Image Map Image Map
Page 1 of 2 12 LastLast
Results 1 to 10 of 11

Thread: PSU problems

  1. #1
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default PSU problems

    This might be wishful thinking but,

    I have the PSU and card frame from an Intel iSBC 635. It powers up fine and the CPU card starts up and gives a green light. All voltages are nominal with reference to 0V, but

    There is a very large AC voltage between every DC supply and earth. First noticed when I was checking the Tx pin of the console serial connection and found a 50Hz AC signal of >100V instead of the 5v DC I expected.

    The output is obviously floating and my Oscilloscope is high impedance referenced to earth, but if I put my Fluke on it, I get 47V RMS between the +5 V connector and earth. My knowledge of PSUs is ok, but I have never seen such a large difference and not really happy to plug the RS-232 connector into my PC ! and normally I would have circuit diagrams and specs.

    The PSU is an Xentec 980298 but I can't find a circuit diagram anywhere. The Intel manual "Intel iSBC 635 Power Supply Hardware Reference Manual, Order No. 9800298" does not seem to be on the net, anyone have a copy ? or ideas.
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

  2. #2
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default

    Hum

    Looks like there is no connection between 0V and earth and the supply is truly isolated. Being industrial, that's maybe not totally surprising.

    Think I need to jumper between 0V and earth.
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

  3. #3

    Default

    It is not necessarily needed. It depends on what you connect to it and what the impedance of the signal is. You might try shunting the 0 V rail, to ground, with a capacitor of say 1 or 10 nF and then measure the voltage.
    Typically digital stuff should have a real ground some place. If you are connecting to something on the same circuit, connecting to local ground is fine. If it is connected to a long wire to some remote location, only one end should be grounded.
    Dwight

  4. #4
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default

    Thanks Dwight, was just jumpy after seeing almost 300V peak to peak on the Tx line of my serial port !

    Earth free stuff always caused me headaches at work, then other parts of the station have two different earths. I think I will connect 0V to ground as everything will be in the same room but via a cap. Electricity is strange stuff. Remember we had a scaffold pole melt once just because it was adjacent to the generator transformer and managed to pickup some real voltage and power.
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

  5. #5

    Default

    Audio stuff is most sensitive to ground loops. Several artist have been killed because they didn't follow the single point ground rules and had too many ground pins cut on their equipment. You always want one ground reference someplace.
    Grounding issues can blow up stuff in areas with a lot of lightning. The ground shifts are usually not that great but have a lot of current. Two grounds being off by 50Volts for a few micro seconds with 100's of amps can blow out stuff that had grounds connected at both ends.
    Dwight

  6. #6
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default

    Lightning !

    We had a strike at our cooling water pump house and we lost random bits of kit throughout the station, spread over a couple of miles.
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

  7. #7

    Default

    When this un-earthed apparatus is connected permanently to another where the zero volt lines are earthed all will be well as the un-earthed device's 0v lines will assume ground potential.

    However, a very big problem remains.

    In the case where there is no connection to another earthed apparatus the 0v, as you have found out, assumes a voltage significantly higher than earth, but not only that the dynamic internal source resistance of that voltage depends on the capacitances that are present. You could find out roughly that value by connecting a resistor between the 0v line and ground, and see when the voltage drops to half. But, the energy stored in the capacitance can be enough to damage the serial transceiver IC's at either end of the RS232 interface if you hot plug your two devices at the moment the plugs connect.

    I'd recommend connecting the 0V line to ground to avoid this possible damage and if you were worried about two devices connected to different earths, connect the 0V line to earth with a robust (at least 2W rated) 10R resistor, that limits the inter earth currents, a technique popular in the audio field to reduce the effects of earth loops with amplifiers connected to different mains power earths. That resistor would also be shorted out say with a shielded RS232 cable where the metal shells at each end are connected together, and the two earths, if they were different, get forced to the same potential that way.
    Last edited by Hugo Holden; July 17th, 2020 at 02:43 PM.

  8. #8
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default

    Yep, I have brought the 0v line to earth and all is good (except the processor isn't booting to the Intel monitor, so I'm not getting anything on the Tx)

    Was surprised that it wasn't like that in the first place which caught me out, but typing the first post made me think and check.

    We dont tend to have many problems with mains earth differentials in the UK, I think we are too small and too wet. I have even seen someone dismantle a live neutral bar at a 3.3kV star point and live due to the balanced load on the transformer.
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

  9. #9

    Default

    In AU & NZ we have the MEN (multiple earth neutral system) where the dwelling's earth stake is connected to neutral at the power box. If the street neutral connection fails, the earth wiring potential at the GPO's can go high above ground, if the earth stake has a high resistance as they often have in AU, as very dry soil etc. A significant shock hazard.

    I designed a device (which was published in Silicon Chip magazine) which uses an independent earth stake as a reference and monitors the dwelling's earth and neutral voltage, if that goes up a latch is set and an alarm sounded.

    One thing that caught out some plumbers is if the neutral connection is dodgy and the earth stake poor, and the earth is connected to metal piping as it often can be in the dwelling, there can be substantial currents in the water pipe leading to the street.Then if that pipe gets cut, say to fit a new water meter, a large potential difference appears across the two cut pipe ends.

  10. #10
    Join Date
    May 2018
    Location
    Lancashire
    Posts
    494
    Blog Entries
    13

    Default

    I think we used to call connecting everything including the pipes equipotential bonding (its been 40 years since I did IEEE regs though), but we tend to have plastic pipes these days and 'earth' everything by connecting the earth to the neutral of the three phase supply rather than having earth rods in houses.

    However, we have a large neutral earthing resistor (as in a physically big tank of liquid with just enough salt to give the required resistance) between this neutral star point of the generator transformer and earth, so if we get a phase fault, the potential of the neutral can rise quite significantly above ground, but we then have protection that will trip out the system as this voltage rises so it should be ok, fingers crossed
    Current fleet
    TRS80 Model 4 - BBC B - Tatung Einstein - PCW9512 - PET 3032 - C64 - ZX81 - Spectrum 48K - Amiga A500 - VAX 3100,3300,4000VLC & 4000 Model 96 - Apple II europlus - Apple iMAC G3. Sharp MZ-80K. - DEC Micro PDP 11/73 - IBM 5160 XT - Multibus 286/10.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •