Oscilloscope Question for DC Ground Noise

Discussion in 'General Electronics Chat' started by EVBuilder1, Jul 18, 2014.

  1. EVBuilder1

    Thread Starter New Member

    Jul 18, 2014
    I appreciate your help!

    I am working on an Electric Vehicle that seems to have an Open in a ground circuit. The keyswitch is sensed to ground and seems to be opening intermittently when it is hot, and also when the motor controller goes into regeneration mode. Thus, we could be getting an induction "pulse" when we go from full power to full regen. This could be a swing of over 1200 amps DC.

    I'm trying to see if noise on the ground circuit could be causing an issue, or if we have an actual open. I have a 2 probe Oscilloscope and I don't understand where to place my probes. Normally I would probe a signal line and reference to ground, but in this case that seems wrong.

    Thank you for your assistance, I am a newbie to Scopes and mine is very cheap. I am an ME, but self taught in EE, so while I can do DC circuits pretty well, much of the EE world is unfamiliar so please be patient and give lots of details.

    Owon VDS2062 60MHz PC-based Oscilloscope, 100MHz Bandwidth, 1GSa/s Sample Rate, 10M Record Length
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Hi EV, welcome to the forums !!!

    When you say "hot" does that mean as in "warm" or as in "on"?

    Do you happen to have a schematic of the vehical?

    Do you connect your control circuit together then attach to the power source at one point? I'm wondering about series resistance giving you spikes. With 1200 amps it doesn't take much resistance to give you a big spike.

    Some of the power supply guru's here keep a battery powered scope on hand just to keep it 100% isolated so it can only respond to differential signals on it's probe input. Honestly, after doing a bunch of work on switching power supplies it seems transient noise is some sort of black art: I've never quite understood why I see some things on my oscilloscope.

    But a switch seeming to be open should be something that can be found.
  3. EVBuilder1

    Thread Starter New Member

    Jul 18, 2014
    When I say hot, I do mean warm. It usually takes an hour or more of working the vehicle before the first occurrence of the open.

    I can't share the schematic of the vehicle, but I will share a schematic of the circuit that is opening. Let me take some time to draw up a circuit that will be helpful.

    In the meantime, can you help me with the probe placement? Right now I get a reading on my scope that is 0V with noise + - 5V in a 12V system, but again, I think I'm placing my probe incorrectly, so just the basic understanding of where to place the probe would be very helpful.

    To be more clear, the hi amp circuit @ 110V dc does supply the DC/DC converters for my 12V system, but is isolated from the 12V system. The DC/DC I'm using is at this link, I actually use 2 in parallel to get the power I need.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    It sounds like your problem may be related to how the power and ground is distributed within the car. The distribution pattern should be in the shape of a star, with the battery being the center of the star. Each device should have its own power and ground "leg" leading back to the battery. Problems can occur when a device on one leg shares a ground with a different device on a different leg.
    When you go through a power conversion, like your 108V to 13.5 volts converter, the 13.5V output now becomes the center of its own star. Two different power systems should never share grounds. Ground systems should only be connected together at one point.

  5. crutschow


    Mar 14, 2008
    An oscilloscope measures voltage between the probe and the ground clip, so you place the ground clip and probe to measure voltage differences between those points. As to where to connect them in you circuit, that's hard to say without seeing the circuit and knowing more details about the problem (which is not completely clear from your description).
  6. EVBuilder1

    Thread Starter New Member

    Jul 18, 2014
  7. to3metalcan


    Jul 20, 2014
    I would guess your ground clip should go at the common post where the other grounds tie together...then probe from there to the ground terminals of the individual circuits in DC mode...if it goes open, you'll just see a 12-volt level. If it's noisy, you'll see peaks dropping across the resistance of the wire.