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EMI on scope probe

Discussion in 'Electronic Design' started by Roland Andrag, Nov 21, 2004.

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  1. Hi everyone,

    I have a circuit which contains a ULN2803, which is used to switch a 12 VDC
    relay (standard 8 pin, around 100 mA). The relay, in turn, switches a
    contactor at 230 VAC (a couple of mA at 230 VAC). The resulting arcing over
    the contacts of the relay is generating a lot of EMI, which is at the moment
    wreaking havoc with the rest of my circuit.

    In trying to solve the problem, my first problem is that I can't accurately
    measure the noise anywhere in my circuit, since my scope probe picks up the
    noise (up to 5 V peak-peak) even when the probe tip is clipped directly onto
    its own ground connection, and to nothing else.

    The scope is a Tektronic TDS1002, with its standard probe. If I disconnect
    the probe at the co-ax connector, the noise dissappears, so I'm sure that
    the nois is not entering via the scope power supply from the AC line.

    Any help very much appreciated.


  2. Ian Stirling

    Ian Stirling Guest

    Arcing of relay, or contactor?
    I'd connect the device to an isolation transformer, (or use a battery
    operated scope (disconnecting the earth of the scope is an option used by
    some, but is a really bad idea safety-wise)) to avoid current flowing in
    the shield, which it otherwise will want to.

    How is the contactor wired, on a PCB, or ...
    Ideally the contactor switch wires should be twisted together, to
    reduce any current loops.

    Cling-film wrapped round the circuit, with a few wraps of foil over the top
    can tell you if shielding might help.
  3. Hi Ian, thanks for the quick reply.

    Arcing on the relay contacts - the contactor contacts are not connected at
    the moment.

    I've had the scope on a UPS (which was unplugged completely to isolate it),
    still have the same problem at the scope side - fairly sure that the noise
    is getting induced in the probe, since the noise becomes much smaller as I
    move the probe further from the relay. I'll get hold an isolation
    transformr tomorrow - it is definately a lot easier than usning a UPS.

    I can't twist the 230 V live and neutral that switch the contactor together,
    since the live signal passes through a couple of relays - I have kept fairly
    close together though.


  4. Ian Stirling

    Ian Stirling Guest

    Does the problem go away if you disconnect the relay contacts?
    You do have snubber components across the contactor coil?
  5. John Smith

    John Smith Guest

    Hi, Roland -

    Contactor coils are notorious for generating noise. I can't remember the
    exact values we used, but something like .01 uF or .1 uF in series with 47
    Ohms around the contactor coil may help. Vary the values and look for
    minimum noise. You might get better values for this snubber from a search on
    the Web or from the contactor manufacturer's Web site.

    All our new circuit designs were placed next to a contactor (sans snubber)
    which was alternately energized and deenergized and the response of the
    circuit was examined. The line cord to the contactor was coiled up and laid
    on the circuit. Sometimes we saw strange stuff, like an opamp scooting off
    to the rail. We then added snubbers, filters, or bypasses to the circuit
    until it was clean. The boards were usually installed in DC or AC motor
    speed controllers operating thyristors on the 460VAC, 3-phase mains. So, the
    boards were required to be as insensitive to noise as possible.

    Here is a site by Douglas Smith, consultant, with lots of helpful probing
    ideas and with information on EMI... But, it will take
    some time to read through all his stuff.

    I understand the problem you're having. Good luck.

  6. John Smith

    John Smith Guest

    Here is a mention of the shorted scope probe problem you have. He gives no
    solution in this brief, however.

  7. mike

    mike Guest

    What's the characteristic of the noise?
    I have the same problem on a TDS540. If I do the FFT, I can clearly
    distinguish all the radio and TV stations in the area. Don't know if
    the TDS1002 has fft, but if it has a channel output, you can run that
    into a spectrum analzyer. Sometimes it's much easier to deduce the
    problem in the frequency domain.

    Metrology in a high noise environment is always a problem.
    Lose the ground lead and use a probe socket grounded at the source.
    You'll have to use a probe with the ground all the way to the tip.

    Probe cable shields have been known to break.


    Return address is VALID.
    500MHz Tek DSOscilloscope TDS540 $2200
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
  8. Roland

    Chasing inductive kick disruption with a CRO is not terribly productive.

    The high, fast turn off transient voltage is getting significant current to
    flow in parts of your circut which are only lightly coupled by capacitance.
    Some of this current is flowing to ground, which is why your CRO shows a
    deflection with shorted probe.

    Even slow signals against ground produce a deflection with shorted probe,
    because current is passing through the CRO on the way to ground, producing a
    voltage drop. In particular, the probe lead has resistance which produces a
    voltage drop.

    A small multimeter type CRO is better, but still not the most useful tool So
    put the CRO away and work on suppression techniques. Generally, I put on
    the "usual band-aids" then mop up any remaining troubles by thinking about
    the symptoms and trying various ferrite and R - L - C parts tacked on.

    To limit the inductive kick, you give the current somewhere to flow which
    makes a nice contained circuit close to the coil. This involves snubbers for
    AC or DC powered coils and can mean a diode across the coil for DC powered

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