A PROTECTION DEVICE FOR THE COMMODORE 64 Latest updates and/or corrections 3-1-2012 This over-voltage protection unit is designed to prevent damage to the Commodore 64 computer due to a failing power supply. One of the ways the Commodore "brick" power supply can fail is from a shorted internal 5 volt regulator. That fault puts excessive voltage into the computer and damages chips such as RAM very quickly... and silently. The protector is essentially a fast acting electronic circuit breaker. It functions to quickly cut off the 5 volt supply line to the computer if the voltage exceeds about 5.3 volts. Built from readily available parts, the protection circuit is installed in series with the 5 volt line between the power supply and the computer. Early versions of this protector were wired "downstream" from the power switch. That method later proved to be problematic because, as the computer was switched on, the relay engaged but its contacts would often "bounce" and the computer would not always start up properly due to the resulting power "glitch". Wiring the saver modification ahead of the switch enables the relay to be engaged all the time the power supply is plugged in, whether the computer is on or off. There are several ways to add the protection device to your system. One way is to install the components inside the computer itself. That way, it is protected from any supply that is plugged into it. If you have more than one computer, a protector can be installed in each one. Cost of parts should be less than US$10 for each unit. Since the parts count is low, I didn't bother making a circuit board for my internal protectors although I did use a "perf" board for my prototype. In later ones, I used some silicone rubber sealer as glue to hold the relay in place on the motherboard, then wired the other components on top of it. Wires to and from the added components complete the installation. I took photos of installations in several versions of the C64 and C64C. To install the protector circuit inside a C64, it is necessary to open the +5 volt line on the circuit board. Input and output lines of the protector are wired to each end of the now-open circuit. If your C64 board differs from the pictures on my site, you will have to locate the correct area of your board to make the connections. In all versions, the correct point is electrically between the power connector and the power switch. There are at least five board versions of that computer, each with a different PC board layout, so the physical connections will be specific for each board. The most common board appears to be the 250407 version B, so I will describe how to implement the modification of that board first because it is the most difficult. A modification must be done to the bottom of the board. Therefore, it's necessary to remove the board from the case bottom and then remove the metal shield (unsolder all tabs) from the bottom of the board. The copper circuit trace between the power connector and the power switch needs to be cut open and the saver connections made to the nearest solder pads of the open ends. There are two different ways you can make the connections. 1. Drill a tiny hole in the space shown, scrape the paint off the copper trace, then install the wires going to the saver mod as shown in the photo, one to the power connector terminal and the other soldered to the trace under the hole. With those wires in place, the shield and board can be re-installed and the saver components wired in. 2. As an alternative to drilling the hole and soldering wires at the top, both wires can be soldered onto the connections on the bottom of the board and routed to the top side through the cartridge port space for connection to the saver circuit. See photo. Other common versions of the C64 include board numbers 326298 (early model), 250425 and 250466. The saver modification can be added to those boards without removing them from the case because everything can be done on the top of the board and no foil traces need be cut. Look for a small coil labeled L5. Disconnect one end of that part and install the input and output connections of the saver across the open circuit as shown in the photos. On a C64C 250469 short board, a connection on the input filter coil can be cut (see photo) to install the saver, which makes removing the PC board unnecessary. The area under that top metal shield is very small and you must mount the saver components as low as possible so the metal doesn't short to anything underneath when it's put back. Alternatively, you can cut a portion of the shield out over the new components, if that's easier. As an alternative to mounting the protector inside a computer, one can be built in a small box as a stand-alone device. That way, you can use it with any supply or computer, but that has the disadvantage of needing two DIN connectors and an enclosure. If the protector box is wired directly onto a power supply cable, that eliminates the need for the added connectors, but it will work with only that supply unless physically transferred. You can choose the implementation that works best for you. The circuit is the same regardless of how it's installed. The stand-alone protector requires a 7 pin male DIN plug on one end that fits the power socket on the computer, and a 7 pin female DIN socket on the other to receive the plug from the power supply. Note that although a 7 pin connector is used, only four pins are active. The socket can be an in-line or chassis mount connector, but the plug should be wired to a length of cable which allows the protector box to be moved out of the way of the user. My prototype model used a cut off 2' length of cable from an old dead power supply. If I were to make another, I would use a longer cable to keep the box off my already crowded desktop. I found a chassis mount female DIN socket from a junked piece of electronics gear and installed it in the protector box. The male DIN plug cable was "pigtailed" out of the other side of the box. A failure indicator LED can be installed in the box if desired. If the protector is going to be added directly to a supply cable: route the cable through the box by notching the ends of the box to allow a loop of cable to go through it when the cover is reattached. Cut open the outer jacket of the loop of cable in the box. Note that there are several different manufacturers of those bricks, and so the wire colors can be different! The most common bricks use the following wire colors: brown or red for +5, blue for - (ground), two black wires or yellow and white for the 9VAC. The +5V wire inside the cable needs to be cut and the ends wired to the protector input and output lines. The end from the supply goes to the input of the protector and the one to the computer goes to the output end. The uncut ground or negative lead can have a small bit of its insulation removed so it can be attached to the protector circuit ground. The other two wires of the supply cable are the 9VAC lines and need not be opened. How it works: The circuit for the protector is simple and strightforward. A 4.7 volt zener diode on the 5 volt line continuously monitors that DC level. The resistor in series with the zener diode limits the current to the base of the first transistor Q1, and the resistor from base to ground keeps the transistor cut off until the threshold voltage is exceeded. The zeners 4.7 volts plus Q1 transistors 0.6V B to E drop totals slightly over 5 volts, the trip point of the device. If the supply voltage rises above that level, the zener diode conducts, turns on the transistor and pulls its collector lead to ground. That lead is wired to a normally conducting second transistor Q2 which drives the relay. When Q2 turns off, the relay opens. The relay is de-energized only during an over-voltage condition. If the supply fails in that manner, the relay drops out and opens the previously closed contacts cutting off the +5VDC supply voltage to the computer. The circuit is designed to keep the 5 volt line open as long as the fault persists. It is self-resetting. There is one fuse inside the C64 that protects the 9VAC line (and another embedded inside the supply) but neither the C64 nor its supply are fused on the regulated 5 volt line. One fuse has been added to the protector circuit. That fuse will open if an external overload or short circuit is accidently applied to the computer ports. The new fuse protects the saver relay (too much current can weld the contacts together) as well as the power supply from excessive current draw. Some notes about the parts used: The relay can be an SPST (single pole single throw) if the indicator LED is not used. An SPDT (single pole, double throw) or DPDT (double pole, double throw) will also work. The relay selected must have a five volt coil and contacts rated at 2 Amps. Since the computer only draws about 800mA (but more when a cartridge is used), a 1 Amp relay would also work but a margin of safety is a good idea. This design is different than a previous one that turned on the relay during an overvoltage fault condition. The main problem with that design is that a 5 volt relay will overheat and eventually burn out if left connected to a failed (internal regulator shorted) power supply. That is because the voltage goes upwards to 11 volts which will be more than twice normal operating voltage. In such a case, if the coil burned out, the relay would deactivate and the computer would not be protected. That's why the design was changed. You need a relay with a 5 volt coil, and one that has a high DC resistance is preferred. Relays used in my early protectors had coil resistances of about 60 ohms. One drawback to that is it puts an additional load on an already marginal power supply. The higher the DC resistance of the relay coil, the less load it represents, so a coil resistance of 150 ohms or higher is preferable in this application. NOTE: small relays often have a coil that is sensitive to the polarity of its input voltage! If the coil is wired backwards, the contacts will not pull in and the computer will not get power. The transistors can be any low power silicon NPN type rated at 500mA or more. The voltage rating is not critical either. Generic types such as the 2N2222 or 2N3565 are OK as is their equivalent such as the old Sylvania ECG123A or NTE123A. A higher power NPN transistor will also work, such as tab mount or TO-220 "flatpack" types, although they are physically larger. Note also that the metal tab or case of such a transistor is normally connected to one internal element... usually the collector, so its metal case must be isolated from surrounding metal or wiring. The zener diode can be a 1/2 or 1 Watt but its voltage rating is important since it sets the "trip" point of the device. Also, the two resistor values at the zener are likewise critical and should not be changed. The silicon diode across the relay coil is a standard power diode (1N2004) but can be a small signal type such as a 1N914. Its current and voltage ratings are not critical as it's only there to absorb "spikes" from the coil when it de-energizes. Polarity is critical for any diode. They usually have a band around the cathode (K) end. The resistors are all 1/4 to 1/2 Watt carbon or carbon film types. 1 Watt resistors of the same resistance value can also be used although they will be physically larger. Space is limited inside the computer, especially in the C64C which has a metal shield over the PC board. Make sure nothing shorts out when you put that cover back on!!! If desired, an LED (shown in the diagram but not seen in my photos) or a buzzer can be used to monitor the relay status and indicate a fault condition, but their use is optional. If the red LED on the computer goes out, it's likely a power supply problem... either low or high level of the 5V line. 1.5A fuse ___[]~~~[]____________________________________ +5VDC | | | C | com from \ | K_|_ O 0 PS / 560 | A /\ I \ relay \ ohms | | L NO 0 \0 NC 4.7V K | / |___| | | zener \__|__ \ 1K | |--------- +5V out to diode A /\ \ / | | computer | \ | / | | | \ | | C__| / 470 ohm | | |/ \ (optional) | |____| Q2 / | C__| B | | | |/ |\__ |A |_______| Q1 E | _\/_ | B | | |K fault / |\__ | | indicator 470 \ E | | | LED ohms / | | | (optional) | | | | GND (-) __|___________|________|_______________|_____ GND NOTE: Use of a power hungry add-on like a RAM expansion unit is not recommended using the original CBM brick power pack. That RAM unit originally shipped with its own power supply, one that provided 2.5 Amps for the 5 volt line. That beefier supply makes the computer saver unnecessary since it's a switcher like the C128 supply, and is therefore not vulnerable to failure like the C64 brick. If you have an expander but no supply for it, consider getting a C128 power supply and installing a power connector that fits your C64. With that hookup, again, you don't need a saver. If a C64 power connector is "pigtailed" onto the existing C128 power cord in a Y formation, that supply could be used with either computer (one at a time, of course). Just make sure to insulate all the wires after making the connections to the cable. Ray Carlsen CET Carlsen Electronics .