Electronics - Power Interface Board
The Power Interface Board is used to bring all electrical connections in the LTC box to one location, create a common ground, and then go out to all the devices in the box. Additionally, this board will allow an operator at the LTC box to visually see that all connections are good and the system is operating.
- This will handle all electrical connection except for the ignition battery which is now located on the Ignition Board.
- All power connections should come through this board.
- Board has LEDs on it to indicate good connections. Previously one could not know that the system was in working order while at the launch tower.
- Power conversion modules (DC/DC converters) should be located on the board.
- This board acts as a common ground plane for every system in the box (although solar panel does NOT ground here - instead grounding into solar/battery charger).
- A couple of extra power outputs should be available for future expansion.
- No dedicated Phidget power monitoring terminals should be on the board. If we want to sample power - take it off the main screw terminal so its obvious what we're sampling. Also keeps board size down. Also allows sampling of other things in the future by not dedicating a Phidget to a specific task.
- Note: Data sheets in the GIT repository "launch-tower"
- 5V DC/DC Converter ==> Murata OKI-78SR-5/1.5-W36-C (1.5Amp is less than the 2Amps we'd ideally have but this part was available already)
- 24V DC/DC Converter ==> Mouser 580-UWE-24-3-Q12P-C (3Amp). This part also has a GPIO for enabling power converter. This frees up a relay from the Phidget board that was going to toggle power. Now we can use a GPIO on the Phidget board to turn off umbilical power.
- LEDs are Dialight 550-xx04 which try to draw 20mA but we are instead limiting them to 5mA. These should be bright enough to see in desert sun.
- There is a push-button switch to monitor the battery voltage. The way the circuit is setup, the battery LED could just be showing a voltage from the solar panel passing through to charge the battery. The switch temporarily int erupts the solar panel pathway and shows that the battery is present.
- TVS Diodes are present to eliminate electrostatic discharge coming into the board from whatever the power connects to.
- Circuit paths are double-sided where possible to allow ample current flow.
- There is a top and bottom ground-plane.
Some of the decisions on this portion of the project were debated - here's the reasoning:
Use this board to star the power
Using this board creates a star topology for power. This is negative because wires could be running extra distances.
- Having all the connections in one place will make the box cleaner.
- Having all the connections here means we can monitor the system health.
Ultimately seeing the connections are good is more important than the starring - so we'll go with this board.
Power converters on the board
In the past power converters have been off-board devices so if they burnt out they could be replaces without making changes to circuit boards.
- No power converter has actually fried so this requirement may be necessary.
- By putting converters on the board we can save a lot of space and get reduction in LTC container box.
- If we're building a "power board" it makes sense to have everything related to power on it.
In the end the small size of power converters we found fit really well with the board size and if something fries - we'll just de-solder it (so all parts have through holes). Also, we should put sockets onto the board and not actually have the converters mounted but we forgot to order sockets.
Umbilical power at 24v
The decision to put power converters on the board resulted in not being able to find an 18v DC/CD converter.
The decision to upgrade to a 24V converter was made because the rocket can handle it and the part was very robust.
Use the solar charger load terminals
The solar charger has three sets of terminals - solar panel, battery, and load. In the old launch tower the load was not used to feed the system - instead system draws right off of the battery.
The load terminals may get cut off by the charger when the voltage drops to (say) 10V. However, we want our system to be able to run itself off the battery even if the battery gets down to 9V or 5V. For this reason, we can't afford the system to cutoff power so we will continue to NOT use these terminals - and instead run the system directly from the battery.
I/O screw terminals distributed around perimeter
The screw terminals are located to put them closest to their actual device - limiting cable length. This required the layout of the box to be figured while this board was developed and could be a slight problem if the layout got changed int he future.
Design Reference - July 23, 2012
Here are the current schematic and layout designs as of July 23rd, 2012. This design was submitted as the final production run. Major changes have occurred on the board compared to June 18 version and final description of board and choices are above.
Eagle files available in the GIT repository "launch-tower".
Design Reference - June 18, 2012
Here are the schematic and layout designs as of June 18th, 2012. This shows the general idea of bringing all the power into one location with LED indicators. Left here as a reference comparison to final version.