Editing Commodore 64 Power-Supply Protector (C64 Saver)

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{{Infobox hardware
| name           = Commodore 64 Power-Supply Protector
| image          = [[File:C64-Saver-Top.jpg|250px]]
| caption        = A popular open-hardware implementation of the '''C64 Saver'''
| developer      = Ray Carlsen (original design)<br />community derivatives (OpenC64Saver, SaV64, SaRuMan-64, C64 Saver 2, etc.)
| type           = External over-voltage / over-current protection adapter
| first_released = 2016 (public release of the Carlsen “C64 Saver 1.2” design)
| latest_release = C64 Saver 2.4 PCB (2022)
| operating_voltage = +5&nbsp;V&nbsp;DC (monitored)  •  9&nbsp;VAC passthrough
| application    = Commodore 64 / 64C / SX-64 / VIC-20 CR and any CBM 8-bit that uses the 7-pin DIN power connector
}}The '''Commodore 64 Power-Supply Protector'''—best-known by Ray Carlsen’s original name '''“C64 Saver”'''—is a small plug-through safety adapter that sits between a vintage Commodore power brick and the computer.  
Its only mission is to '''sacrifice itself if the +5 V rail from the ageing “epoxy-potted” supply drifts above a safe level''', instantly disconnecting the load and saving irreplaceable MOS chips such as the VIC-II, SID, CIA and RAM.

== Why a Protector Is Needed ==
Original Commodore “brick” PSUs were cost-reduced, epoxy-potted switchers with no serviceable parts or active regulation feedback.  
With age the internal linear regulator or crowbar diode can short, driving the +5 V DC rail to 7–12 V—high enough to destroy ICs within seconds.  Modern collectors therefore avoid the stock PSU '''unless''' a C64 Saver-class device is in-line.

== Operating Principle ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Function block !! Component&nbsp;example !! Description
|-
| Over-voltage sense || TL431 programmable shunt (earlier boards use 5 V zener) || Monitors the 5 V rail; trips at 5.3–5.5 V (adjustable ± 50 mV).
|-
| Crowbar / latch || SCR (e.g. C106D) or P-channel MOSFET + relay || When tripped, shorts the rail through the SCR or mechanically opens the circuit; resets only when input power is removed.
|-
| Fuse element || Resettable polyfuse 1.1 A or fast-blow 1 A || Limits fault current so the PSU shuts down instead of the computer.
|-
| Visual alert || Bi-colour LED || Green = normal; Red = tripped/over-voltage.
|-
| Passthrough lines || 9 VAC pair, Sense line || These are '''not''' switched—protector covers only +5 V DC.
|}
*OpenC64Saver* introduces reverse-polarity protection, replaces the SCR with a low-R<sub>DS(on)</sub> MOSFET for cooler running, and adds a precision trim-pot for exact trip voltage set-up.

== Design Evolution ==
{| class="wikitable styled-table" text-align:center;"
! Variant !! Year !! Key features !! Trip point (nom.)
|-
| '''C64 Saver 1.x''' (Ray Carlsen) || 2016 || Through-hole, SCR crowbar, 5 V6 zener || 5.4 V
|-
| '''C64 Saver 2.x''' (RC) || 2019 || Smaller PCB, TL431 reference, polyfuse replaces wire link || 5.25 V
|-
| '''OpenC64Saver''' (SukkoPera) || 2018 || Fully open hardware, MOSFET disconnect, reverse-polarity diode || User-set
|-
| '''SaV64 / SaV64-II''' (SharewarePlus) || 2019 || Commercial kit, optional OLED volt-meter, user-replaceable fuse || 5.25 V
|-
| '''SaRuMan-64''' || 2020 || SMD, auto-reset, fold-back current limit || 5.30 V
|}
All variants honour Carlsen’s open-distribution request that the design remain non-proprietary for the benefit of the community.

== Typical Bill of Materials (through-hole C64 Saver 2.3) ==
{| class="wikitable styled-table" text-align:center;"
! Qty. !! Reference !! Part
|-
| 1 || F1 || Polyfuse 1.1 A (RXE110)
|-
| 1 || SCR1 || C106-D or BT151-500R
|-
| 1 || U1 || TL431 (2 %) shunt reg
|-
| 2 || R1, R2 || 1 kΩ & 4.7 kΩ (trip divider)
|-
| 1 || R3 || 220 Ω (LED limiter)
|-
| 1 || C1 || 100 nF decouple
|-
| 1 || D1 || 1N4148 (gate clamp)
|-
| 1 || D2 || 5.6 V Zener (reference bias)
|-
| 1 || LED1 || 3 mm bi-colour (GRN/RED)
|-
| 1 || J1 || Male 7-pin DIN (to PSU)
|-
| 1 || J2 || Female 7-pin DIN (to computer)
|}

== Installation ==
# Power '''off''' and unplug the PSU.  
# Connect the PSU’s 7-pin DIN plug into the '''male''' side of the Saver.  
# Plug the Saver’s '''female''' 7-pin into the computer.  
# Power up. LED should show '''green'''.  
# If LED immediately turns '''red''' or no power reaches the C64, the PSU is already faulty—'''do not keep using it'''.

The device draws under 5 mA and introduces ≈ 15 mΩ series resistance, insignificant even for sensitive Ultimate-64 boards.

== Testing the Trip Circuit (Bench) ==
* Dial a bench supply to 5.0 V and feed the Saver through the DIN pins (+5 V → pin 2, GND → pin 5).  
* Increase voltage slowly; at 5.3–5.5 V the LED flips to red and the output collapses to ≈ 0 V.  
* Remove input power for 5 s to reset the latch.

== Limitations ==
* '''Does not regulate ripple'''—a PSU with excessive 50/60 Hz ripple under 5.2 V will still pass.  
* '''No 9 VAC protection'''; an internal short in the PSU transformer can still inject noise or excessive VAC.  
* '''Not a substitute for a modern PSU'''—it is a last-line “crowbar”, not continuous regulation.  
* If the SCR version trips it may blow the PSU’s internal fuse; replace that fuse before re-testing.

== Where to Buy or Build ==
* '''DIY PCB/gerbers:''' OpenC64Saver repository (KiCad + BOM). 
* '''Kits / assembled:''' SharewarePlus SaV64 series, RETRO Innovations, and many eBay/Tindie sellers.  
* '''Integrated solutions:''' All Ray Carlsen modern PSUs ship with an internal Saver circuit plus precision linear regulation.

== Related Pages ==
* [[Commodore 64 Power Supply Guide]] – failure modes of the original bricks  
* [[Commodore 64 Capacitor Replacement Guide]]  
* [[Commodore 64 Troubleshooting Guide]]

[[Category:Commodore Systems]]
[[Category:Commodore Maintenance Guides]]
[[Category: Commodore 64 Modifications and Enhancements]]