Electronics > Beginners

Tapping into the ~400vdc from an active pfc circuit

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SeanB:
With active PFC the input goes through a bridge rectifier, so the negative of the capacitor after the active PFC stage is going to float at roughly half mains voltage ( actually will swing from negative mains peak to 0V per cycle, average voltage is half the mains voltage) and the positive side will be 400V above that.

Your PC power supply is designed to have the secondary voltages referenced to mains ground, and there are multiple connections to there on the board as well. disconnecting mains earth creates a lethal voltage on the case, the wires carrying the 5V and 12V supplies, and is generally not a good idea. Will create a lot of RF interference to every thing in the vicinity as well, and might cause improper operation of poorly shielded equipment. Again, the lethal voltages on a big case thing out in the open right by you is a bad idea.

Better for your design is to get a separate smaller SMPS module ( two wire supply type, so the output is floating and not mains ground referenced) and use that as your power supply for the circuit. That way you can program it with the main input PFC side disconnected and non powered.

justinjja:
Hum, that doesn't quite make sense to me.

If cutting the ground created a lethal voltage on the case,
then the ground would have to normally be carrying some current.
and if the ground was carrying current, it would trip when plugged into a gfi.

grounding the case is to protect from something like a lose wire touching the case,
and then the case becomes lethal.

EDIT:
You can find anything on the internet:
https://www.rcgroups.com/forums/showthread.php?1170241-Using-two-power-supplies-for-higher-voltage-capacity-chargers-safety-issues

Read through a few pages, nobody died lol.

ogden:

--- Quote from: justinjja on September 15, 2018, 05:29:18 am ---Hum, that doesn't quite make sense to me.

--- End quote ---

Then drop your "project" as it is said multiple times here already. You have my vote as well.

If you are not able to get that you must not connect DC negative rail of full-bridge rectified AC mains to ground (where neutral is tied to), then this project is not for you. You can [safely for yourself] try, but you will blow fuses or rectifier diodes or PFC circuit.

justinjja:
That isn't what I said at all.
That is the reason I originally was going to cut the ground cable.

drussell:

--- Quote from: justinjja on September 15, 2018, 05:55:15 am ---That is the reason I originally was going to cut the ground cable.
--- End quote ---

Yeah, but this is where things go off the rails.  That's not how you do something like this, even for experimentation.  You don't lift the chassis from ground, you modify the rest of your apparatus' circuitry to work properly while still maintaining safety and sanity.

Before you even think about running something like this directly off the mains, you need to build your required style of circuitry in a low voltage version and thoroughly test everything.  Then, once you have everything working properly in a low-voltage version, you can move on to making a safe version of the motor power side and test it with your high voltage motor.

If everything were isolated properly with a transformer, you could theoretically modify the secondary, low voltage side with your control circuitry to run at the same potential as the motor side, provided that everything to the control side was properly isolated with transformers and optocouplers with proper isolation and insulation.  I'm not sure you understand the intricacies of doing that safely yet.

You could also just as easily design it with them them both isolated from each other (essentially as your computer PSU is originally, in stock form), again, using proper optocouplers and transformers, etc. so that it would always be properly isolated, regardless of what the potential at the actual motor is relative to your control circuitry.  Then you still have danger in your motor area, potentially more dangerous than regular mains connected stuff since we're talking high current 400V, but can be done, provided you take all appropriate precautions.  Again, I'm not sure you appreciate the danger involved with high energy, high voltage circuits in a configuration like this.  This could easily be far more dangerous than the 120 volts that comes out of the wall.

One way or another, you need proper isolation!  Removing the ground and running your control circuitry at 400 volts without isolation is not the way...  That is lunacy.  In some ways, that is even worse than the old hot-chassis TVs and radios.  :)

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