasn’t a solution. When the ambient temperature dropped below about -15°, the heater couldn’t keep up and the pocket of air in the bay where the computer sits was still about -5°, too cold for the computer.

The deep dip on this graph was an observing run with the interior heater switched off (once the PC is running, its internal heat keeps it healthy). But you can see that the temperature was sitting at -5 to -8, even with a “no freeze” space heater running in the observatory. (Outdoor temperature was about -20 at the time.)

So, I decided to try installing a small, low-power heater inside the cabinet of the PC, with a thermostat to keep the cabinet interior just above freezing.

I ordered a small, thermostat-controlled “enclosure heater” from Omega Industrial. This small device has a built-in thermostat that keeps the temperature at about 10° C.

After the heater arrived, I brought the PC indoors for construction and wiring. This is a photo journal of the project.

Opening the side panel of the PC, there is a fairly large open space in the centre, above the PCI cards and to the right of the disk drive. I want to mount the heater so that it sits in that open space.

The heater is a thin sheet of flexible material bonded to a simple L-shaped bracket of aluminum. It’s quite compact.

There aren’t any good mounting surfaces inside the cabinet. But it seems that if I were to mount it to the cabinet cover, it would fit into the desired space when the cover was closed.

I temporarily mounted the heater in a likely-looking spot with a bit of double-sided tape, . . .

and carefully closed the cabinet cover, inspecting the fit and clearance from multiple angles, to ensure that the heater would fit into the free space in the PC without touching anything. Once I found the perfect spot, I marked the position of the heater and its mounting holes with a black felt pen.

Now, how to power the heater with 110V?

My original thought was to splice into the PC’s power supply. However, I found two problems with this plan.

1. It’s hard. The PC power supply is a sealed unit, not easy to get into. That’s by design – they want to keep the dangerous high-voltage part well separated from the low-voltage interior components. Getting wires out of there was going to be ugly.
2. Wiring from the PC to the heater would mean wires connecting the PC main body to the cabinet cover, which would make removing the cover for maintenance awkward. I would prefer the cover remain an independent unit.

So, I decided to mount a separate power cord into the cover itself. This would allow the cover to remain a completely independent unit.

Thinking that through, I decided to install a proper 110V plug receptacle, a lighted power switch, and a fuse holder. These parts were a couple of dollars each at a local electronics store.

I determined that the components would fit in the space under the main heat sink, at the bottom of the cabinet, and marked these positions on the cabinet cover.

Warning: this location didn’t work and had to be corrected: if you happen to have excactly this PC don’t copy this design until you read the rest of this article.

Also shown here is a “nibbling tool”. This is used to cut precise rectanglular holes in sheet metal. If you plan to do any mounting of rectangular components in cabinets, you need this tool. Trying to cut rectangular holes by drilling and filing, or using standard metal shears, will drive you crazy.

Here is the cabinet cover after cutting the holes for the power plug, switch, and fuse holder.

The switch, fuse holder, and power connector are mounted in their holes, and I’m feeling very proud about how nice it all looks.

This interior view shows the mounted heater and connectors. The heater is mounted with two screws, and is held off from the cabinet by insulated spacers. (I wanted the heat to be dissipated into the air, not into the cabinet cover).

Then it hit me. I have had a brain fart of some kind and, despite all the measuring and testing, the switch doesn’t fit in that position. When closing the cabinet, it hits the main processor heat sink. I had intended to mount the switch horizontally, not vertically.

Poop.

So, I used the nibbling tool to extend the rectangular hole in the horizontal direction. Fortunately there is still enough surrounding metal to support the snap-in switch.

I put a layer of duct tape over the embarrasing hole, both sides, and then cut out the desired rectangle with a sharp knife. Fortunately the tape is about the same colour as the cabinet, so it doesn’t look too foolish.

Oriented this way, everything fits and the cabinet will close safely.

Now, let’s wire the power, fuse, and switch to the heater.

The heater is 40 watts at 110V. That’s 40/110 = about 1/2 amp of current, so I installed a 1-amp fuse in the fuse holder.

Consulting the manufacturer’s web site, I found a wiring diagram for the switch (since it has a built-in “on” lamp, it is more than a trivial two-connector device.).

I wired everything point-to-point, through the fuse and switch. Crimp-on spade connectors are used for the switch and power plug (crimped and soldered).

Since this is 110V sitting inside the PC cabinet, I insulated every connection with heat-shrink tubing, so there are no exposed 110V surfaces that might contact a PC component.

Here is the result with everything closed up. A standard power cord fits in the plug, and the illuminated switch turns the interior heater on and off. (The heater also has a built-in thermostat so it can be left on all the time during cold season.)

So how does it work?

I installed a temperature logger inside the PC cabinet and left it running, with the heater on, overnight on a cold night.

I let the PC cool down for a couple of hours before switching the heater on, to provide a low reference point. In this graph you can see the zig-zag as the thermostat cycles on and off. For much of the night it successfully kept the PC interior above freezing.

You can see, however, that when the temperature reached -25 in early morning, it wasn’t able to keep up and the interior temperature was still below freezing.

However, this is a big improvement, and is keeping the interior at a useable temperature over a much broader range of temperatures. Essentially any temperature I can stand is now in the useable range.

And this test was without the observatory interior “no freeze” heater running. I’m now running a test with that heater running as well, and it’s very cold this weekend (-25 both nights). Those data will follow after I get them collected.