737 Overhead Power Distribution Rewiring

Filed under Overhead
February 15, 2019
8 mins to read

Lately I have been in the process of disassembling the entire sim to paint the floor and shell, so while I had everything apart I decided to do some updates on my home-built overhead.

Goals

Goal #1 was to reduce the number of wires coming out the back of my overhead. Between the power adapters, HDMI cable, ethernet cable, and CPU power supply input, it was just too unmanageable.

Goal #2 was to re-wire the power in such a way as to allow me to control, via USB relays, when the power was turned on and off per the aircraft DC and AC power buses.

Re-wiring Power to Reduce Wires and Control Power Distribution

I have the following items in the overhead that require power:

FlightDeck Solutions FDS Board to control most switches and LEDs (12V)
Phidgets LED card to control dual-brightness LEDs (12V)
PoKeys Ethernet board to handle remaining switches FDS board couldn’t (5V)
Phidgets Relay Card (no power required; determined by input at each relay)
Polulu Mini Maestro 12-servo Controller Card (5V)
FlightSimParts.eu 7-Segment displays for ALT/LDG (5V)
TFT controller board (12V)
Full complement of CustomSimParts servo gauges (5V)

That’s a lot of stuff to power, and many of those items had bulky power adapters that came with them. I needed to consolidate as much as I could to simplify my overhead installation later. I already had 12V and 5V power coming in from a modified CPU power supply which was good since everything in the overhead is powered by either 12V or 5V.

The tricky thing is that some OH systems are powered when AC power buses are active. For example, the 7 segment displays and back-lighting only come on with AC power. The starter solenoids obviously only come on during engine start to hold the knob in the start position and then they release. So this required some things to be controlled by a relay card. Using a relay I can turn on/off those relay-connected items via ProSim using gates.

Below is a diagram of how I wired up my OH power. All the items that need constant power can get it from the moment the CPU power supply is turned on, and the relay-connected items would only receive the correct power when switched on by the software.

In the above diagram you can see the constant 12V and 5V power comes in from the CPU power supply and goes to terminal distribution blocks. I purchased terminal blocks on Amazon that had plastic covers to reduce the chance there could be short circuit. I also made sure all ends were terminated properly with fork connectors so none of the power wires can easily come free.

From the distribution blocks constant power is distributed to the various cards that need it, or it is passed to the relay card which, when activated, feeds the input power to the respective connected devices.

The 7-Segment displays only come on when AC power is turned on. The relay allows that to happen.

The same with the panel back-lighting. It only comes on when AC power is on. You’ll note in the diagram above that I added the dimmer POT for the panel and gauge back-lighting brightness after the relay. This allows me to feed a constant 12V to the relay and control the on/off of the back-lighting, but then when the relay is turned on I can then also control the variable panel brightness from zero up to full bright. If I put the POT before the relay, when the dimmer is set to zero no power will flow to the relay and I believe the relay needs at least a minimum amount of power to activate.

On a related note…the CustomSimParts gauges I use have back-lighting that will work with both 5V or 12V. The gauges have a built-in resistor to keep the 5V LEDs from burning out. However, I was told by CustomSimParts that although the gauges will work fine with 12V, if you choose to go that route it’s better to add an in-line 150-200 Ohm resistor in front of them to reduce wear. Since I’m using 12V for the backlighting I soldered in a 200 Ohm resistor in-line to each gauge which turned out to work perfectly. The brightness doesn’t overpower the case back-lighting.

All said, doing this allowed me to get rid of 4 power adapters (FDS card, Phidgets LED, TFT power, Pokeys card). I am still left with one power adapter for the 4-port USB hub. For some reason it would not accept power from my buses, so I am stuck using its adapter for now and it is necessary to ensure the USB-connected devices have enough USB power to run.

Below is a picture of how the wiring turned out. The power is not placed as nicely as I would have liked, primarily because I lost space due to the hump in the middle of my overhead cabinet, limiting where I could put components.

View of the inside of my overhead with re-wired power, relays, and covered distribution blocks.

The hump in the middle was necessary to clear the FDS cockpit shell crossbeam. If you plan on never using the FDS drop-down overhead frame in your FDS shell then you can remove the crossbeam and thus not need the hump freeing up valuable space. However, in the future I may switch to FDS panels, so I left the cross beam in place. You can see the OH crossbeam in the image below.

My FDS shell. Note the cross-beam where the overhead goes. If you leave beam in, you will need to account for this in your overhead frame design You will lose some space in your overhead case, so decide if you really need the beam or not.

Conclusion

I hope this article gave you some ideas on how to wire power for your overhead.

One thing to mention…I haven’t added in any fuse blocks which really should be done to prevent any shorts from burning up the overhead components or starting a fire. While the FDS card has a built in fuse the other cards and components don’t. Fortunately the CPU power supply has short protection so it will cut out if a short is detected. And I’ve been very careful to make sure all my connections are solid and protected. I have been lucky so far, but you never know. Better safe than sorry. If you are planning a build, I recommend you consider putting in fuse blocks as well.