General idea for a repairable heavy duty power supply

[eslapion]

Yes, I know, I know, we've heard a lot of stuff, some of it not so pleasant, about power supplies lately.

Still, there is a strong demand for power supplies that last long, can provide extra power when you plug a lot of stuff to you VIC or 64 and won't destroy our precious antique computer when they fail (everything eventually fails).

The boosted power supply that came with the 1764 was, more or less, a C128 power supply with a round DIN connector instead of the square 5 pin weird thing.

Some people have modded C128 PSUs to put a round DIN connector and effectively do what Commodore did with the 1764 PSU. From a technical point of view, that's great. From a historical and collector's point of view, its a catastrophe.

What is there then? What could we use that's made up of new stuff instead of recycled old stuff to power adequately our old machines?

Switching PSUs usually have a high voltage section in them that makes them officially unrepairable... unless you want to have a new porcupine style hairdo when you stumble upon a charged capacitor. These can give electrical shocks up to 350 volts... NOT good. Repairing switching PSUs can also represent a tremendous fire hazard if you don't know what you're doing. Sometimes all you have to do is replace the fuse... sometimes not.

Transformer based power supplies are safe because the high voltage is directly attached to the transformer and its easy to completely isolate that part. However, they can be very bulky when they are rated beyond a certain power. If they have a regulated output, they can produce a tremendous amount of heat and have terribly low efficiency because of the linear regulators they use.

But what if we could keep the transformer and replace the linear regulator with something else?

I think it is perfectly possible to replace the linear regulator in a transformer based power supply with a low voltage switching DC to DC converter.

Explanation:
A linear regulator will reduce the voltage fed to it (usually a rather unstable source of voltage) to the desired voltage by bleeding off as heat the difference between input and output voltages.

For example, lets say your C64 consumes 1.5A on the 5Vdc (A c64 with a Super snapshot V5 plugged in, more or less) and the transformer inside the Commodore brick feeds the regulator an average of 9Vdc. The voltage drop is therefore 4 volts with a current of 1.5A, this means the regulator will produce 4*1.5= 6 watts of heat.

If you have one of those more powerful power supplies with big regulators and draw 2A (A C64 with a 1750 and SSV5), that will be 8 watts of heat for the regulator alone. Using the same power supply to feed both your C64 and a 1541-II or 1581 will raise the 5V consumption to 3A and therefore the dissipated heat will be 12 watts. Now you got to the point where dissipated heat is just below that of a small soldering iron.

If we use a device known as a switching buck instead of a linear regulator to drop the unstable voltage to 5V, the amount of heat dissipated becomes close to zero because it uses no dissipative component.

A switching buck acts like a very rapidly opening and closing switch that connects, via an inductor. This generally describes how a buck converter works.

As a gross oversimplification, one could say that a buck converting 9V into 5V will act as a switch that is connected 5/9 or 56% of the time. This switch will usually turn on and off thousands of times every second so the result is smooth. And as we all know, switches, unless they are faulty, produce no heat.

There are many products that use buck converters. For example, many PC video cards have a special hard drive connector (my radeon 9800 does) that take in 12Vdc to produce extra low voltage power for its high end circuits. These chips usually operate between 1.4 and 2.5 volts. It is a buck converter that takes in 12V and outputs the low voltage.

PC motherboards also have many powerful buck converters that take 5V or 12V to convert it in the special low voltages required by the processor or RAM. The 3.3V coming out of ATX PSU is generally not low enough for more modern chips.

All in all, buck converters are highly efficient voltage regulators and converters that present considerable advantages over linear regulators. They are a sort of low voltage switching power supplies.

Another characteristic of buck converters is if you feed them higher voltages, they produce less heat and become more and more efficiencient because the switch they use has to be turned on for a shorter period of time. This is exactly the opposite behavior of linear regulators which produce more heat when they are required to drop a higher voltage.

That's because, if the buck is a switch, imagine what portion of the time the switch has to be on if you drop from 12V to 5V instead of the 9V to 5V mentioned above. That becomes 5V/12V = 41.6% of the time and this means the average current coming into the buck has proportionally dropped.

Now imagine we feed that same buck 18V, then its switch is now on only 28% of the time to output 5V. Since produced heat is calculated as the square of current times the resistance (all components, even "non dissipative" ones have a small parasitic resistance), a buck converter fed with 18V will produce 4 times less heat than one fed with 9V.

Also, by using a transformer that provides a higher voltage, we can use one that's rated for a lower current and it too will produce less heat. With a linear regulator, a designer wants to have the lowest possible voltage that's above the output voltage to reduce the heat lost by the regulator. With a buck converter, you want the highest possible voltage to reduce current.

So the general idea is, why not create a power supply, that is fully repairable, will not expose anyone to high voltages and produce very little heat by using a transformer and a buck converter instead of a linear regulator?

A power supply like that would probably be so safe that anyone could buy it as a kit and build it themselves. If the voltages on the secondary of the transformer do not exceed 24V, it just might be possible for it to have its CSA/UL certification still valid.

Presently Lemon64 apears to be down but I intend to mention this idea there too when possible.