ALL ABOUT THE COMMODORE POWER SUPPLY Latest updates and/or corrections 2-5-2014 Why was the factory power supply (the black or white "brick" PS) for the C64 designed the way it was and why does it have such a bad reputation? To begin with, a separate power supply for any electronic device means the powered device itself does not need to pass UL (Underwriters Laboratories) approval for safety. An external power supply isolates the computer or other product from the power line and possible shock or fire hazard so only the PS need be certified for safety. Most low voltage and relatively low current power supplies built back in the 1980's were of the analog or linear design, such as the popular "wall wart" you see everywhere. Older versions (like the C64 PS) use a step-down transformer to convert the AC line voltage to a lower voltage, rectify the AC (alternating current) to DC (direct current), filter it with electrolytic capacitors and regulate it to a precise voltage with a simple circuit (transistor or IC) to properly feed the device. Because it uses few components, the linear design is simple, relatively reliable and low in cost. The downsides of all linear power supplies: they generate quite a bit of waste heat because their design is not very efficient and the internal transformer is large and heavy. The more power (current in Amperes) they are required to supply, the larger and heavier they are and the more heat is generated by the regulator and other components. The use of a heat sink (a metal "fin" physically attached to the regulator) helps with heat dissipation, assuming the metal is exposed to the air. Note: not all small AC to DC supplies are regulated internally. With those, voltage regulation is done inside the powered device. Although the Commodore PS was designed with a current capacity of 1.5 Amps, it runs hot inside with a load of less than 1 Amp (C64 with cart plugged in) because it's a sealed-in-epoxy unit. This prevents user tampering and electric shock hazard but it holds the heat in and "cooks" those internal components eventually. It cannot be repaired if it fails and the heat cannot easily escape except through the epoxy case which is a relatively poor conductor of heat. All that said, look how long most of those power supplies have lasted. There are several modes of failure for a linear PS. The electrolytic (filter) capacitors eventually dry out inside resulting in excessive "ripple", an AC component on what should be pure DC voltage. That often results in a lower than normal DC output which the computer cannot tolerate. It will fail to boot or run properly but will suffer no damage; it just doesn't work. If any PS component fails by opening up, the output voltage drops to zero, again without damage to the computer. The worst and most destructive PS failure is caused by a bad regulator that shorts out internally so its input voltage (more than 11 volts) is passed directly to its output! Most semiconductors (transistors and IC's) inside the computer run at 5 volts DC and can withstand a minor voltage overload without damage... but some cannot. The eight RAM chips in a C64 are especially vulnerable to over-voltage damage. Those IC's have an "absolute maximum rating" of 5.5 volts and are quickly destroyed above that maximum level. I've seen several CBM bricks that always worked normally when cold but would output excessive voltage when warm. Those supplies went on to damage several users computers because the owners didn't realize the fault was repeatable. Any PS brick failure means it must be replaced, preferably with something better if available. Why was the Commodore power supply and the C64 designed without a fuse in the 5 volt line? There is one for the 9VAC source inside the computer and another in some power supplies, but none for the 5 volt source. Answer: The regulator IC inside the PS is designed to shut down (called foldback current limiting) if overloaded. The PS output voltage drops down to protect itself and the computer if the load exceeds 1.5 Amps. The problem with that is if the regulator itself shorts out, there is nothing to prevent damage to the computer chips from excessive voltage and nothing to limit the current when it happens... except a thermal fuse inside the supply transformer to cut off AC power to the supply. That thermal fuse opens when the transformer temperature exceeds a preset limit so it can't burn your house down if left on by accident. If all you have is the Commodore brick, there are some things you can do to make it last longer. Since internal heating does eventually destroy it, keep it cool. Run it on the floor where cooler air can circulate around it. Don't put anything on or around it that would block the airflow. Don't try to use that PS to run power hungry devices such as a RAM expansion unit (REU) or carts like the 1541Ultimate2. Lastly, consider a Computer Saver for your system. If your supply does fail, the Saver will prevent damage to your computer chips. Commodore did later provide C64 power packs that didn't have epoxy inside and that did allow for air cooling of the regulator heat sink inside. Their cases were held together with screws which made those supplies repairable. They were more reliable but could still fail in a number of ways like the earlier bricks because their electrical design was the same. Some after-market linear supplies have a strange quirk of putting out a slightly higher voltage when the supply is unloaded (computer turned off). That made my Computer Saver device "trip" (fail-safe) when it wasn't supposed to. As it turned out, those supplies used thinner wires inside their power cable causing increased resistance and allowing for a drop in output voltage under load. To compensate, the designers added a resistor to the regulator to up its voltage a bit under the normal load of a computer. The final result was that the regulation from no load to full load changes by almost 0.3 volts. To compensate for that effect, my later Savers trip point was raised to 5.4 volts, still within safe limits for computer protection. A later Commodore PS was designed for use with their 17xx series RAM expansion units (REU). It is a switching type rather than the earlier linear design, and the 5 Volt output can supply 2.5 Amps. Those supplies are rare and hard to find. Commodore realized at the time that although their standard brick PS had a rating of 1.5 Amps, it couldn't stand up under the added load of an REU. The same is true with after-market devices like the 1541Ultimate2. Some power supplies will shut down when that added load is put on an otherwise working power supply. So why should a PS rated at 1.5 Amps fail to work with a load of only 1 Amp or so? Answer: The start-up current draw is more than the running current and that makes the supply go into shutdown. Many devices draw more current when starting up than when running. Have you ever seen your lights dim when the refrigerator starts up or you run your power saw? The same startup surge is normal with electronic devices. The C128 series computer power supplies and those of the 1571 disk drive are of the newer "switching" or digital types. Those supplies are generally smaller for the current they can supply and use more (but smaller) components to do the same job. Even so, they are inherently more reliable and generate less waste heat because their design is more efficient. Recent switcher designs are better than what was available back in the 1980's. Important note: although it looks identical to a factory C128 PS and uses the same five pin square power connector, an Amiga power supply will not work with a C128 because its output voltages are different! Feedback from users report that using an Amiga PS on their 128 results in almost normal operation but an annoying buzz or whistle in the audio output, most notibly in 128 mode. I don't have any reports of damage to the computer but I'm not ready to try it myself to find out. Power supplies for the C128 and C64's are hard to find now and reliability is a major issue, especially for the C64 brick. Note that some users are modifying their C128 PS and adding a C64 power cable so it can run that computer as well. At least one vendor is making a 128 to 64 adapter that just plugs onto the 128 PS cable. Both the C64 and the C128 need a regulated +5VDC (at 1.5A and 4.5A respectively) as well as a 9VAC source at 1 Amp directly from a transformer. Due to that unusual dual-voltage requirement of those computers, an off-the-shelf commercial supply is not available. I understand some hobbyists are making their own out of "spare" parts. If I were to start from scratch, I would want a 5 volt AC to DC switching PS module that could handle up to 5 Amps. That would be the basis of a universal PS good for any 8 bit Commodore computer that needed an external supply. The 5V switcher should be able to accept line voltage (120V for USA and 230V for Europe). It would have to maintain good regulation under all load conditions (including no load at all) and preferably shut down if overloaded without destroying itself or the load. That would eliminate the need for a fuse. It should be efficient enough to be used in a sealed case without overheating. A separate transformer (120 or 230V) would be needed to generate the 9 volts AC source for the computer. Lastly a case, power cables and connectors would be needed to complete the project. These components could all be purchased new for perhaps $50. As an alternative, the appropriate DIN plug could be installed directly onto the cables of two "wall warts" and then no case would be needed. It's not very eligant but would work as well. Ray Carlsen