Power supply over voltage protection retrofit circuit ideas.

[Steve Jones]

Hi chaps, it's been a while! I have been pondering the dreadful power supplies in some vintage synthesizers such as the Oberheim OBxa and the Prophet 5 and since I repair them quite often I have been wondering about what I can do to retrofit the power supplies in them to protect the somewhat irreplaceable analog and digital IC's that these machines are full of. Even synths with well built supplies such as the Yamaha CS-80 are so valuable now and so full of unavailable chips I think that it is time to begin routinely upgrading their power supplies with some kind of surge and over voltage devices. I have seen a few failures lately, for example an E-mu Drumulator where the failure of a diode that was on the earth pin of a 5V regulator to lift the output a little allowed the regulator to pass full regulator input voltage to the 5V rail of the machine. I would be interested in anyone here's thought on the topic, or information on any new clamping or shunting devices for low voltage rails that may be available other than the old favourites (like using a zener diode and an SCR) which would be easier to fit.

 

[JohnRoberts]

The simplest is a fuse in series and Zener diode shunt a volt or two higher than regulated voltage. Of course an active adjustable shunt clamp could be designed, but that introduces another bit of complexity to perhaps fail.

JR

 

[LeeYoo]

I think Transzorbs or TVS diodes are made for this.
Zeners on steroids.
Select slightly above "maximum standoff voltage".

 

[volker]

Look for crowbar circuits.

 

[Steve Jones]

Awesome, thanks guys!

 

[JohnRoberts]

Another protection scheme is a polyfuse or PTC device. These are placed in series with the load (like a fuse) and when they get hot they turn high impedance. This may be too slow or too late for your application. The nice thing about PTC fuses is that they conduct again after they cool down.

JR