Out of interest - another DSLR cooling project - #5, I think. This should be entertaining reading...
It's PWM using a Teensy 2.0 with Arduino code. The Teensy native PWM frequency is much higher than the Arduino and more suitable, I think.
The idea was to cram all the electronics for cooling, dew heater and camera sensor defogger into one relatively small box (which hinges as shown for battery replacement - yet to fit the velcro strap), to make temperature control accurate and human friendly - a single switch and LED provide all the control and feedback necessary.
Cooling capacity is 20C max differential - cooling is zoned within this range to operate at 7, 5, 0 and -5C (7 is a bit arbitrary at the moment) depending on air temp. Once setpoint is assigned it doesn't change (based on temperature reducing at night). I'm not going to use it at > 25C.
Within a few degrees it will accommodate an increase in air temperature. A new session can be started within 5 -10 minutes of shutdown, giving the cold finger time to warm up to ambient allowing setpoint to be recalculated with the push of a button.
The push button on the front of the box is depressed during power-on, disabling cooling, while registering ambient temperature and assigning setpoint temperature - using a single temperature sensor attached to the cold finger. The red LED flashes morse code to report the selected setpoint - I kid you not. ... ..-. --.. -- S F Z M ??? you guessed it Seven, Five, Zero and Minus (5 only)
. Releasing the button activates cooling - on-temperature is indicated by the LED.
Now I want to try it out. I have fiddled with the snubber network to get the smoothest MOSFET switching possible, based on oscilloscope output. Virtual design tools are amazing.
FYI - high power, logic level MOSFET, Vgs 4.5 and very low Rds(on) - IRF2804 from utsource. I was using a NXP PSNM1R1-PL30, but these are difficult to source and expensive to ship. Both run cool without a heatsink - barely warm to touch. However, I've used a Freetronics SMD MOSFET in this project, with similar characteristics.
I know that there are differing views on the use of PWM, but I like it. Linear is naturally noise free but, having spent many hours working with it, it's a pain to manage in a nights imaging, where consistency is needed, particularly for calibration libraries.
It's been fun...