Isolating ground with Diode

Discussion in 'The Projects Forum' started by alyeomans, Jul 21, 2013.

  1. alyeomans

    Thread Starter Member

    Sep 13, 2010
    29
    2
    I'm developing a power supply circuit which has high and low voltage and a common ground. I have isolation of the switching control line via an optoisolator and a separate low voltage power supply. See attached circuit.

    The remaining linking factor is the common ground - I have placed a diode to block any current flowing into the low voltage section.

    Alternatively I could remove the linking ground connection all together.

    Any thoughts on making this safer would be appreciated.
    Al
     
    • cct.png
      cct.png
      File size:
      18.2 KB
      Views:
      168
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,564
    2,379
    All the symbols used are for Earth Ground, are they supposed to be?
    Max.
     
  3. alyeomans

    Thread Starter Member

    Sep 13, 2010
    29
    2
    Quite so MaxHeadRoom, the earth symbol is actually the DC common (0v) and is different to the power plug earth - an important safety point.

    I don't see any reason why both DC 0v's need to be connected and to maintain isolation they should be separate - thus negating the need for a barrier diode between them.

    Cheers
    Al
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    If you intend To have the "switching control" operate the MOSFET Then gnd and digital ground MUST be directly connected.

    You would need strong reason to connect the power line earth to ground as you indicate.
     
  5. alyeomans

    Thread Starter Member

    Sep 13, 2010
    29
    2
    Quite so ErnieM, the optoisolator (not shown in the snippet of the schematic) takes care of the MOSFET.

    Thanks for your comments.
    Al
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    Not quite so fast there...

    So basically, you've snipped away the relevant portions of the circuit you wish us to comment upon?

    I have strong doubts this will work, and that is taking into considerations I don't have the slightest idea what you're doing, though I do believe we share that idea in some strange way.

    I'm off to the junction of the Caribbean and the Atlantic for the remainder of the week... when I return will I see celebratory confetti or smoke and ash?
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Is the AC into Header 1 supplied by a transformer ?

    Why not show all of the circuit details.
     
  8. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    You better not look too close, you may see both. :rolleyes:
     
  9. alyeomans

    Thread Starter Member

    Sep 13, 2010
    29
    2
    Thank for your replies. Attached is the circuit with the optoisolator.

    ErnieM: The part of the circuit for particular comment was using a barrier diode for the common DC ground. I would welcome further comment on the optoisolation and with hands and fingers away I do hope for no smoke or smoking confetti.

    I have planned this circuit to drive a 3kW resistive heating element. The control of the mosfet is through another board which uses a PIC18F4620 and one of the PWM outputs. The full circuit also has a temperature bridge (for feedback loop), buttons, lcd screen and serial comms (TX&RX).

    tubeguy: No transformers needed here as i'm not isolating from the mains supply, just a large amount of noisy switching. As per above, there a lot to the full circuit but once complete without smoking i'll happily share.

    I intend to add an inductor to help with switching spikes (if I need to).

    A
     
Loading...