Thermocouple surge protection

J

JTM

Guest
We have some long runs of Type-T thermocouple and would like to protect them
from nearby lightning strikes with a TVS or similar.

If I simply connect a bidirectional TVS between each lead of the TC and
ground will that affect the readings? Is there a better, but still compact
and inexpensive, solution for TCs? Modifying the reading equipment itself
is not an option.
 
"Soeren" <Look@iNO-SPAMt.dk.invalid> wrote in message
news:Xns94FED671D9187o8oLOOKatHOMEo8o@212.242.40.196...
Hi,
I would guess "no", since it will be high impedance while not
"activated", and the TC is very low impedance, but I fail to see the
purpose ?
Lightning goes to earth by the lowest impedance path and unless you
actually earth one of the TC-leads, the TVS will do nothing and the
lightning will possibly go to earth through your measuring device,
killing it very very very dead in the process.
Click to expand...
Nearby strikes are taking us down so there must be an 'easy' route through
the equipment. I need to route the surge somewhere besides the equipment so
am considering a TVS or such.

I would try an earthed wire mesh around it, like the shield in a coax
cable (isolated from the TC leads of course).
You could test it with an ESD-generator (or an empty lighter with piezo
ignition).
Regards,
Soeren
Click to expand...
The unshielded cable is in place and, like the reading equipment, cannot be
modified.
thanks
 
Hi,


"JTM" <jtm@yahoo.com> wrote in
news:NH2wc.14$w65.12@fe37.usenetserver.com:

We have some long runs of Type-T thermocouple and would like to
protect them from nearby lightning strikes with a TVS or similar.

If I simply connect a bidirectional TVS between each lead of the TC
and ground will that affect the readings?
Click to expand...
I would guess "no", since it will be high impedance while not
"activated", and the TC is very low impedance, but I fail to see the
purpose ?
Lightning goes to earth by the lowest impedance path and unless you
actually earth one of the TC-leads, the TVS will do nothing and the
lightning will possibly go to earth through your measuring device,
killing it very very very dead in the process.


Is there a better, but
still compact and inexpensive, solution for TCs? Modifying the
reading equipment itself is not an option.
Click to expand...
I would try an earthed wire mesh around it, like the shield in a coax
cable (isolated from the TC leads of course).

You could test it with an ESD-generator (or an empty lighter with piezo
ignition).


--
Regards,
Soeren

* If it puzzles you dear... Reverse engineer *
New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
 
Hi,


"JTM" <jtm@yahoo.com> wrote in
news:j14wc.18$w65.9@fe37.usenetserver.com:


Nearby strikes are taking us down so there must be an 'easy' route
through the equipment. I need to route the surge somewhere besides
the equipment so am considering a TVS or such.

[Snip]

The unshielded cable is in place and, like the reading equipment,
cannot be modified.
Click to expand...
Bummer !

Well, it might be worth trying a TVS from each lead to earth (a Y-
coupling) like this.
____________
_____________SS______________| |
/ | | |
* TC | | instrument |
\_____________SS______________| |
. | |____________|
| |
[TVS] [TVS]
| |
+--+--+
_|_
/// Earth

(View in notepad or similar)


--
Regards,
Soeren

* If it puzzles you dear... Reverse engineer *
New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
 
On Fri, 4 Jun 2004 15:23:30 -0400, "JTM" <jtm@yahoo.com> wrote:

"Soeren" <Look@iNO-SPAMt.dk.invalid> wrote in message
news:Xns94FED671D9187o8oLOOKatHOMEo8o@212.242.40.196...
Hi,
I would guess "no", since it will be high impedance while not
"activated", and the TC is very low impedance, but I fail to see the
purpose ?
Lightning goes to earth by the lowest impedance path and unless you
actually earth one of the TC-leads, the TVS will do nothing and the
lightning will possibly go to earth through your measuring device,
killing it very very very dead in the process.

Nearby strikes are taking us down so there must be an 'easy' route through
the equipment. I need to route the surge somewhere besides the equipment so
am considering a TVS or such.

I would try an earthed wire mesh around it, like the shield in a coax
cable (isolated from the TC leads of course).
You could test it with an ESD-generator (or an empty lighter with piezo
ignition).
Regards,
Soeren

The unshielded cable is in place and, like the reading equipment, cannot be
modified.
thanks


Here's an untested idea: Add a resistor in series with each
Click to expand...
TC lead at the equipment, followed by reverse-biased
diodes to the positive and negative rails of the equipment
circuit. That will clamp the incoming spikes to the rail
voltages, same as the ESD protection circuits built into
CMOS logic ICs.

If you don't want to open the equipment to tap into
the rails, then instead follow the series resistor with
parallel back-to-back diodes to ground. The TC
output is so small the diodes will have no effect in
normal operation since you'll be so far from forward
conduction.

You might need to fool around with the input resistors.
Also, that might screw up some cold-junction compensators
that use resistive networks right on each TC input.





Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
"JTM" <jtm@yahoo.com> wrote in message news:<NH2wc.14$w65.12@fe37.usenetserver.com>...
We have some long runs of Type-T thermocouple and would like to protect them
from nearby lightning strikes with a TVS or similar.

If I simply connect a bidirectional TVS between each lead of the TC and
ground will that affect the readings? Is there a better, but still compact
and inexpensive, solution for TCs? Modifying the reading equipment itself
is not an option.
Click to expand...
Would optical isolation with a transmitter would be too expensive? I
guess that would just put the problem into losing the transmitters
rather than losing your current instruments. That may not be any more
acceptable to what happens now.

Are the TCs sheathed and ungrounded?

I don't think a high impedance TVS as you describe would affect the
reading but a quick test would tell.

Bill Schuh
 
Hi Bob,


Here's an untested idea: Add a resistor in series with each
TC lead at the equipment, followed by reverse-biased
diodes to the positive and negative rails of the equipment
circuit.
[...]
Also, that might screw up some cold-junction compensators
that use resistive networks right on each TC input.
Click to expand...
And the resistors (acting with the reverse leakage of the diodes) would
throw the measurements so far off, that guessing a random number would
be more reliable ;)


--
Regards,
Soeren

* If it puzzles you dear... Reverse engineer *
New forum: <URL:http://www.ElektronikTeknolog.dk/cgi-bin/SPEED/>
 
On Mon, 7 Jun 2004 22:47:57 +0000 (UTC), Soeren
<Look@iNO-SPAMt.dk.invalid> wrote:

Hi Bob,


Here's an untested idea: Add a resistor in series with each
TC lead at the equipment, followed by reverse-biased
diodes to the positive and negative rails of the equipment
circuit.
[...]
Also, that might screw up some cold-junction compensators
that use resistive networks right on each TC input.

And the resistors (acting with the reverse leakage of the diodes) would
throw the measurements so far off, that guessing a random number would
be more reliable ;)
Click to expand...
TCs are normally read with high-Z input measuring circuits, so there
should be no appreciable current flow, hence no drop across the
resistors. TC outputs are typically 50 mV or less, so the
reverse-parallel shunt diodes wouldn't really be into conduction.
You could use reverse-series zeners in shunt as well. (Or probably
red LEDs, which have a super-sharp knee at 1.4 V forward bias.)


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
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