ADN8831

[shivansh]

I am using ADN8831 to control the TEC.
i did small board just like evealuation board and it worked just fine
now i designed the ckt for controlling two thermisters ...and i just
stuffed 1 stage out of two..and what i see is pin2 and 3 are not same
..there is some bias current...
I tried to do this thing on the new board as well but its just the
same...those chopper opam doesnt work at all...
i dont understand what wrong i am doing?
If anyone has any clue pls reply on this...
Thanks.

 

[Joerg]

I am using ADN8831 to control the TEC.
i did small board just like evealuation board and it worked just fine
now i designed the ckt for controlling two thermisters ...and i just
stuffed 1 stage out of two..and what i see is pin2 and 3 are not same
.there is some bias current...
I tried to do this thing on the new board as well but its just the
same...those chopper opam doesnt work at all...
i dont understand what wrong i am doing?
If anyone has any clue pls reply on this...
Thanks.

If you use two thermistors you might be best off providing en external
opamp. The ADN8831 isn't geared to do that.

 

[[email protected]]

I am using ADN8831 to control the TEC.
i did small board just like evealuation board and it worked just fine
now i designed the ckt for controlling two thermisters ...and i just
stuffed 1 stage out of two..and what i see is pin2 and 3 are not same
.there is some bias current...
I tried to do this thing on the new board as well but its just the
same...those chopper opam doesnt work at all...
i dont understand what wrong i am doing?
If anyone has any clue pls reply on this...
Thanks.

The ADN8831 is designed to control a TEC in a rather simple-minded
fashion.

The Linear Technology LTC1923 chip does the same job, and the Jim
Williams application note AN-89 is pretty good.

It isn't as good as the comment I published in Rev. Sci, Instrum. in
2004 - volume 75 pages 788-89, but you've have to e-mail me (at
billdotslomanatieeedotorg - my e-mail address is real) if you wanted a
copy of the pre-print.

 

[Ban]

I am using ADN8831 to control the TEC.
i did small board just like evealuation board and it worked just fine
now i designed the ckt for controlling two thermisters ...and i just
stuffed 1 stage out of two..and what i see is pin2 and 3 are not same
.there is some bias current...
I tried to do this thing on the new board as well but its just the
same...those chopper opam doesnt work at all...
i dont understand what wrong i am doing?
If anyone has any clue pls reply on this...
Thanks.

There is no significant bias current. Pin2 is non-inverting and Pin3 is the
inverting input of the first stage. They have similar input characteristics,
but still cannot be exchanged, or you have it latching to the rail.
I also quite don't understand what you want to do with the second
thermistor. Isn't it already built into the Laser-diode or Tec assembly? A
second thermistor might measure some other temp, but it cannot be used to
regulate the tec.
The second opamp stage is hard wired to the controller and is used to filter
the control voltage. Read about the non-linearity in thermistors and how to
linearize it.

 

[[email protected]]

inverting input of the first stage. They have similar input characteristics,
but still cannot be exchanged, or you have it latching to the rail.
I also quite don't understand what you want to do with the second
thermistor. Isn't it already built into the Laser-diode or Tec assembly? A
second thermistor might measure some other temp, but it cannot be used to
regulate the tec.
The second opamp stage is hard wired to the controller and is used to filter
the control voltage. Read about the non-linearity in thermistors and how to
linearize it.

A second thermistor can be useful to monitor the temperature at the
exhaust side of the TEC. The amount of heat per unit current
transferred across a Peltier junction depends heavily on the
temperature difference across it, and this can have a dramatic
influence on the gain around the thermostatic feedback loop.

Jim Williams Linear Technology application note AN89

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1095,P1893,D4178

discusses this in qualitative way, but doesn't include the quadratic
equation relating heat transfer in watts, current through the junction
in Amps and the temperature difference across the junction in degrees
Kelvin which allows you to deal with this in practice. As mentioned
above, I have got the equation into the literature - most recently in
2004 in Rev. Sci, Instrum. - volume 75 pages 788-89.

 

[Ban]

A second thermistor can be useful to monitor the temperature at the
exhaust side of the TEC. The amount of heat per unit current
transferred across a Peltier junction depends heavily on the
temperature difference across it, and this can have a dramatic
influence on the gain around the thermostatic feedback loop.

Jim Williams Linear Technology application note AN89

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1095,P1893,D4178

discusses this in qualitative way, but doesn't include the quadratic
equation relating heat transfer in watts, current through the junction
in Amps and the temperature difference across the junction in degrees
Kelvin which allows you to deal with this in practice. As mentioned
above, I have got the equation into the literature - most recently in
2004 in Rev. Sci, Instrum. - volume 75 pages 788-89.

But do you use the second sensor to change the gain/dynamic settings of the
PID-control loop, so the ripple of the temperature is minimized or the
polarity change avoided?
This seems to be rather a uP job than those simple controller chips.

 

[[email protected]]

PID-control loop, so the ripple of the temperature is minimized or the
polarity change avoided?
This seems to be rather a uP job than those simple controller chips.

We did use a uP to change the gain around the PID loop to keep it
fairly close to critically damped over a wide range of temperature
differences across the Peltier junction.

It is something that you could theoretically do with an analog
multiplier or two, but I've not heard of anyone doing ti that way. The
simple controller chips can't manage this, but aren't usually used in
circumstances where it is a problem.

 

[Joerg]

We did use a uP to change the gain around the PID loop to keep it
fairly close to critically damped over a wide range of temperature
differences across the Peltier junction.

It is something that you could theoretically do with an analog
multiplier or two, but I've not heard of anyone doing ti that way. The
simple controller chips can't manage this, but aren't usually used in
circumstances where it is a problem.

The larger Cypress PSoC might come in handy. You can program the
behavior of some of the analog sections on the fly. Should certainly be
fast enough in TEC applications.

 

[shivansh]

Ok let me describe in details ...
I have 2 TEC controller on board and each of them controlling one
Thermister...
The problem is on bth the chips at pin 2 and 3 i m getting 3 mV offset.
the second problem is one chip is properly sending heating and cooling
signal ...but the other one is just sending heating signal but not
cooling.
an I have connected TEC in propotional gain