THE COMPUTER SAVER A PROTECTION DEVICE FOR THE COMMODORE 64, LATE VERSION VIC20 & PLUS/4 Latest updates and/or corrections 12-28-2019 Note the special instructions at the end of this article!!! 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 Saver 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 5.4 volts. It is not a "crowbar" device but opens the line from the supply to the computer before the voltage gets high enough to do damage. That's the safest way to do it. After-market power supplies, even switching types, can fail at any time. Switchers are more reliable and much less prone to catastrophic failures.This Saver is designed mainly to protect any computer that uses the standard Commodore "brick" power supply. Computers include all versions of the C64, the later CR version of the VIC20, and the Plus/4. Because of the Plus/4 square power connector, a different external Saver is made to work with its four pin square DIN connector.Since replacements are not available, I make my own plugs. Built from readily available parts, this protection circuit is installed in series with the 5 volt line between the power supply and the computer. A protector module can be installed inside the computer or can be purchased as a separate plug-in device that connects between the computer and its supply. I make three models: an assembled and tested module that can be installed by the user (or myself, if desired) inside a C64, VIC20 or Plus/4, an assembled in-line cable (with module inside) and a small box with the Saver inside. Each Saver version has the trip point set and has a failsafe LED. The complete external cable and plastic box Saver versions are plug-and-play ready. There are several ways to add this protection device to your system. One way is to install the components or a completed module inside the computer. That way, it is protected from any supply plugged into it. If you have more than one computer, a protector can be installed in each one. 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 open the line and make the connections. In all internal 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, and the physical connections will be specific for each board. A Saver is the best way to ensure the computer is safe if the brick PS must be used. 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. Rather than removing the entire bottom shield, you can unsolder just one of the tabs at the right rear side of the board and bend the shield up to access that area of the board. A Dremel tool with a thin cutting blade or just a small file or knife can be used to open the copper circuit trace between the power connector and the power switch. The shield can then be put back and all Saver connections made on the top of the board. The Saver input connects to one pin at the rear of the power connector, and the Saver output to one leg of the power switch. The third wire is connected to a convenient ground point. Other common versions of the C64 include board numbers 326298 (very early model), 250425 and 250466. The Saver modification can be added to those boards without removing them from the case or removing the bottom shield 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 (there may be a jumper wire only) 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 trace must be cut on the bottom of the board to open the line between the DC input connector and the power switch. See photo. NOTE: as an alternate to cutting the bottom board trace, a connection on the input filter coil can be cut (see photo) to install the Saver, which makes removing the PC board unnecessary, although there isn't much coil wire to work with. When adding individual Saver components to computer boards that use a metal shield over the motherboard, note that the area under that top metal shield (heat sink) is very small and you must mount the components as low as possible so the metal shield doesn't short to anything underneath it when it's put back. Alternatively, you can cut a portion of the shield out over the new components, if that's easier or you can purchase and use a pre-wired module and mount it over the cartridge port. See photo. 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 that 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 only as long as the fault persists. It is therefore 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 can be added to the protector circuit if desired. That fuse will open if an external overload or short circuit is accidentally 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. However, that failure is unlikely and a fuse isn't really needed since the power supply regulator IC itself has a feature called "fold-back current limiting". If too much current is drawn for whatever reason, the voltage automatically drops down to limit the current. On the other hand, if the regulator shorts and its output voltage rises, the Saver will "trip" and protect the computer and itself from damage. 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 850mA (more when a cartridge is used), a 1 Amp relay would also work but a margin of safety is a good idea. The relay I use is a DPDT with two sets of 2 Amp contacts. Wired together, the total is 4 Amps, greater than any load could impose, so it provides a good safety factor. You need a relay with a 5 volt coil, and one that has a high DC resistance is preferred. 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 applied voltage! If such a relay coil is wired backwards, it will not energize. 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, 2N3565 and 2N3904 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 basic "trip" point of the device. The two resistor values at the zener are likewise critical. See special note at the end of this article!!! The silicon diode across the relay coil is 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 Watt carbon or carbon film types. 1/2 or 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 metal cover back on!!! If desired, an LED or 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. In newer Savers, I use a dual-LED (red and green) that has three legs. One leg is wired to relay NO (normally open) contacts and the other to the NC (normally closed) contacts. That way, the LED will light up green if the +5VDC is normal and change to red if the PS fails via over-voltage. The center leg of that LED goes to ground through a 1K resistor. This is the basic circuit but later modifications were made and are shown on "saver schematic.jpg". Most notable are the 220K resistor from the relay coil driver collector Q2 to the base of the trigger transistor Q1. The resistor adds a bit of feedback hysteresis (snap action) to the relay circuit. The added 0.2uF capacitor from the driver transistor Q2 collector to ground keeps that device from oscillating. These mods stop the momentary relay "buzz" that some users reported when using the Saver. A continuous buzz from the Saver relay indicates an overload. It can happen if there is some kind of short in the computer and can also happen if the power supply starts to fail (output voltage too low). Don't let the relay buzz continue or the relay contacts will be damaged. 2Amp fuse (optional) ___[]~~~[]____________________________________ +5VDC | | | C | com from \ | K_|_ O 0 PS / 1K | A /\ I \ relay \ ohms | | L NO 0 \0 NC 4.7V K | / |___| | | zener \__|__ \ 2.2K | |--------- +5V out to diode A /\ \ / | | computer | \ | \ | | | / | | C__| \ 1K | | |/ / | C__|____| Q2 | | |/ B |\__ |A |_______| Q1 E | _\/_ | B | | |K fault / |\__ | | indicator 820 \ E | | | LED ohms / | | | (optional) | | | | GND (-) __|___________|________|_______________|_____ GND NOTE: Use of a power hungry add-on like a RAM expansion unit is not possible 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 a Computer Saver less necessary since it's a switcher like the C128 supply, and is much less vulnerable to over-voltage failure as 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. A C64 power connector can be "pigtailed" onto the existing C128 power cord in a Y formation or wired directly into the supply case like the 128 cable. With both cables attached, 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 if you decide to pigtail the 64 power connector. SPECIAL NOTE: Because of fairly wide Saver component tolerances and to accommodate the normal voltage variations of even good power supplies, the basic Saver circuit must be "trimmed" so the voltage cuts off at about 5.4 volts DC. If set too high, it will not protect the computer; if too low it will trip prematurely even with a good PS. Feedback from a few users indicated the Saver would trip when they turned their computer off, and it remained in failsafe mode until unplugged from the PS. Apparently some power supplies output can jump up past the failsafe point when the load on them is suddenly removed (computer off). That's why the trip point is set as high as it is. 5.4 volts is still under the "absolute maximum rating" specification of 5.5VDC for the RAM chips. The PS in question was the Commodore repairable one. It's about the same size as a brick but is a different shape and has vent holes in its plastic case. To ensure correct operation, I use a variable bench DC supply and a very accurate voltmeter to test and adjust all my Saver circuits after construction and before connecting to a computer. The Saver trip point is often too high (or can be too low) with the standard resistor values shown. To trim the voltage trip point, I add a higher value resistor in parallel with the 1K ohm one, and in some cases the 820 ohm one. That added resistor value normally varies between 2.2K and 10K with the parts I used, but mostly because of the actual conduction point of each zener diode. Note that the transistors used can also affect the trip point. A different resistor pair (different values) can be used as long as their combination sets the trip point at the correct level. Note: S.I.T. means Set In Test. To test the circuit after construction, I slowly increase the supply voltage until I hear the relay cut in (voltage appears on the relay output side) and continue advancing it until the relay cuts out again. The voltage at that instant is the trip point of the device. Note that the zener diode is somewhat temperature sensitive. If very cold, the trip point will read high, and if hot from soldering, the trip point will be low. Therefore, the mounting of the Saver components, if inside the computer, should be away from all heat producing components. If building your own -external- Saver: since the 5VDC minus line is usually tied to ground of the three wire AC cord, it's a good idea to include that connection. That ensures the computer is properly grounded in case the transformer in the PS goes electrically "leaky". To implement, tie the DIN socket negative pin (2) to the shield tab and to chassis ground. If you use an LED to monitor the 9VAC source from the power supply, use a diode such as a 1N914 across the LED to protect it from reverse current "spikes" from the transformer. A standard power supply diode is not fast enough. Wire the diode as follows: anode to LED negative and cathode to LED positive. Use a 470 to 1K ohm current limiting resistor in series from the 9VAC to the LED. I have a European power supply that uses pin 4 rather than pin 5 for the 5VDC into the computer. Most C64's have those pins tied together inside the computer but I found just one that didn't. So, either PS will work if those two pins are wired together externally. Addendum All of my external Savers will now include the dual function LED. It lights up green if the PS is working normally and goes red if the Saver senses overvoltage and cuts off power to the computer. Ray Carlsen CET Carlsen Electronics