Part 2:
I envisage that I will develop this circuit further possibly incorporating current fold back and other changes. When I can get my hands on some of the newer protected power switch FETs. But at the moment these designs are released for use.
I have done many hrs of testing to ensure the circuits are capable.
Circuit Specifications (an overview):
- Will operate between 10V and 18V DC. No over voltage protection. It’s not needed here, ATM.
- 4 Independent channels with control of pulse width mark space ratios from 1% to 99%.
- Can deliver 3 Amps (plus) per channel but recommend all 4 channels in operation do not exceed 8 amps total. You have to remember this is a cheap design and 3 amps is around 36 Watts of energy. That is a lot! 99.9% of users will never use anywhere near this amount of energy to keep dew at bay!
- FETs run at around 55ºC (case temp) at max I draw of 3A (with Heat-sink).
- (Simple) Reverse polarity protection (short circuit diode).
- Over current protection (8 amp fuse) main fuse.
- Over current protection (4 amp fuses) on the 100mm x 80mm PCB design for each individual FET.
- Pulse width (energy output settings) can be easily identified by the use of a flashing LED (inbuilt in each switch) on each channel.
- Low frequency PWM. Around 1Hz of operation which should not induce “spike” power supply noise. IMHO you do not need higher operating frequencies here.
- Low power consumption. Around 4 mA at idle.
- Uses a “P” channel high side switch with low “on” resistance (Rds). You cannot short +12V. The FET, because of its low Rds, will only be dissipating a few milli-watts of energy over its package, meaning the majority of the energy is delivered across the heater strap thus maximum efficiency.
- Cheap design. Can be made for around $100 or less depending upon your access to odds & ends.
Attached are pics of my 1st version PCB & design. These pics are just to show how I fitted the PCB. A lot has changed/improved since this version however; it is still running well & without fault for over 1 year.
When all is said & done I’ll fill in the ground planes on the PCBs and transport them to the projects & Articles.
edit:
the box design is for the small PCB only. I have never built the large PCB...but will do so soon.