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A1200 Power Requirements
To build a suitable PSU, I first needed to know what the A1200 power requirements are, and what the car power system can provide. I started with the A1200, and with the help of my friend Paul Harding tested what voltages the A1200 needed. We determined that to run the computer needs +5V and +12V. The -12V that the standard Amiga PSU provides is only needed for sound and serial comms, however since I needed to use the serial port I was going to have to provide the -12V power. The other thing I needed to know about the A1200 was it's current requirements. The A1200 PSU stated that the +5V line needed 3A, the +12V line 650mA, and the -12V line 100mA. The two lower current lines were not a problem, but the higher current +5V line would be. It needs to be more than the specified 3A because it is used to power the LCD display backlight. In total, I estimate that about 35W is needed to run the system.
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Car Power
As anyone who has dealt with car power systems will tell you, they can be a real nightmare to work with. Voltage can vary a lot, earthing can be difficult to get right, and all sorts of noise creeps in everywhere. The basic car supply gives you around 12-13V when the engine is off, and 13-14V when it is running. However, it can go as high as 16V on some cars. Also, when the engine is on, alternator whine gets into the supply. Cars can supply large amount of current, which is why you can run a 1000W stereo system in one. However, depending on the size and condition of the battery, it might go flat very quickly if you aren't careful. I used to have a 150W heater in my old Fiesta (my first car), and it didn't take long to start dimming the lights. On the other hand, my Alto has never had that problem, even running a 600W sound system. Earthing can be a problem in cars too. In order to reduce audible noise, it's often a good idea to earth all components as near as possible to each other. Otherwise, you can get alternator whine and all other sorts of noise entering the system.
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Power Supply Unit MK1
My first attempt at a power supply used simple voltage regulators. In the end, it didn't work, because it couldn't regulate the power enough to prevent crashing in some very extreme cases. However, for a car with better electrics, it might well suffice. Anyway, I document it here as a point of interest and so you can see how I developed it into the MK 2. The basic idea was to use simple voltage regulators. These little devices take an unregulated voltage in (usually at least a few volts above the target voltage) and output a regulated voltage. They are fairly simple in design, and are limited in what they can do. In particular, they can only deal with certain amounts of current and often get fairly hot. I had to cool mine with heatsinks and a fan. I decided to use two, one for the +12V line and one for the +5V line. The +12V regulator worked perfectly, being able to supply enough current. The +5V regulator just couldn't put out enough juice though, so I added a power transistor circuit to boost it. That way, the voltage regulator only has to regulate a small current and the real power flows through a 40W transistor. Needless to say, the transistor required a really large heatsink to keep it cool. While the regulators have thermal shutdown protection, transistors usually don't, instead choosing to simply explode! To get the -12V I needed, I used an ICL7660 IC. This handy little device is a voltage inverter. You feed in up to 12V, and it outputs up to -12V. I often only get -11.5V from it, but this seems enough for the A1200 to work with. I simply used a reference design from the datasheet. Once again, Paul Harding and Mike O'Hara helped me out with making the case for the PSU. This was basically a plastic box from Maplins with holes for the cables and a grill for the fan. The fan itself was glued into place and taped to make it airtight at the sides. The box was actually a lot larger than it needed to be, but because of the way I had laid the components out it needed to be quite long, and that was the only box that fit. With the MK2, my layout was much better.
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Power Supply Unit MK2
The MK2 PSU is basically an upgrade to the +5V line. The +12V and -12V lines stay the same as the MK1. The idea is to give the +5V line more current, better stabilisation and keep the whole thing a lot cooler. To do this, I used a Maxim MAX787 switch mode power supply IC. I had originally intended to build my own switch mode PSU, but the MAX787 seemed to fit the task perfectly so I decided to save myself the hassle. The benefits of using a switch mode PSU are many, the most important of which are the fact that the output is totally isolated from the input, and that a lot less heat is produced. Switch mode supplies work by turning the input voltage into a square wave using some kind of oscillator. This is then rectified and smoothed just like a typical AC input supply (for instance, a mains to DC supply). The output is monitored and regulated by varying the frequency of the square wave (typically around 100KHz). I used the standard +5V circuit from the MAX787 datasheet, which worked perfectly. The MAX787 and the large diode needed for the circuit don't give off a lot of heat, only requiring small heat sinks to operate inside a closed case. The actual case is a standard PSU case from Maplins, a little larger than the PSU requires and without any ventilation holes. The only airflow is where the cable entry and exit holes are, but this seems to be enough. The new MK2 supply works perfectly in my car, and the system is now entirely stable thanks to it. I must at this point thank Ian Greenway for his help with the MK2 supply. In particular, his suggestions about grounding the supply and my car amp to the same place cured the final problems I was having. Many thanks Ian!
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