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Commodore 64 Power Surge Protection Using Diode

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Although I owned only a Colecovision Adam during my youth and recently got into the 8-bit Atari retro vibe, I was gifted a C64 breadbox by my best friend ( he rescued it from it's owner throwing it away ) since he knew I'm into retro stuff ( have a lot of PowerPC's also ...).

However since ,currently due to the lockdown, I can only use it's original power brick I thought to use the diode method as shown here 

and here


I ordered these from Amazon.de but there is no indication wether they are uni or bi-directional , does it matter ?


Anything else I should pay attention before soldering this on and plugging in the original power brick ?

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Ok, I understand that the diode will become conductive at voltages above 6.8V. Jan Beta explains that it will kill the fuse in the power supply while the other guy seems to have misunderstood and mentions the fuse in the computer. As far as I know, the fuse in the computer only protects the 9V AC line, and I've tried myself to successfully power on a C64 without a fuse inserted, though you wouldn't get sound or use 9V on the user port without it.


Also Littelfuse is the brand name, not a spelling error. :)



The ones you found on Amazon seem to be uni-directional, whether it means you should install it with the marking towards the ground plane?


I've seen custom made PCBs to be inserted in the power supply after you remove all the epoxy. While any such hack voids any safety marking, generally it seems like a good solution if you know what you're doing as the transformer part of the PSU often is fine, only the circuit that rectifies 9V AC to ~12V DC and then regulates it down to 5V that tends to break.

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You can build this overvoltage circuit below for relatively cheap. I modified my Atari and C64 power supplies using a variation of this circuit which introduces a voltage clamp and fuse on the 5v output.




Instead of the 7805 I used a DC-DC module and adjusted its output to around 5v, then trimmed the board to fit inside the case. Most of the parts are hidden under the DC module but it's essentially the same circuit. The only changes I made were to the zener diode value from 6.2v to 5.6v and I also added a choke ( not pictured here ) to reduce the switching noise exposed on the 5v line.  Winding the pot on the module past 5.7v blows the 2a fuse and protects my machines. Have been running this setup happily for about a year now with no problems. You could also use a TPSM84205EAB, which is a drop in replacement for the 7805.






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