Subject: Re: Temperature Probe Interface / Coax cable Issues
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Date: Mon, 20 Jan 2003 23:57:31 -0500
NNTP-Posting-Date: Mon, 20 Jan 2003 23:57:38 EST
Organization: Bell Sympatico
Place a capacitor across the sensor and observe the change, if any.
If it is oscillating it will change , if the problem is the extra
capacitance it will also change.
If it doesn't change the problem is something else.
On Mon, 20 Jan 2003 23:51:47 -0500, Vlad wrote:
>I have a thermometer that has this IN and OUT temperatures reading.
>I extended the length of the OUTsensor by about 15 feet and the
>reading changed by about 2 degrees C.
>Connected the X10 probe to the input connector of the meter and
>observe two things :
>a) The signal is measured once a minute by a short pulse ( may be to
>reduce the heating effect on the sensing element). and probably using
>a sample and hold circuit.
>b) The waveform of the pulse lost some of the high frequency
>components when the longer cable was used, probably caused by the
>increased capacitance of the cable. Rounded top corners.
>In this case the most importante caracteristic of the cable his it's
>On Tue, 21 Jan 2003 02:43:03 GMT, John Popelish
>>Kevin Walton wrote:
>>> Summary - is this just impedence matching problems from using the
>>> wrong coax?
>>> I wanted to replicate a temp probe interface that I am using, which I
>>> have done and it works - ish.
>>> I didn't try to understand the circuit but it uses an ICL7660 and a
>>> TS912 IC, 2 variable resistors, a zenier reference, resisters,
>>> capacitors and 2 1000UH SMD Inductors - one on each input wire.
>>> The sensing is done by a remote 1K0 3900ppm NTC sensor, connected by
>>> coax and a bnc plug/socket. The original coax is unmarked, 3.2mm
>>> external diameter, rubbery coated, 2.5m long.
>>> In my replacement probe the coax is RG 58 C/U, 5mm dia., 3.5m long.
>>> I calibrated the temp reading succesfully using the new interface and
>>> probe with the coax rolled into a say 20cm roll on the table, but then
>>> unrolling the cable caused significant change in the reading. Placing
>>> 2 sensor rolls of wire on top of each other caused the readings to go
>>> None of this occours in the original sensor, it can be rolled,
>>> unrolled, placed ontop of new sensor cale rolls with no problems.
>>> Is this an obvious problem that I just don't appretiate, such as
>>First off, coax impedance is not involved in low frequency signals
>>(approaching DC), like your thermistor signal. For that type od
>>signal, all that might matter is DC resistance (not at all likely a
>>factor at 3.5 meters), shielding effectiveness (if in a very high
>>noise environment) and possibly, total capacitance, if the circuit is
>>poorly designed, and this is a factor in an opamp stability. In other
>>words, about the only way you can get it wrong is of you connect the
>>outside of the coax as your signal input. If not, and your circuit is
>>still acting up, you have some other problem that you have created
>>while changing the coax.