Part 1, see next post for part 2.
I am putting these designs up in the forum prior to placing them in the Projects section to get feedback.
Attached are circuit designs for a 4 channel PWM dew heater controller. This circuit basic design has been around for some time, however, principally each previous design has a major flaw in that these circuits utilise a low side switch device. This design means that the outside shell of your RCA connector can effectively become +12V. Touch this against any earth (which is easy to do with RCA connectors) and you will pass 12V thru to earth with possible consequences to your valuable equipment.
My circuit utilizes a “P” channel FET acting as a “high side switch”. In this manner, the outside of the RCA connector will always be earth. You can not short the +12V because in my design the outside of the RCA connector will always be 0V as it connects the RCA jack outer sheath to earth.
I have provided this project in two circuit PCB versions. Both are 4 independent channel PWM dew heaters: one fits on a single side 100mm x 60mm PCB; the other has fuse protection for each FET and fits on a single side 100mm x 80mm PCB. This allows other users to amend my circuit easily, print and make their own PCBs in the future if they so wish. These PCB designs are within the Eagle freeware maximum PCB size parameters of 100mm x 80mm.
I have made some small changes to my previous design attempts and have trialled many other changes but settled on: a simple design; reliable design; cheap with easily available parts; that fit on a PCB using Eagle PCB freeware! As well the design is a 4 independent channel PWM heater. You can control how much energy you apply to keep dew at bay at independent levels for each individual channel.
edit:
Oh the drawing is showing a dual channel only just to show the design.
Eagle free-ware is available
here