| This is the temperature regulator circuit I use with my CB-245 camera.
Is is designed to operate directly with the original Peltier power supply
found in the Cookbook. This circuit is driven from a National LM35 temperature
sensor which is mounted in a hole on the cold finger of the CCD chip. The
voltage output of the LM35 is in Degrees Centigrade and has a normal range
from +100 Deg C to -40 Deg C. The output voltage range is 10 mv/Deg C which
means the device puts out a value from 1.0 Vdc at +100 Deg C, to -400 mv
at -40 Deg C. That meant I had to put a analog inverter to flip the LM35's
voltage output to a positive range when the device was operating with the
CB-245. That is what the LF356 opamp is used for. It is a Unity gain inverting
amplifier, with a 20 K ohm pot to adjust the gain to unity. The output
of the LF356 drives the minus input of the LM339 comparator device. This
device is used to sense the difference between the temperature set point
and the LM35's output. When the temperature of the cold finger is below
the temperature set point, the LM339 device is turned "OFF".
That means the Peltier voltage is running at what ever the voltage adjustment
pot was set for. But when the temperature trip point is reached the LM339
conducts, pulling the Peltier voltage down to about 1.25 Vdc, the reference
voltage of the LM317 regulator. The Peltier voltage is never really OFF,
but rather idles along at about 1.25-2.0 Vdc when the temperature set point
is reached. When the Peltier starts to warm up, the temperature trip point
falls, and turns on the Peltier voltage again. The causes the Peltier to
cool down again, and reach the temperature trip point again. After awhile
the the voltage on the Peltier starts to look like a square wave with a
varing pulse width. That is actually the voltage on the Peltier changing
to keep the temperature at as constant a temperature around the trip point
as possible. Theis circuit can regulated the Peltier temperature to within
+/- .1 Degrees Centigrade.
If you decide to use this circuit please make note the the 0.1 uF capacitor across the LM317 voltage adjustment to ground will have to be removed. Otherwise the temperature regulator will have a hard time reaching a stabilized temperature. The capacitor will cause the voltage to ramp instead of turn ON/OFF, which is needed to keep the temperature changes as small as possible. Removing the 0.1 uF capacitor will not degrade your filtering in any way. The LM317 regulator has a very high ripple rejection already. I have been using this circuit for about 6 months now and it has worked perfectly. Adjustment is easy. Just turn on the power supply like before. Set the temperature adjust pot for what ever temp you want and then start adjusting the Peltier voltage pot until you reach about 7 to 7-1/2 volts. Do this in reasonably small steps and let the temp stabilize a little before increasing the Peltier voltage up. Shutdown is the reverse process. Bring down the Peltier voltage in small steps until you reach the lowest you can get to. Then turn the temperature adjust pot down. |