Need design for pos/neg voltage source

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I need to control a bath circulator/chiller. The analog input control
accepts 10mV per degree C. Positive voltages result in temperatures
above zero C, negative voltages (referenced to analog ground) result
in temperatures below zero C. I've build positive voltage supplies,
and negative voltage supplies, but no supplies that swing both ways.
What's the best way to do this? I was thinking of using a positive
supply but instead of connecting the ground directly to the analog
ground, connecting to a voltage divider with a 10:1 split. Would this
work?

 

[Dan Major]

If you don't have to generate the supply for the input control (i.e. if
you're just getting a voltage from somewhere else), _and_ you can stand
a hit to your common-mode rejection, you don't have to have a negative
supply.  You just need a differential amplifier on your front end
that'll shift the voltage up to whatever you want the zero reference to
be, and won't violate your op-amp's common mode input requirements while
it's doing it.

Dang I hate getting old. I've done something like this a few (ok,
about 8) years ago. I used an instrumentation amp to create an offset
voltage for an accellerometer used to meastre tilt. But I'll be
switched if I can remember how I did it. Don't have my design notes
from that far back. Could you please explain in simple words of one
sylable or less (grin).

 

[whit3rd]

I need to control a bath circulator/chiller.  The analog input control
accepts 10mV per degree C. ...  I've build positive voltage supplies,
and negative voltage supplies, but no supplies that swing both ways.

But, what's your input coming from? Do you want a knob? What
range does it cover? And, what is the input impedance you have to
drive? What power sources are available?

Probably you want to start with a regulated supply, and float it
(don't
connect either the (+) or (-) terminal to ground). Then, one
resistor divider connects to the ground reference, and another,
adjustable divider, connects to the analog input.

 

[Spehro Pefhany]

I need to control a bath circulator/chiller. The analog input control
accepts 10mV per degree C. Positive voltages result in temperatures
above zero C, negative voltages (referenced to analog ground) result
in temperatures below zero C. I've build positive voltage supplies,
and negative voltage supplies, but no supplies that swing both ways.
What's the best way to do this? I was thinking of using a positive
supply but instead of connecting the ground directly to the analog
ground, connecting to a voltage divider with a 10:1 split. Would this
work?

Yes, maybe, depending on what the input control impedance looks like.

Something like this:








.----------.
+-----------------------------------+----------|+5 |
| | | |
| | | |
| .-. | |
| | | | |
| | |249R | |
| 2K00 200R '-' | Isolated |
.-. ___ ___ | | |
| |<-----|___|---+-----|___|--+------+ | PSU |
500R | | | | .-. | |
'-' | | | | | |
| | | | | | |
| | | '-' 249R | |
| | | | | |
| | | | | |
| o o | | |
| + to ctrl - | | |
| | | |
| | | |
+-----------------------------------+----------| 0V |
'----------'


This should give you around (a bit more than) +/- 20°C range depending
on the load impedance. You can connect a voltmeter across the output
terminals (with the controller attached) to "calibrate" the pot angle
vs. output voltage.

 

[Dan Major]

I have read the other responses and now i know i have a different take
on this.

Some questions:
What is the temperature range of the fluid bath apparatus?
What is the maximum current required at the control input?
Have you considered normal DC amplifiers?
Do you need to provide more advanced control than setpoint?
Do you need to provide an error signal instead of a setpoint voltage?
Do you want to have controlled temperature vs. time profiles?
What is the temperature drive capability of the bath?
What is the heat load capacity of the bath?
etc.,
  .

I'm sorry. I was checking for responses for a while, didn't see any
and gave up. Now here, a month later, I find that some people *did*
peply and I missed them. Again, please forgive my rudeness!

What I have is a Thermco Scientific Neslab RTE circulating bath
chiller. It has provisions for a remote control of temperature. The
voltage input for the setpoint is 10 mV per degree C. We need to
chill down to about -10 degrees C (machine limit is +200 degrees C to
-20 or -25 degreed C). This will provide circulating fluid to chill a
seres of condensers. The ovens that will work in conjunction with the
chillers will operate at up to 800 degrees C. Although the main
controls on the chiller DO provide for temperature profiling or
ramping, we will operate at a single temp setpoint. Considering the
operating temp of the ovens and the mass of the reactors inside them,
plus the mass of the condensers, I think we'll be able to chill down
the condensers a lot quicker than the ovens heat up. The chiller also
has an output of 10mV/degree C of actual temp. I plan on using some
inexpensive LCD panel meters to monitor the voltages. I have a fairly
well regulated 12v DC supply that is powering other instruments/stuff
on the panel. I appreciate any comments or suggestions you can give.

 

[Dan Major]

This should give you around (a bit more than) +/- 20°C range depending
on the load impedance. You can connect a voltmeter across the output
terminals (with the controller attached) to "calibrate" the pot angle
vs. output voltage.  

Thanks. That looked a terrible mess until I cut/pasted it into
notepad and used the right font. I don't know what the input
impedance is, I assume it's high using such small values for control
voltages. I don't know if I can easily find these resistor values,
but I'll play around with what I can get, keeping close to the ratios
you mention.