Commodore 64 Power-Supply Protector (C64 Saver)

Commodore 64 Power-Supply Protector

Caption	An open-hardware implementation of Ray Carlsen's Computer Saver
Type	Over-voltage cut-out for the +5 V rail
Designer	Ray Carlsen (original design); OpenC64Saver (SukkoPera) among community derivatives
Manufacturer	Unknown
First released	Unknown
Latest revision	—
Operating voltage	+5 V DC monitored; 9 V AC passed straight through
Layers / PCB	—
Compatibility	All versions of the Commodore 64, the VIC-20 CR and the Plus/4
Features	—
Model No.	—

The Commodore 64 Power-Supply Protector, universally known by Ray Carlsen's name for it — the Computer Saver or C64 Saver — is a small circuit that sits in the +5 V line between a Commodore power brick and the computer, and opens that line if the rail rises above about 5.4 V. Its purpose is to stop a failing supply from destroying the machine's custom chips.

Everything on this page about how the circuit behaves is Carlsen's own description, reproduced in full in the OpenC64Saver documentation.

Why it is needed

The original Commodore bricks are potted in epoxy, cannot be opened or serviced, and give no warning before they fail. Carlsen names the failure mode directly: a shorted internal 5 V regulator, which puts excessive voltage into the computer and "damages chips such as RAM very quickly... and silently".

On a Commodore 64 there is nothing on the board between the brick's 5 V output and every logic device in the machine — Commodore's own circuit theory has the 5 V go through a choke, a pair of filter capacitors and the power switch, and then straight to the chips. The same is true of the VIC-20 CR, which likewise has no on-board regulator.

Carlsen is explicit that this is not only an old-brick problem: "After-market power supplies, even switching types, can fail at any time." Switchers are more reliable and much less prone to catastrophic failure, but a Saver protects against both.

What it actually does

It is not a crowbar. This is the single most common misdescription of the device, and Carlsen addresses it head on:

"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."

A crowbar shorts the faulty rail to ground and relies on a fuse to clear; a Saver opens the line instead, so the fault energy never reaches the machine and nothing has to blow to stop it.

Behaviour

Property	What Carlsen and OpenC64Saver document
Trip point	About 5.4 V, set by trimming
Action on trip	The +5 V line is opened, not shorted
Output element	A relay in the OpenC64Saver implementation
Reset	The device stays in failsafe until the supply is unplugged
9 V AC	Passed straight through, unprotected

Why the trip point is as high as 5.4 V

Carlsen explains this, and it matters because a reader who sets it lower will get nuisance trips:

"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 for the RAM chips."

Temperature sensitivity

The Zener reference drifts with temperature: cold reads high, hot reads low. Carlsen's consequence is practical — if the Saver is fitted inside a machine, keep it away from anything that produces heat, and do not judge the trip point on a board that has just been soldered.

Which machines it suits

Carlsen's own list:

• All versions of the Commodore 64
• The later CR version of the VIC-20 — not the early 2-pin-connector boards, which regulate their own 5 V on the board and have no 5 V rail coming in to protect
• The Plus/4, though its square four-pin DIN has no external equivalent, so an external unit needs the socket changed to the round C64 type

Fitting

Internally

Carlsen's preferred arrangement: fit the circuit inside the machine, so it protects against any supply plugged in. That means opening the +5 V line on the board and wiring the protector's input and output to the two ends of the break. The correct point is electrically between the power connector and the power switch.

There are at least five C64 board layouts and the physical location differs on each, so work from your own board rather than from a photograph of someone else's. On a VIC-20 CR the modification requires cutting a trace.

Externally

A box with a male and a female DIN-7 connector (formally DIN 45329) lets one unit serve several machines.

Get the pinout right. On the Commodore 7-pin DIN, pins 1, 2 and 3 are all ground and +5 V is on pin 5 — on the C64 E board it is on pins 4 and 5 as well. Pins 6 and 7 carry the 9 V AC. Measuring or feeding between "pin 2 and pin 5" as though pin 2 were the positive is wrong in both directions: pin 2 is ground.

Setting the trip point

OpenC64Saver carries a trimmer for this. The documented procedure needs a variable bench supply and a voltmeter:

1. Raise the supply slowly until the relay cuts in — voltage appears on the output side.
2. Keep advancing until the relay cuts out again. The reading at that instant is the trip point.
3. If it is too high, bring the voltage back down until the relay cuts in, adjust the trimmer, and repeat.

Aim for about 5.4 V. Too high and it will not protect the machine; too low and it will trip on a good supply.

Limitations

• It protects the +5 V rail only. The 9 V AC passes through untouched.
• It does not regulate. A supply with heavy ripple that stays under the trip point will pass.
• It is a last line of defence, not a replacement supply. A brick that has already drifted needs replacing, not protecting.
• If it trips, stop using that supply. The trip is the evidence.

What this page does not claim

A number of derivative designs circulate — commercial kits, SMD versions, variants with meters — and this wiki has not been able to verify per-variant trip points, release dates or bills of materials for them from a primary source. Those figures have been removed rather than repeated. The two designs documented above are Carlsen's original and SukkoPera's OpenC64Saver, both of which publish their own descriptions.

If you are building one, work from the repository's own schematic and BOM for the revision you are building, not from a table on a wiki.

Related pages

• Commodore 64
• Commodore 64 General Maintenance
• Commodore 64 Troubleshooting Guide
• Commodore 64 Capacitor Replacement Guide
• Commodore VIC-20
• C64 Reloaded MK2