Thursday, June 16, 2011

Field desk - part 3 - Electrical System and lighting

Electrical services into, and within, the field desk.

Electrically speaking, the field desk will have one power lead into the desk, with a double switched GPO, and a variety of DC power sources available. A task light will also feature in the design.

Electrical enclosure

The electrical enclosure was designed to contain all components, and attach in the top of the RHS bay. The front panel will contain all interfaces - plugs, switches, etc. The original design was to use an old PSU from a 1900 series switch, but  upon checking the PSU, it was found to have some of it's pins non-commissioned (-12V and - 5V) - thank fully the size of the enclosure was dictated by the GPO, and banana sockets, this meant I had room to look at alternate options.
Figure 1 - Basic Enclosure unpainted
The front panel holds all connections, and has the receptacle for the light in the top RHS. - The receptacle is simply a short piece of  DIN rail, and the lamp holder sits inside the rail.
The front panel of the power box is made of 3-4mm thick plexiglass, drilled, cut filed to hold the GPO, IEC socket, and banana posts. To prevent scratches showing on this panel, I marked the terminal values on from the back, and then spray painted over them from the back - this means the paint cannot be scratched from outside the case. Interesting note was when the paint dried, I could suddenly see this invisible cracks around the banana posts - it looks almost surreal to see "reversed cracks" filled with paint.
The enclosure is designed to sit in the top of the field desk, therefore all ventilation is through the floor (or the front) - I simply replaced the floor with some punched mesh, and then used sheet metal shields to redirect any airflows from the back, through the PSU fan, through the PSU, and then into the front section, through the enclosure floor. Each "side" of the PSU has around 12 sq inches of floor vent available to induct, or expel air.
The enclosure (power box) is held in the top of the desk by means of some brackets, and a folded lip at the back. None of the retaining hardware obscures the ventilation grid, and removal of the power box is accomplished with the removal of one screw, since the forward brackets tilt to permit removal.
Figure 2 - Close up of front panel, with first sheet metal divider removed
The only parts of the enclosure which are painted are those parts visible in normal use - ie the front panel/s, and the bottom. The sides were deliberately not painted since the paint would simply rub off on the walls of the cabinet during insertion, or removal.
Figure 3 - Mesh base to enclosure used for ventilation of PSU
 A 12V LED lamp was purchased from the local variety store (KMart) - I had looked at an incandescent lamp, but compared to the LED lamp, it was pale and yellow. I considered one of the halogen lamps I use when I sew, but they do throw some heat, and I considered that would not be wise in the planned location - not to mention wasted energy as heat.
The lamp was gently disassembled (no warranty voiding yet) and tested for it's ability to "hold up" from a horizontal plane - it was discovered that if held at a 45 degree angle, the lamp's "flexible arm" would support the lamp to the maximum reach. Based on that, a bracket was made (from plexiglass) to hold the base at 45 degrees. The bracket slides into a short length of DIN rail which is used as a track. This track is part of the enclosure and is accessible from the front panel.
Figure 4 - Power box in place in field desk with lamp inserted
The lamp will be removed to close the door, so the plug which supplies power to the lamp was cut through the opened switch, and additional wiring soldered on. The wiring is then terminated to the connections for 12VDC (and COMmon) so the lamp will run whenever the PSU is on. This should have worked but during final testing it was found that the light actually needed more than 12 VDC - the "wall-wart" power pack put out 14VDC unloaded - typical for a 12VDC cheap supply, so I connected it to 12VDC - the light was so dim, you'd have thought it was off. I moved the negative cable from the COM to the -5VDC (giving 17VDC) and she lit up beautifully. - Now the cables are between the -12VDC and the 3.3VDC connectors giving 15.3VDC for the light.
Since the planned 1900 PSU was abandoned, the next most affordable option was to use a surplus ATX PSU. There are a number of articles on the web which discuss the conversion - most centre on forcing, or redirecting the softpower "On/Off" wire, and providing a load to stabilise the regulation circuitry. I started going through my collection of surplus ATX PSUs looking for a reasonably low powered unit which worked, and could be used in this project. I tested some of my surplus ATX PSUs and found a 450W which worked OK. I originally planned on using  a "wiring harness" to connect everything up, but the space was too tight for that option.
Figure 5 - Wiring harness (Mk 1) which was too big for use
What I ended up doing was opening the PSU case, removing the IEC socket (and other mains supply switch and components) and soldering in a hardwired cable. At the same time I cut the ATX plug off, and trimmed all HDD/FDD cables at the first Molex connector. This gave me a bundle of wires about 350mm (14") long. I tied the green wire (PSU_ON) and one black wire (COM) to a toggle switch, and then grouped all other wires together based on their colours and connections within the PSU. Purple (Standby 5V) and the Grey (PSU_OK) cables were tied off inside the PSU since I didn't need them.
Figure 6 - Internals of ATX PSU being modified for use.
The rest were soldered to brass tabs made to suit the backs of the banana sockets (with 2 sets each for COM (Black) and 5V (Red)) The blue and white wires (-12V and -5V) looked so lonely on their tabs when all other tabs had 5 or more cables soldered in. A 7W wirewound 10 ohm resistor was added across the 5VDC rail for a regulating load (although most supplies theses days don't seem to need that - I'll remove the resistor for now and see if the PSU behaves)
Figure 7 - Modified PSU in enclosure with earthed divider panel.
As mentioned previously, the enclosure needed some way to direct air through the PSU fan, and back out of the enclosure. I accomplished this by bending up sheet metal dividers - the one closest to the front also being Earthed, and the one at the back being used to restrain the PSU, and block the holes in the casing where the IEC sockets were removed.

Figure 8 - Completed power box with second divider in place for ventilation redirection.
All Done!!! (although I haven't put in the 2 self tappers in the top corners yet!!)
Figure 9 - Completed power box ready for use.
As can be seen in the background of some photos, the desk cabinet is already being painted. The next article will cover the painting and trimming of the desk.

1 comment:

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