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Using ferrites on thermocouples

Discussion in 'Electronic Basics' started by jacksan, Jan 16, 2007.

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  1. jacksan

    jacksan Guest

    I have incoming 8 type T thermocouples (they share a common constantan leg)
    and I wish to filter EMI noise coming from a nearby large variable-frequency
    drive. Omega sells TC connectors for this purpose with built in ferrite
    cores (90 ohm @100Mhz), they also sell some separate cores (155 ohm
    @100Mhz). Google-ing turned up a couple of users who run the TC through a
    figure 8 dual-hole core, presumably for better "matching". I'm guessing
    they ran one lead through one hole (simply pass it through, not multiple
    windings) and the second through the other hole...does this sound correct?

    What technique is better? (1) run each leg through a separate core. (2) Run
    both leads through one core. (3) Use a 2-hole core.
    Since all the TCs share a common "leg" options #2 and #3 are possible but
    will not be too pretty (the common leg will have 8 ferrites on it).

    Other questions on ferrites:
    Is bigger (ohms) better? Note that the measured tempearture is relatively
    constant (changes 5F in one minute).
    Should the core be relativley snug around the wire(s)?

    Thanks in advance
  2. John  Larkin

    John Larkin Guest

    I've used both fig-8 ferrites (one wire through each hole) and
    surface-mount beads in thermocouple inputs, and both work well for
    high-frequency rejection. A lot of low-offset opamps make really good
    RF detectors, and a ferrite can improve that situation by 10:1 or
    more. But ferrites start to be useful in the 10's of MHz, and the
    noise from a VFD is likely to be mostly lower-frequency stuff, where
    the ferrite will be pretty much a dead short. Maybe capacitance to
    ground would help more, or better a ferrite followed by capacitance.
    Tc's are slow, low impedance gadgets, so a lot of capacitance -
    microfarads range - wouldn't slow down temperature measurement much.

  3. jacksan

    jacksan Guest

    Depending on the type of VFD it looks like there can also be some relatively
    high frequency components:

    I'll expirement (relying on the empirical since I can't do the
    calculations!!!) and test some big low-frequency Stewards ferrites along
    with some .1, 1.0 and 2.2uf ceramic caps.

  4. jacksan

    jacksan Guest

    Do ferrites have to fit snug on the wire?

    I have 22AWG wire and the ferrite holes are .2" dia.
    Is this OK?

  5. Sure.
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