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I have this circuit built and it keeps blowing up my bridge rectifier when I power it.

Is it because I have connected the chassis/system ground to earth ground? I cannot figure out what is happening and every time it blows a fuse and destroys the rectifier.

click for schematic

enter image description here enter image description here

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  • \$\begingroup\$ I can't read that schematic, any chance you could crop a higher resolution pic? \$\endgroup\$ – RedGrittyBrick Jul 16 '13 at 14:05
  • \$\begingroup\$ Sure, thanks for letting me know. Sorry about that. \$\endgroup\$ – EwokNightmares Jul 16 '13 at 14:06
  • \$\begingroup\$ Sorry, I didn't mean to remove the schematic, I've edited the Q to add in some slices of your schematic at a size I can read. If you don't like what I've done, please use the revision history ("edited" link) and rollback to an earlier version if you prefer. \$\endgroup\$ – RedGrittyBrick Jul 16 '13 at 14:50
  • \$\begingroup\$ No, I like what you did, thanks! I just didn't know how to. \$\endgroup\$ – EwokNightmares Jul 16 '13 at 14:51
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Yes, you cannot connect either of the DC terminals of the bridge rectifier to the mains ground. Remember, one of the two "active" mains inputs (usually called "neutral") is also tied to mains ground. This means that when the other mains input ("line") goes negative, it is forcing unlimited current through one of the diodes in the bridge.

Both sides of the bridge rectifier output must be allowed to "float" with respect to ground.

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  • \$\begingroup\$ What you say makes sense. I had read about grounding for power supplies and I was under the impression that earth ground should be the same as chassis ground incase something inside shorts the chassis to a lethal voltage, and then also that chassis ground should be the same as system ground so that it can serve as an EMI shield, and from those two requirements, then it implies that system ground would be connected to earth ground at some point (they called it a star ground). What do I have wrong in what I just described? \$\endgroup\$ – EwokNightmares Jul 16 '13 at 15:07
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    \$\begingroup\$ "chassis ground should be the same as system ground so that it can serve as an EMI shield" is a context-dependent statement. Since your power supply is non-isolated, you cannot use this technique. Your "system ground" needs to be isolated from earth/chassis ground by an impedance that doesn't pass the power line frequency. Most people use something like a parallel combination of a 1Mohm resistor (to drain static charges) and a 1nF capacitor (to bypass RF signals to ground). \$\endgroup\$ – Dave Tweed Jul 16 '13 at 15:14
  • \$\begingroup\$ The unlimited current would also explain why my 150uH inductor in the schematic also burned up once where the enamel on the coils was burnt. It must have created a very large voltage across it. \$\endgroup\$ – EwokNightmares Jul 16 '13 at 15:34
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While it is not clear to me from the schematic, here is a likely scenario:

There is a non-isolated feed from the mains, going into the bridge (through whatever circuitry there may be). The negative line on the rectified side of the bridge is your circuit ground.

Connecting this circuit ground to earth ground brings the mains voltage into conflict with the bridge, since relative to circuit ground, the earth ground is a mains-voltage AC signal.

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Is it because I have connected the chassis/system ground to earth ground?

Yes. The 3rd pin on the power connector is mostly equivalent to the neutral line. The neutral line should be Earthed, but since it's done at some distance from the house wiring, and may have all sorts of faults, and since it carries the return current, and it's possible with some connectors to connect them backwards with neutral and live reversed, the 3rd pin is provided for things that need a safe Earth connection.

You could try powering this device from a GFCI as you are testing it. Instead of blowing your rectifier, you will probably just trip the GFCI until you get it right.

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