Protect modules from reversed power connections

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Muff Wiggler
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Protect modules from reversed power connections

Post by Muff Wiggler » Fri Nov 30, 2007 1:01 am

Loads of thanks to John Blacet for sharing this scheme on his tech page at http://www.blacet.com/tech.html
John Blacet wrote:Protecting Modules from Power Supply Problems....

Most synth modules use + and - supplies. Accidentally reversing these supplies, for instance when hooking up the power to the module, is generally fatal to most ICs and can pretty much spell the end of the board!
The failure of some board components can cause excessive current flow, sometimes blowing traces and components off the PCB!

What to do? The first thing of course is to use a power supply connector that is polarized so that it is difficult to improperly plug in the board power. Someone will eventually find a way to defeat this however, so we need a back up system. One common technique is to put a series diode in each power supply line. Reversing the power will result in negligible current flow. The downside of this is a significant voltage drop on the output, and no overcurrent protection.

Enter the PTC or Positive Temperature Coefficient resettable circuit protection device. These are a fairly recent invention, available from a number of companies, including Littlefuse. They are rated for a certain current, below which they function like a very low value resistor. Go above this rated current and the PTC starts to heat up, changing to a very high value resistor in a few seconds. Lower the current and the PTC cools off, returning to normal operation. No fuse to replace!

If we want to protect a module from reversed supplies or overcurrent conditions, the PTC sounds ideal. It is a bit slow however, so we need to both speed up it's response and protect our ICs at the same time. We can use a variation of the above diode trick and connect reverse bias diodes to ground on the + and - supply inputs downstream from the PTCs. The circuit diagram shows the idea.

Image

D1and D2 will be off in normal operation. If we place -15V at the +15V input, however, D1will basically short to ground, causing PS1 to see big time overcurrent and clamp it's little sphincter shut, the ICs on the board not even having time to get worried. Same thing happens at the -15V input if we reverse the voltage.

If a component on the board fails (or a kit builder installs an electrolytic bypass capacitor the wrong way), PS1 and/or PS2 will simply heat up in response to the overcurrent, limit the current to negligible levels and prevent smoke and flames. Perhaps the only downside of this is that the usual smoking resistor or blown up cap will not be there for the troubleshooter....

There you have it; works like a charm, costs a couple bucks. We use it . Why doesn't everyone?

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Kwote
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Post by Kwote » Fri Nov 30, 2007 1:13 am

yeah. i remember reading that when i was about to buy my first modules and i was paranoid that the blacet PS500 might not be setup with it already. but i guess it is. must be mostly advice for DIY'ers
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Muff Wiggler
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Post by Muff Wiggler » Fri Nov 30, 2007 1:25 am

it's not on the PS500, it's on the power input of each module

so even if you use a different PSU, the Blacet modules are still protected.

I believe it is meant as advice for both DIY'ers and other modular manufacturers - I believe that MOTM puts this on their boards as well, but I don't think anyone else does. Stay paranoid ;)

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Kwote
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Post by Kwote » Fri Nov 30, 2007 10:04 am

yeah. i totally didn't understand that shit back then. but i reread it. makes more sense to me now. i find myself having to read things several times coupled with actual experience before i actually understand it. hope that's not just me. haha.
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synthetic
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Post by synthetic » Fri Nov 30, 2007 1:04 pm

Plan B sells a power distribution board with this built-in for Eurorack systems:

http://www.ear-group.net/model_1.html

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