Richard said:
I need to build a thermocouple simulator for the test bench. It will be
for type K and will simulate 32 F to 1600 Deg F or there abouts.
It is NOT a calibrator, just to be able to simulate some adjustable
inputs. I have to build a box with 24 pots in it for 24 channels.
No whiz at all on this so give me hand if I'm getting in trouble. I'm
probably doing the hard way. My supply is from a 7805 so I was going to
take 5 volts to each pot like this..
5v ---10K----100---- Ground
Between the 10 k resistor and the 100 ohm resistor, I would put a 10 K
pot. so that one end of the 10 pot went to ground and my center tap would
be an adjustable 0 to 50 millivolt output.
Is there a better arrangement of resistors to use?
Thanks.
Richard
I used to calibrate thermocouple potentiometers in a cal lab (late 60's)
with a voltage standard such as the Fluke 332. If you have access to a lab
voltage standard, all you need is a temperature-voltage table for the
thermocouple types you need to simulate. Set the voltage standard to the
voltage corresponding to the desired temperature and there you are.
Using your method, I can see a problem immediately. The circuit that you
are going to drive with the output of your resistor string will need to have
a very high impedance in order to avoid loading problems. A better way, if
you insist on using this method, is to follow the resistor string output
with a quiet, low-drift op amp that can easily drive the circuit that will
measure the voltage. You can find a good assortment of those at Linear,
Analog Devices, Maxim, etc. Make sure that you select a model that has
offset voltage adjust facilities, or provide another method to adjust the
output offset to zero volts for zero volts input. You also would need to
use a bipolar power source for the op amp.
A third method would be to use a stable power source into a 15- or 25-turn
wirewound pot. Follow the output of the pot with a stable op amp with
suitable gain to drive a 3 1/2- or 4 1/2-digit digital panel meter to give
you a scaled reading. Use the voltage table to adjust to the voltage you
need.
Whichever method you decide to use, you need to pay close attention to the
quality of the components. Use low tempco resistors and pots. Use
low-drift op amps. Use very stable voltage sources. Otherwise, all the
detail you put into your simulator will be wasted by temperature drift and
instabilities.
All that said, if you're doing this for your company, I'd suggest that
buying a simulator might be cheaper than building one.
Omega has a cheap one at
http://www.omega.com/ppt/pptsc.asp?ref=CL601_PHCL-602&nav=temk10. It
doesn't have the accuracy of the higher-priced models, but you didn't state
that requirement in your post. Googling for "thermocouple simulator"
returned a lot of possibilities.
Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)
Never take a laxative and a sleeping pill at the same time!!