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Isolating Oscilloscope Channels

Discussion in 'Electronic Design' started by EdV, Apr 18, 2007.

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  1. EdV

    EdV Guest

    I have a dedicated production test system that uses a Tektronix
    oscilloscope which does not have isolated channels. I have to measure
    the "lag time" between the falling edge of one IGBT H bridge drive leg
    and the rising edge of another. It is specified to be 3 to 5
    microseconds. Of course these signals are referenced to to different

    I thought a simple opto isolator circuit would do the trick but as I
    look at parts I get the feeling I am going to stack up enough
    propagation delay that measuring 3 to 5 microseconds is not going to
    have the needed preciscion.

    Maybe drive a resistor and then use diff amps?

    Your thoughts as always are welcome.

    Ed V.
  2. GregS

    GregS Guest

    Nice to have the good old scopes with differential plug-ins and

    10 m probes. That would work.

    Are any of the grounds, grounded?

  3. EdV

    EdV Guest

    No the "grounds" are the emitters of the upper leg and lower leg of
    the H Bridge. Maybe I should just arbitrarily assign "ground" the
    upper emitter which is also the lower collector invert one of the
    signals and tweak the scope vertical. I am still waitng for my test
    fixture to get built so I can snoop these signals.

    Thanks for helping me start thinking again.

    Ed V.
  4. John  Larkin

    John Larkin Guest

    Get one of the Tek isolated scopes? They're fabulous.

    If it's just an edge, transformer couple it.

  5. Paul Mathews

    Paul Mathews Guest

    See Tektronix P5200 series of HV diff probes and similar products.
    That's the right way to do it.
    Paul Mathews
  6. Joerg

    Joerg Guest

    As John mentioned transformer coupling can achieve that. Mini-Circuits
    has lots of those. Possibly you could also get away with LAN
    transformers from an old Ethernet card. But make sure nobody needs that
    card anymore ;-)
  7. Guest

    Wont work I.M.O.

    The current through the primary - secondary capacitance will screw your
    measurement over AND if the (on the day they used the right goop for
    encapsulating) 500 V (or so) isolation level in the cheap(!) Chinese LAN trafo
    from "Electronics Factory No 5, Huawei" ever goes away then the H bridge -
    off-mains since the O.P. needs isolation - might then make lots of colours with
    your scope (before the muffled Boom from the sub-basement fuses cuts the show

    The O.P. should buy the proper differential / isolated probes for the voltage
    level. They will cost money. But so will the Health and Safety incident, the
    trip through court, and compensation payments should some worker get injured.
    The word "production" ups the stakes quite a bit.

    The Chinese will print whatever safety labels are requested from the user of the
    part - it does not follow that any of that actually applies; as f.ex. HP found
    out the hard way with a run of power supply "Y" capacitors ...
  8. Tim Williams

    Tim Williams Guest

    - Turn off power (to the bridge) and measure directly
    - Disconnect gate drives from bridge and measure at ground
    - Couple with C, R and transformer (you're looking for the edges, right?) --
    with care you can also measure the edge height
    - Watch carefully at high voltage (15V of gate drive does kind of get lost
    in 600V or whatever though)
    - Build your own differential probe ;-)

  9. EdV

    EdV Guest

    This link looked interesting:

    I am going to experiment with these circuits. Unfortunately the
    suggested transformers are not meant to withstand some of the voltages
    that might appear
    if something went wrong. :-(
    I think spending $1K each for two good active diff probes from
    Tektronix is going to make the most sense.
  10. Hawker

    Hawker Guest

    I do a pile of consulting for a company that often needs the same thing.
    They have some fancy large Tektronix 8 channel isolator they use.
    Me I us an old THS series battery powered Tektronix scope for this.
    It has two channels, each is isolated from ground and from each other.
    It's not fancy but it would pretty good for that purpose.
    They run about $800 used and I think $2500-$3500 new depending on sample

  11. Joerg

    Joerg Guest

    Just don't use audio mic transformers for microsecond stuff. Also, their
    breakdown voltage isn't very high, or there is no rating for that at all
    on some.

    You can always roll your own or buy from a reliable source where there
    are breakdown specs. If you roll your own you could get some Rubadue wire:

    Certainly makes sense, especially if you have to do this often or if you
    must conduct documented test. Saves a lot of ECO writing for test jigs
    which probably costs more than $1k in EE hours.
  12. EdV

    EdV Guest

    I am stuck with the oscilloscope that is in the tester because it used
    to test many different baord level assemblies. Kind of remeber asking
    "why don't we have isolated channels"? I believe it was cost. Oh well.
  13. EdV

    EdV Guest

    A shame about the microsecond thing. The signal should always be 20
    Vpp now that I review the circuit, when it is working correctly. I
    could protect the transformer with clamp diodes but not much point if
    the timing would not be preserved.

    Ed V.
  14. Joerg

    Joerg Guest

    You can get fast transformers at places like Mini-Circuits. I don't know
    how high LAN transformers go and where they saturate but a datasheet of
    one should tell. Reason I don't know is because I always keep a pound of
    #43 ferrites in my lab so I can quickly wind one up if in need. Faster
    than filling out a Digikey order ;-)
  15. Tim Williams

    Tim Williams Guest

    Heh yep. I've had excellent response from even really crappy transformer
    designs. 100 turns of 30AWG on a high-permeability core (toroid or
    etcetera) will get you there. Primary then secondary, don't really even
    need to interleave them. Use *two* layers of masking tape between windings
    if insulation spec is really high. ;-) My experience gives bandwidth into
    the 10s of MHz (risetime comparable to the driving circuit: 100-200ns).

    I would use a series capacitor to block DC and a (parallel) resistor to
    dampen oscillations. With small C, you can get away with pretty wimpy
    windings, since the pulse ends up really narrow. With large C (and L!), you
    can see the whole square wave with maybe just a little droop along the way.

  16. Joerg

    Joerg Guest

    Looks like Ed is only interested in the transition times. My staple for
    that looks like this: About 6" PTFE wire, 6" of a CAT-5 strand or
    whatever, twist the two together in 1/2" turns or so, wrap four times
    through a 1/2" OD toroid of #43 material. I get sub-10nsec transitions
    through those. The PTFE insulation is only needed where there is a lot
    of bzzzt between primary and secondary.
  17. Joerg

    Joerg Guest

    Oops. I've done it a few hundred times by now. Or maybe thousands. Do I
    have to go to confession for that now?

    Well, you have to do it right. Transformer coupling can be very helpful
    especially in production because space it usually cramped and it doesn't
    exactly help to cram two or more diff-probes in there. That's one reason
    why there is PTFE cable available where they test and certify every foot
    of it for dielectric strength.

    Anyhow, that's also how we design patient interfaces. Lots of them out
    there in the field. So if you really don't trust such transformers it
    would be best to eat healthy, exercise, and absolutely not develop a
    coronary artery problem because chances are they'll use one of those
    machines on you. But rest assured, the ones I designed are all
    defibrillator proof and tested for that. I wouldn't do them any other way.

    Yep, that sure can happen. Know thy sources ;-)
  18. Hawker

    Hawker Guest

    I have often told clients "if you want the work done you need the
    correct tools" if I can't get the correct tools I can't get the work
    done. If they need this tested then they need to get the correct tool
    for the job no two ways around it.

    It amazes me how often a client is willing to pay me $5,000 - $25,000
    for consulting work, but won't spring an extra $500-$1000 for the
    specialized tools I need for the job. I understand that basic tools are
    my responsibility to supply (a standard scope is a basic tool) but since
    every job is unique there are often a few special tools required (esp
    ICE tools for whatever processor I am using or HV tools for HV products).

  19. Joerg

    Joerg Guest

    From a business point of view this is often the reason: Our consulting
    services are billed just like materials. They get purchased and then
    consumed. A tool becomes inventory and must be entered into the
    amortization tables. Depending on cost and type it has to remain there
    for several years. Inventory Dollars is a number that needs to be as low
    as possible so the CFO can leave the next board meeting a happy camper.
    Payroll Dollars is the other number that needs to be low and that's why
    US companies like consultants. Which is a good thing :)))

    This is also one reason why businesses often spend more money on long
    term rentals than we would, beyond the point where an outright purchase
    would have been a better deal.
  20. EdV

    EdV Guest

    Thanks! I'll try this one.

    Ed V.
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