Power supply for the arm.

     Long time no see, a while back I mentioned that I had upgraded the elbow servo from a analog HS-645MG to a digital HS-5685MG which runs at 7.4V instead of the 6V most analog servo's run on. It can run at 6V but it is at the very bottom end of its working limits. I do eventually want to upgrade all the servo's to digital so i can make use of the advantages they offer.
 
    For now though it posed a interesting problem, did I let it run at 6V with the other servo's at a reduced capacity or do I find a way of using them all at their prefered voltages. I could easily purchase a dual output power supply which would server me well in many ways besides the arm but where is the fun in that! I would be passing off a perfect opportunity to learn about the design and construction of a larger dual output linear power supply. The supply itself will be powered from another DC supply, I am not confident enough to make a device that runs from the household 240V AC supply.

    So back at the start of November or (possibly the end of October) I started designing my own dual output Linear supply with the following design goals in mind

1: Semi fixed dual channel output.
    The voltages I will be using are likely to remain fixed for a long period of time so I want to avoid the possibility that the voltages might be altered without my knowledge by accident. I will include the option of adding a multiturn potentiometer at a later date to make it more flexible and widen its area of use.

2: Modular Construction
    I have no doubt I'll find design flaws and areas I can improve upon so I want to break up the primary areas of the power supply so I can easily upgrade them. should I ever decommission the power supply I can reuse some boards in other projects.

3: Run from a 12V DC supply
     I have chosen 12V because it gives me the option to power it from a Lead Acid battery and offers a wide range of voltages from 1.25V to roughly 10V. The ability of running the power supply from a car batter means I can finally take the arm away from my desk and make it more mobile, this brings me one little step closer to mu final goal for the arm.

4: Monitor output and input voltages
    Despite the output voltages being semi-permanent I still want to monitor them to ensure stability and reliability over long periods. Because of the option to run the PSU from a battery I also want to monitor the input voltage to make sure it stays within 11-13V input.

5: Capable of supplying 5A of current per channel
    The arm currently draws 2.5A when all the servo's are under full load, this will increase as I change the servo's from analog to digital servo's. 5A per channel seems like overkill but I would prefer the regulators to be working well within design tolerances than working on their limits.

     These were the original five design goals which I set myself  but as time passed I added several more goals to the list as it became clear they would be required or simple to include as additional features.

6: Thermal management
    Asking the PSU to regulate 5A with a input / output difference of 10V  (example: 12V input with 2V output) would produce a insane amount of heat possibly more than my design could handle, even 2.5A with a In/out difference of a few volts would make it too hot to touch after half hour or so.
   Because of this I had to include thermal management but I decided not to just screw a 12V fan on the back running full pelt when the supply is idle, I decided to use a thermal management IC that will monitor the two heatsinks and speed up or slow the fan accordingly.

7: Low current fixed output regulators.
    Fixed value regulators are easy to implement on their own board for supplying smaller devices, I decided on 5V, 6V, 8V and 9V selectable outputs mainly because they are what I had in stock, I considered a second board with a small LM317 for selectable low current output but shelved this as I already possess several LM317 boards I made in the past for such a purpose.


   Right now I have seven PCB's I have designed and manufactured to meet these design goals, I have found some small mistakes and other improvements to be made but nothing that cripples the boards themselves.

The boards I have can be named as,
1: Regulator #1
2: Regulator #2
3: Fixed output Regulator
4: Thermal management
5: Processing (by arduino)
6: Input power control
7: Front panel display

    Because of a small oversight in the design of the front panel display I placed the mounting holes in the wrong place so I will have to re-design this board again. I am pleased that all the boards are working as designed so far with no faults in the circuitry.
 
    I have taken a fair few photos of the design and construction of the boards to date, far to many to post here so I am instead sharing a link to the gallery with the pictures taken so far, I will update this with more pictures as time passes, check the picture annotations for more info on each picture.

https://picasaweb.google.com/102902771360767231940/DualChannelPowerSupplyProject

Thats all for now, I will write again soon with a circuit diagram of the lot, right now I am keeping it all in my head!