Using ferrites on thermocouples

[jacksan]

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
jj

 

[John Larkin]

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
jj

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.

John

 

[jacksan]

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.

John

Depending on the type of VFD it looks like there can also be some relatively
high frequency components: http://www.belden.com/pdfs/Techpprs/capwmtp.htm

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.

thx

 

[jacksan]

Do ferrites have to fit snug on the wire?

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

jj

 

[Homer J Simpson]

Do ferrites have to fit snug on the wire?
No.

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

Sure.