Feedback Temperature Regulator:
I've had many request for information about a feedback temperature regulator for the Cookbook camera. Here is a design by Roy Tucker of SouthWest Cryostatics which he uses in his camera. Roy has released this design into the public domain and has given me permission to share it with anyone who is interested, so here it is.
I've made some minor modifications to Roy's design. Roy used a 4.7V Zener in his original design and a resistor network which gave a working temperature range of 188 degrees Kelvin to 243 degrees Kelvin or -85 C to -30 C. This is fine in Roy's Cryocam, but I fear that many of our Cookbook camera's may not be able to reach Roy's maximum temperature. That and the fact that the 4.7V Zener is difficult to find prompted me to replace Roy's Zener with the more easily obtainable 5.1V Zener. This diode is available off the shelf at Radio Shack. I modified the resistor network to provide a working range of 190 K to 270 K or -83 C to -3 C. I think most of our camera's are capable of maintaining -3 C and I doubt any of our camera's are capable of reaching -83 C so I think this range should cover us pretty well. I chose to use a 100K pot because at some point I plan on replacing the pot with a DS1267 dual digital potentiometer. I'm working on a circuit which will allow Win245 to utilize software controllable temperature regulation and the 100K pot will make the regulator circuit very easy to upgrade. I plan on using the remaining pot on the DS1267 to replace R43 on the pre-amp card. This will allow for a software selectable gain control for Cookbook camera's. More to come on this later.
I would recommend turning your TEC regulator voltage to maximum and simply leaving it there. The temperature regulator circuit will do the rest. You may even want to eliminate the voltage regulation portion of the Power Supply completely and tie your TEC positive voltage lead directly to the output side of the R1 and R2 parallel power resistors in the stock Cookbook power supply. There's plenty of room to experiment here. Roy states that his TEC power supply does not need to be precisely voltage regulated, but it should be well filtered and have less than 10% ripple. So, I think you'll do no harm by bypassing the voltage regulation portion of the Peltier supply.
Good luck with your temperature regulation experiments. Be sure to let me know about anything you discover along the way. I'll be sure to pass along your experiences so that we all may benefit.
Jerry Mulchin has an alternative temperature regulator circuit that has several advantages over the above. It uses the LM35 temperature sensor which reads out in direct degrees Centigrade and uses the existing LM317 voltage regulator in place of the ECG261. I invite you to visit his web page at http://www.employees.org/~jmulchin/tempreg.html and check it out. Excellent job Jerry!
Another interesting design is given by Scott Kroeger. His circuit is a proportional controller which uses a Radio Shack thermister as the temperature sensing unit. Check out his circuit at http://www.execpc.com/~kroeger/Peltier.GIF
